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8/10/2019 365 Days of Medical Physics_ May 2012
1/16
Nuggets of knowledge about medical physics.
365 Days of Medical Physics
24 May 2012
IMAT/VMAT basics
One of the newest and most interesting external beam delivery techniques today goes by many
names: intensity modulated arc therapy (IMAT), volumetric modulated arc therapy (VMAT), etc.
To make things more confusing, vendors have each given this technique their own proprietary
names: RapidArc (Varian), SmartArc (Philips), and VMATTM[didn't I already say VMAT?]
(Elekta). Maybe the most general and correct name for IMAT would be conebeam dynamic angle
fluence modulated xray therapy(CBDAFMXT), but you might confuse that acronym with a
chemotherapy drug name... In this post I'll discuss some of the ba sics of this arcbased form of
IMRT (and just call it IMAT to keep things simple).
At its most basic IMAT is essentially conventional IMRT, but with the gantry moving in one or
more rotating arcs, rather than delivering from a small number of fixed angles. This means that
most of the concepts and advantages and disadvantages of IMRT apply to IMAT (detailed below).
IMAT was developed (and marketed!) as a conventional linacbased alternative to helical
tomotherapy and as a more conformal / lower critical structure dose andfaster version of static
angle IMRT.
In the figure showing the IMRT hierarchy, IMAT is on the branch of conebeam, dynamic gantry
IMRT. In order to deliver IMAT, a linac must have some of the following capabilities: gantry
motion with beam on, dynamic MLC (i.e. leaf motion with beam on and gantry rotating), and
variable dose rate.
Planning of IMAT is very similar to conventional IMRT. The plan is determined by inverse
planning methods. The degrees of freedom are increased by considering gantry rotation speed,
dose rate, number of field shapes, number of arcs, etc. For planning, arcs are usually
approximated with a f inite number of angles (e.g. 36). Constraints can be more tightly matchedwith multiple arcs at the expense of delivery time. Another important aspect in IMAT
optimization is that MLC leaf speed limits the beam shape "distance" from one angle to the next,
i.e. the MLC leaf positions cannot vary greatly from one angle to the next and thus beam shape
"interconnectedness" must be taken in to account.
Advantages of IMAT include:
Highly conformal target volume dose with lower dose to critical structures than IMRT
or 3DCRT, as dose is spread over more angles.
Faster delivery times and lower MU's (especially single arc IMAT) when compared with
IMRT.
Noncoplanar arcs possible.
Comparable plans to helical tomotherapy, but performed with a conventional linac.
The hierarchy of IMRT techniques.
Roy
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About Me
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365 Days of Medical Physics blogging
About this blog:
Dosevolume histogram basics
IMAT/VMAT basics
Comparing dose distributions: Thegamma test
Multileaf collimators: modern beamshaping
The many faces of bolus
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bolus (2) cDVH (2)DTA (2) DVH (2)
imrt (2) intro (5)MLC
(2) overview (2) QA(2) statistics (3)
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2012 (15)
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IMAT/VMAT basics
The many faces of bolus: Part 2
Dosevolume histogram basics
Compensatorbased IMRT
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2/16
Posted by Roy at 11:52 PM 3 comments:
Labels: IMAT, intro, VMAT
Disadvantages of IMAT include:
Higher cost of hardware and software licensing relative to IMRT.
Increased complexity of plans makes QA a poor diagnostic tool (i.e. hard to
determine source of QA failures).
IMAT delivery techniques are the obvious(?) next step following IMRT. In fact, it's hard to come
up with a list of concrete disadvantages of IMAT over IMRT. (Please comment if you feel
otherwise.) In our clinic it's one of the few new techniques that everyone seems to have adopted
with open arms.
Further reading:
Cedric X Yu and Grace Tang, Intensitymodulated arc therapy: principles, technologies
and clinical implementation, 2011 Phys. Med. Biol. 56 R31 doi:10.1088/0031
9155/56/5/R01(open access).
David Shepard, Clinical Implementation of Intensity Modulated Arc Therapy,
presentation, 2009,
http://www.medicaldosimetry.org/meetings/2009handouts/Shepard_VMAT.pdf
Recommend this on Google
20 May 2012
The many faces of bolus: Part 2
PreviouslyI discussed the role of bolus material in radiation therapy and some of the forms it
takes. This post shows a couple of other examples.
Super Stuff bolus, also known generically as pink bolus, is a moldable bolus material with the
consistency of gelatin. The material is described by the manufacturer as a "hydophilic organic
polymer" and is sold in individual powder packets. Pink bolus is supposed to have a density of
1.02 g/cm3. To use the bolus, you add the necessary amount of water, allow the material to set,
i.e. coming to its gelatinlike consistency, and then knead it into the shape you want. Over time
pink bolus will lose its shape and must be reshaped. Eventually it will lose some consistency due
to moisture loss and a new batch must be made. Care must also be taken to remove as many air
bubbles as possible.
Pink bolus molded into shape.
A packet of pink bolus powder.
Medical physics journals
Comparing dose distributions: Thegamma test
Comparing dose distributions: DTA anddosediffere...
Radiation therapy availability aroundthe world
New medical physicists in the US:Crunching the nu...
The many faces of bolus
April (5)
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3/16
Posted by Roy at 2:45 PM No comments:
Labels: bolus
Recently in our clinic we treated a patient with classic (i.e. nonHIV related) Kaposi sarcoma of
the leg with photons. For this we decided to use rice grains as the bolus material. As with all
bolus, the idea of using rice is to simulate tissue and modify the dose distribution as desired. In
this case, increase of skin dose is desired.
For this patient we built a polystyrene foam box and filled it with loose rice grains. It
took approximately 10 kg of dry parboiled rice to fill the box with the patient's leg. The patient
plus rice box was then scanned with the CT and planned as normal.
The open access article linked below from Ahn et al. shows some dosimetric comparisons
between the use of rice as a bolus and a water bolus for irradiating extremities. I will warn you
that both methods create a mess at best :)
Further reading:
Ahn SK, Kim YB, Lee IJ, Song TS, Son DM, Jang YJ, Cho JH, Kim JH, Kim DW, Cho JH,
Suh CO. Evaluation of a Waterbased Bolus Device for Radiotherapy to the
Extremities in Kaposi's Sarcoma Patients. J Korean Soc Ther Radiol Oncol. 2008
Sep;26(3):189194. http://dx.doi.org/10.3857/jkstro.2008.26.3.189 (Open access.
In Korean with abstract and figure captions in English.)
Rice bolus box for treating a patient's leg/foot.
Some leftover rice (not) used as bolus material.
Recommend this on Google
17 May 2012
Dosevolume histogram basics
A dosevolume histogram (DVH) is a mathematical tool to assess the appropriateness of a given
radiation therapy plan. It can be used to assess whether a plan meets desired constraints for a
voulme of interest, within certain limitations. DVHs are widely used and understanding how
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4/16
Posted by Roy at 11:54 PM 9 comments:
Labels: cDVH, dDVH, DVH
they work is a basic skill for treatment plan assessment. In this post Ill discuss some DVH basics.
A DVH is nothing more than a histogram, but it is important to understand where the data comes
from and how the DVH is representing the data. Modern treatment plans are created based on 3D
image sets created using CT, MRI, etc. These data sets consist of voxels (the 3D equivalent of
pixels). A volume of interest, e.g. a PTV, consists of a subset of these voxels. The basic data in a
DVH is generated by binning the dose values from each voxel in the volume. (Interpolation may
be necessary if the bound of the volume intersects a voxel.) This binned dose frequency data
comprises a differential dosevolume histogram, or dDVH, which I will discuss in more detail in a
future post. The dDVH looks like a common histogram and gives you an idea of how many voxels
receive a certain dose, e.g. the dDVH might show that 85% of the PTV voxels received 98%
102% of the prescribed dose and 46% received exactly 100% of the prescribed dose.
The more familiar form of DVH is the cumulative dosevolume histogram, or cDVH. This DVH is
calculated by summing the dDVH starting at the dose of interest, D, up to the max dose, Dmax
(Eq. 1).
The cDVH displays the percent/number of voxels in a volume which receive at least a dose D,
i.e. the cDVH of a volume irradiated perfectly uniformly to 100 cGy will show that 100% of the
voxels received at least 30 cGy, 50 cGy, 80 cGy, etc, but 0% received 105 cGy. Thus for an ideal
treatment plan, the cDVHs of the target volumes will have a rectangular, stepdown function
appearance and the cDVHs of critical volumes will drop immediately to zero.
In the real world treatment plans are not ideal (I know, its sad). Instead acceptable dose
constraints are set for targets and critical structures. DVHs can be used to determine if these
constraints are mets. One caveat is that standard DVHs do not directly provide spatial
information about the dose distribution. One less than ideal method is to create subvolumes,
but creating useful/meaningful subvolumes is a nontrivial exercise.
Top image from Vorwerk et al. Radiation Oncology 2008 3:31, doi:10.1186/1748717X331, used under CC Licenseterms.
A typical cumulative dosevolume histogram (cDVH).
Eq. 1
Recommend this on Google
13 May 2012
Compensatorbased IMRT
Intensity modulated radiation therapy (IMRT) is almost always performed with the use of a
multileaf collimator (MLC). This is, however, not the only way to deliver static angle IMRT.
Another method is with the use of compensator blocks. In this post I will talk a little bit about
this less common IMRT technique.
http://medphys365.blogspot.com/search/label/cDVHhttp://creativecommons.org/licenses/by/2.0http://medphys365.blogspot.com/2012/05/compensator-based-imrt.htmlhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=4052496099203236182&target=pinteresthttp://medphys365.blogspot.com/search/label/DVHhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=4052496099203236182&target=twitterhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=4052496099203236182&target=facebookhttp://www.blogger.com/profile/14293477186383292023http://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=4052496099203236182&target=bloghttp://medphys365.blogspot.com/2012/05/dose-volume-histogram-basics.html#comment-formhttp://www.ro-journal.com/content/3/1/31/http://2.bp.blogspot.com/-QH1LXw2hb40/T7XfTBBs0EI/AAAAAAAAAHw/PZjmkLrHZJ0/s1600/Vorwerk-cDVH1.jpghttp://medphys365.blogspot.com/2012/05/dose-volume-histogram-basics.htmlhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=4052496099203236182&target=emailhttp://medphys365.blogspot.com/search/label/dDVHhttp://3.bp.blogspot.com/-insnvXwHvks/T7Xywgz16nI/AAAAAAAAAH8/hXh_Vk9sBHs/s1600/cDVH-eq1.png8/10/2019 365 Days of Medical Physics_ May 2012
5/16
Posted by Roy at 11:56 PM No comments:
Labels: compensator, imrt, MLC
As discussed in a previous post, IMRT requires fluence modulation not possible with conventional
poured / handcut blocks. This fluence modulation is necessary to achieve the desired target
matching and critical structure sparing via inverse planning optimization. This fluence
modulation is typically achieved using an MLC, which has many advantage as well as
disadvantages. An alternative method, in use since at least the mid 1990's, is fluence modulation
via solid compensator blocks designed for each individual field. The above image shows a sample
compensator made of milled brass.
Compensatorbased IMRT is purported to have several advantages over MLCbased IMRT,
including:
Being static, each field is delivered more quickly (also lower MU's).
Fluence patterns can be closer to the ideal, i.e. not limited by leaf size, speed, or
leakage.
Potentially cheaper.
Avoids field splitting. (Did I ever mention I hate split fields?!?)
Along with these advantages come possible drawbacks, including:
Long fabrication times, versus automated MLC patterens.
Therapists must change compensator for each field.
Potential for beam hardening.
Large size / weight to achieve low dose regions.
Compensators can be fabricated from a range of materials, including brass, Wood's metal
(Cerrobend), PMMA (Plexiglas), and tungsten powder composite. Milling. molding, or stacking
and bolting are possible fabrication techniques. A handful of companies sell custom fabricated
IMRT compensators on demand, delivering within one or two days of order.
Do you have any experience with this technique?
Further reading:
Chang, S., Cullip, T., Deschesne, K., Miller, E., & Rosenman, J. Compensators: An
alternative IMRT delivery technique. Journal Of Applied Clinical Medical Physics, 5(3),
2004. doi:10.1120/jacmp.v5i3.1965 (open access)P.C. Williams, IMRT: delivery techniques and quality assurance, British Journal of
Radiology (2003) 76, 766776, doi: 10.1259/bjr/12907222(open access?)
Brass IMRT field compensator from .decimal, Inc.
Recommend this on Google
11 May 2012
Medical physics journals
If you want to keep up to date on the latest developments in medical physics, journals are one of
the best resources. In this post I'm going to compile a list of medical physics journals and
http://bjr.birjournals.org/content/76/911/766.fullhttp://medphys365.blogspot.com/2012/05/compensator-based-imrt.htmlhttp://medphys365.blogspot.com/2012/04/basics-of-imrt-overview.htmlhttp://medphys365.blogspot.com/search/label/compensatorhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=8314635032260864515&target=bloghttp://medphys365.blogspot.com/2012/05/compensator-based-imrt.html#comment-formhttp://www.blogger.com/profile/14293477186383292023http://medphys365.blogspot.com/2012/04/multileaf-collimators.htmlhttp://medphys365.blogspot.com/2012/05/medical-physics-journals.htmlhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=8314635032260864515&target=twitterhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=8314635032260864515&target=facebookhttp://medphys365.blogspot.com/search/label/imrthttp://medphys365.blogspot.com/search/label/MLChttp://4.bp.blogspot.com/-Gs6Gb6tq6nc/T7CTEAHgzgI/AAAAAAAAAHY/g4ifqrIXOls/s1600/Decimal_compensator_nima.JPGhttp://www.jacmp.org/index.php/jacmp/article/view/1965http://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=8314635032260864515&target=pinteresthttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=8314635032260864515&target=email8/10/2019 365 Days of Medical Physics_ May 2012
6/16
Posted by Roy at 12:36 AM 4 comments:
Labels: journals, open access
journals with medical physics related content. I will also mention the degree of open access for
each journal (that I'm aware of) and the hindexas computed by Google Scholar.
Medical physics specific journals:
Medical Physics, published by AAPM. Some article types made open access. h5index:
45.
Physics in Medicine and Biology, published by Institute of Physics. Open access option
for authors with publishing fee. h5index: not available.
Journal of Applied Clinical Medical Physics, published by AAPM. Fully open access. h5
index: 15.
Journal of Medical Physics, published by the Association of Medical Physicists of India.
Fully open access. h5index: 7.
arXiv.org medical physics category. Fully open access. Overall for arXiv.org, h5index:
256.
Magnetic Resonance in Medicine, published by the International Society for Magnetic
Resonance in Medicine. Paid access only. h5index: 50.
Other journals with medical physics content:
The Red Journal(International Journal of Radiation Oncology * Biology * Physics),
published by ASTRO. Paid access only. h5index: 68.
The Green Journal(Radiotherapy and Oncology), published by ESTRO. Paid access
only. h5index: 48.
Radiation Oncology, published by BioMed Central. Fully open access. h5index: 23.
Practical Radiation Onoclogy, published by ASTRO. Paid access only. h5index: N/A.
Medical Dosimetry, published by the American Association of Medical Dosimetrists.
Paid access only. h5index: 15.
More info can be found on the state of open access and medical physics publications in my post
about open access on Will Work for Science.
Any other additions to add?
+1 Recommend this on Google
10 May 2012
Comparing dose distributions: The gamma test
In my last post I discussed dose distribution comparison with dose difference and distanceto
agreement (DTA) tests. Another widely used and closely related method for comparing dose
distributions is the gamma test.
The gamma test was first introduced by Low et al. in 1998 as a single metric that combined
features of both dose difference and DTA, while performing robustly in the regions where those
are prone to failure. Conceptually, gamma is very similar to dose difference and DTA, but
combines them into an abstract metric resembling a distance (Eq. 1). In this way both dose
difference and DTA are taken into account for every point compared (rather than eitheror as
previously discussed).
In the above equations I have used somewhat different notation than Low et al. in an attempt to
make things slightly clearer.
If we wish to compare two dose distributions, e.g. a measured versus a calculated distribution,
we will have a dose, Da(ra), in the first distribution at point ra, and a dose, Db(rb), at the
Eq. 1
Eq. 2
http://www.practicalradonc.org/http://www.jacmp.org/http://medphys365.blogspot.com/2012/05/comparing-dose-distributions-gamma-test.htmlhttp://www.sciencedirect.com/science/journal/09583947http://www.ro-journal.com/http://www.blogger.com/profile/14293477186383292023http://medphys365.blogspot.com/2012/05/comparing-dose-distributions-dta-and.htmlhttp://medphys365.blogspot.com/2012/05/medical-physics-journals.html#comment-formhttp://4.bp.blogspot.com/-pjT5xSJPTDY/T7CD00gG95I/AAAAAAAAAG4/uhIV3uHm_Ps/s1600/gamma-eq1.pnghttp://iopscience.org/pmbhttp://www.medphys.org/http://willworkforscience.blogspot.com/2011/01/open-access-medical-physics-and.htmlhttp://medphys365.blogspot.com/2012/05/medical-physics-journals.htmlhttp://en.wikipedia.org/wiki/H-indexhttp://www.aapm.org/http://www.jmp.org.in/http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1522-2594http://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=4014840933681586672&target=twitterhttp://scholar.google.com/intl/en/scholar/metrics.htmlhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=4014840933681586672&target=facebookhttp://medphys365.blogspot.com/search/label/open%20accesshttp://medphys365.blogspot.com/search/label/journalshttp://arxiv.org/list/physics.med-ph/recenthttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=4014840933681586672&target=pinteresthttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=4014840933681586672&target=bloghttp://2.bp.blogspot.com/-xERYTfAD83I/T7CD1LNvJSI/AAAAAAAAAG8/3GzsPXEy3kw/s1600/gamma-eq2.pnghttp://journals.elsevierhealth.com/periodicals/radohttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=4014840933681586672&target=emailhttp://www.sciencedirect.com/science/journal/036030168/10/2019 365 Days of Medical Physics_ May 2012
7/16
Posted by Roy at 12:40 AM 5 comments:
Labels: DTA, gamma, QA
corresponding point rbin the second distribution. The DTA condition is fulfilled when Da(ra) =
Db(rb+r), where ris an arbitrary point a distance |r| away from rb. This condition defines an
isodose contour in distribution baround point rb. Away from this contour the DTA, dDTA, is
undefined. DTA is used with a threshold passing value, DTA, e.g. 3mm. A DTA smaller than the
threshold is considered passing for a simple DTA test. For gamma, DTAis used to normalize the
DTA value, such that a normal passing value would then be unity.
Dose difference is simply the difference of the two doses at the corresponding points: |D a(ra) =
Db(rb)|. As with DTA, a pass/fail threshold, DD, is used in the simple dose difference test, but
is used to normalize the result in the gamma equation, such that the normal "passing" value
would be unity.
We now have two components: normalized DTA and normalized dose difference. By squaring
these values, adding, and taking the square root, we have a distancelike metric, , shown in
Eq. 1. Because DTA is only defined for values of r, such that Da(ra) = Db(rb+r), is only defined
when that condition is met (geometrically located along the DTA isodose contour).
Finally, the actual gamma index, , is determined by finding the minimum value of by varying
r. This essentially means traveling along the isodose contour and finding the point at which DTA
is smallest.
The convention is for passing to be 1 and failing to be > 1. You will notice that a point
yielding normalized DTA = 1 and normalized dose difference = 1 would now fail, since the
corresponding would be 2.
What provides is a single value to evaluate, versus using separate tests and then consideringboth. As with DTA, presents challenges in efficient implementation (clearly Eq.'s 1 and 2 are
not hand solvable).
Your comments (especially corrections) are appreciated.
Roy
Further reading:
D. A. Low, W. B. Harms, S. Mutic, and J. A. Purdy, A technique for the quantitative
evaluation of dose distributions, Med. Phys. 25, 656 (1998);
http://dx.doi.org/10.1118/1.598248
Recommend this on Google
06 May 2012
Comparing dose distributions: DTA and dosedifference
Radiation therapy plan quality assurance often hinges on comparing calculated dose distributions
with measured dose distributions. One of the most common techniques to compare dose
distributions is the combined use of distancetoagreement (DTA) and dose difference. In this
post I will give an overview of these concepts.
Dose difference is a very straight forward comparison of dose at corresponding points in two
distributions. Given a point apin the planned distribution and the corresponding point amin the
measured distribution, the dose difference is simply D(am) D(ap). A passing criterion is used,
e.g. 3% of planned dose, such that if the measured dose difference is
8/10/2019 365 Days of Medical Physics_ May 2012
8/16
Posted by Roy at 3:00 PM 7 comments:
Labels: DTA, QA
Distancetoagreement (DTA) is also very straight forward conceptually. Given a point apin the
planned distribution and the corresponding point amin the measured distribution, the distance
toagreement is the nearest point in the measured distribution from am, such that D(am+ r) =
D(ap). As with dose difference, a passing criterion is chosen, e.g. 3 mm. If the matching dose
level is found within a radius of
8/10/2019 365 Days of Medical Physics_ May 2012
9/16
Posted by Roy at 1:00 AM No comments:
Labels: IAEA, statistics, world
The above map color codes each country as a function of radiation therapy machines (linacs,
teletherapy, or HDR) per capita as of 2010. A more uptodate, interactive version is found here.
The DIRAC site also provides the raw data and even information on individual clinics in each
country.
While there are certainly nonnegligible error bars on their data, the numbers are revealing,
though largely what you'd expect. The highest GDP per capita (or likely highest health spending
per capita) countries show up in green on the above map (5 or more machines per million) and
the poorest countries show up in dark orange or red ( < 1 machine per million). Togo is red
because it has zero machines.
I think the main implications on the medical physics end is with regards to education
and dissemination of current knowledge to countries with few physicists. The US, population
approximately 3.1x10^8, is listed as having 1728 therapy physicists (probably a low estimate)
versus India, population approximately 1.2x10^9, listed as having 144 therapy physicists. Clearly
the manyears of experience are highly concentrated in the green countries. I'll claim that it's
our duty in the green countries to help educate our colleagues in the countries with less local
access to their professional and academic peers.
IAEA statistics on radiation therapy machines per capital in 2010.
Recommend this on Google
03 May 2012
New medical physicists in the US: Crunching the numbers
If you are entering the field of medical physics or have been around for a while, you might be
wondering "how many new medical physicists are joining the ranks each year?" In the US this is
an especially important question in light of the 2014 ABR residency mandate and possible effects
of an aging population on cancer incidence.
As a recent graduate, one of the topics fresh on my mind is the number of jobs available to
graduating students. This is of course a supply and demand game (or possibly a supply and supply
game when you consider number of residency spots). Since potential medical physicists in the US
can come from many different "sources" (i.e. accredited medical physics grad programs, non
accredited medical physics grad programs, nonmedical physics grad programs), it would be
somewhat difficult to directly count the number of new grads. I think it is therefore instructiveto look at the raw stats of the number of people taking the ABR medical physics board exams,
which are (recently?) available on the ABR website.
http://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=6652680838481765659&target=facebookhttp://medphys365.blogspot.com/search/label/statisticshttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=6652680838481765659&target=emailhttp://2.bp.blogspot.com/-hoy8U00dBQk/T6NxD9cJpeI/AAAAAAAAAFs/7QMmVUA2_zg/s1600/IAEA-Worldwide_therapy_access-2010.pnghttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=6652680838481765659&target=pinteresthttp://medphys365.blogspot.com/2012/05/radiation-therapy-availability-around.htmlhttp://medphys365.blogspot.com/search/label/IAEAhttp://www.blogger.com/profile/14293477186383292023http://www-naweb.iaea.org/nahu/dirac/map.asphttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=6652680838481765659&target=twitterhttp://medphys365.blogspot.com/search/label/worldhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=6652680838481765659&target=bloghttp://medphys365.blogspot.com/2012/05/new-medical-physicists-in-us-crunching.htmlhttp://www.theabr.org/ic-rp-scorehttp://medphys365.blogspot.com/2012/05/radiation-therapy-availability-around.html#comment-form8/10/2019 365 Days of Medical Physics_ May 2012
10/16
Posted by Roy at 12:04 AM No comments:
Labels: ABR, statistics, workforce
What we see in the data is a marked increase in the number of people taking all three parts of
the ABR certification exam over the period of 2006 2010, with a slight downtick in 2008. The
increase in the number taking Part 3 (Oral) in all specialties is +45% (220 in 2006 to 319 in
2010). The increase in the number taking Part 1 over the same time period is approximately
+35%.
This presents the obvious question of what will happen to this trend when the ABR residency
requirement takes full effect in 2014.
For more info on this topic:
Mills MD, Thornewill J, Esterhay RJ. Future trends in the supply and demand for
radiation oncology physicists. J Appl Clin Med Phy. 2010 Apr;11(2) (open access!)
Jean Moore, Medical Physics Workforce Study: Overview, AAPM presentation, 2010
Final report AAPM Workforce Study Report (AAPM login required)
Recommend this on Google
02 May 2012
The many faces of bolus
Bolus is a simple, yet important technology used in radiation therapy. The most basic function of
bolus material is to shape the dose distribution in a desired way. This generally falls into two
categories: compensating for "missing" tissue and enhancing the buildup effect of MeV energy
photon beams.
The bolus itself can be made of a huge variety of materials depending on the application. Below
are some materials used as bolus, all of which are applied directly to the skin surface.
http://www.aapm.org/AAPMUtilities/download.asp?file=surveys/workforce/Synthesis.pdfhttp://medphys365.blogspot.com/2012/05/new-medical-physicists-in-us-crunching.html#comment-formhttp://medphys365.blogspot.com/search/label/statisticshttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=7243474880050880667&target=pinteresthttp://www.jacmp.org/index.php/jacmp/article/viewArticle/3005/1881http://www.aapm.org/meetings/amos2/pdf/49-14390-20662-212.pdfhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=7243474880050880667&target=twitterhttp://1.bp.blogspot.com/-CsXnH0iz7uU/T6IflhHek2I/AAAAAAAAAFg/aKWXGbuZT8k/s1600/ABR-stats1.pnghttp://www.blogger.com/profile/14293477186383292023http://medphys365.blogspot.com/search/label/workforcehttp://medphys365.blogspot.com/search/label/ABRhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=7243474880050880667&target=facebookhttp://medphys365.blogspot.com/2012/05/new-medical-physicists-in-us-crunching.htmlhttp://medphys365.blogspot.com/2012/05/many-faces-of-bolus.htmlhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=7243474880050880667&target=emailhttp://www.blogger.com/share-post.g?blogID=5277658315766141584&postID=7243474880050880667&target=blog8/10/2019 365 Days of Medical Physics_ May 2012
11/16
"Superflab" vinyl plastic bolus in 5mm and 3mm thicknesses.
Edge view of Superflab sheets.
Wet towels can provide bolus with tissue like properties.
http://2.bp.blogspot.com/-PswfhKbRW04/T6DU3TY1jVI/AAAAAAAAAFA/h2spwnEqucw/s1600/towel.jpghttp://3.bp.blogspot.com/-GjfNvYrC5GY/T6DU4JfhyBI/AAAAAAAAAFI/oXJ9qBu9jRo/s1600/vaseline_gauze.jpghttp://2.bp.blogspot.com/-WCgf2FCzj5c/T6DU1vrT5II/AAAAAAAAAEw/erUUhVDrMBo/s1600/superflab_bolus1.jpghttp://4.bp.blogspot.com/-3Y9W0Jy4Wic/T6DU2aG0tJI/AAAAAAAAAE4/yy-n5lUyyVQ/s1600/superflab_bolus2.jpg8/10/2019 365 Days of Medical Physics_ May 2012
12/16
- Pgina 2 -
Nuggets of knowledge about medical physics.
365 Days of Medical Physics
30 April 2012
Multileaf collimators: modern beam shaping
One of the most important aspects of therapeutic radiation delivery is beam shaping. The most
common technology currently used to shape xray beams is the multileaf collimator or MLC.
Roy
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13/16
Posted by Roy at 11:59 PM 1 comment:
Labels: intro, MLC, overview
MLC's consist of dozens of independently moving slats, or leaves, which can collimate the beam
into nearly arbitrary shapes. MLC's were first introduced in the 1960's in Japan as a means of
replacing conventional blocks in 3D conformal radiation therapy (3DCRT). Today they are
standard on most therapy linacs and enable modern techniques based on intensity modulation.
Key advantages of MLC's include:
Dynamic movement of leaves during delivery allows for intensity modulated fluencepatterns not achievable with conventional blocks.
Finite set of possible aperture shapes results in a reasonable solution space for inverse
planning optimization for IMRT, VMAT, etc.
Significantly more convenient than cutting custom blocks for every field.
Drawbacks of MLC's include:
Large number of leaves increases possibility of mechanical failure (i.e. reliabili ty
issues).
Nonzero interleaf leakage.
Smoothness of shaping dependent on size of leaves and speed of leaf motors.
Almost everyone can agree that MLC's have been a huge advance in radiation therapy, with the
advantages far outweighing the disadvantages. I plan to discuss MLC alternatives in a future
post.
Image courtesy of Varian Medical Systems, Inc. All rights reserved. (source)
120 leaf multileaf collimator.
29 April 2012
The basics of IMRT: an overview
Intensity modulated radiation therapy(IMRT) is one of the workhorse delivery methods in
current radiation therapy. In many ways, it is a significant step up over the previous standard
technique of 3D conformal radiation therapy (3DCRT). I'm going to start talking about IMRT with
a very basic overview of how it works.
IMRT is a technique to plan and deliver MeV range xray therapy. The main advance with IMRT
over 3DCRT is the algorithmic optimization of dose from all delivery angles at the same time to
meet a predefined set of objectives, socalled inverse planning. This is accomplished by using
multileaf collimators, which can create arbitrary aperture shapes (within reason), thus
modulating the dose to the targets. In turn, this gives the dose optimization algorithm a large
number of parameters to work with to achieve the desired dose shape.
Multileaf collimators: modern beamshaping
The many faces of bolus
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14/16
Posted by Roy at 11:28 PM 1 comment:
Labels: imrt, intro, overview
Key steps in IMRT planning and delivery:
Acquisition of patient geometry (via CT, MRI, etc).
Delineation of targets and avoidance volumes.
Beam angle and energy selection.
Optimization of fluences to desired prescription
and avoidance objectives.
Assessment of DVH's.
QA via secondary calculations and field measurements.
Further reading:
Bortfeld, IMRT: a review and preview, Phys. Med. Biol. 51 R363,
2006 doi:10.1088/00319155/51/13/R21
KY Cheung, Intensity modulated radiotherapy: advantages, limitations and future
developments, Biomed Imaging Interv J 2006;2(1):e19 (open access)
Top image by TaheriKadkhoda et al. Radiation Oncology 2008 3:4 doi:10.1186/1748717X34 licensed under CC licensing.
Bottom image by ZEEs and licensed under CC licensing.
Diagram of a multileaf collimator.
Measurement theory in the clinicCollecting data is one of the main daytoday activities of the clinical physicist. Examples from
the radiation therapy clinic include PDD's, mechanical alignment parameters, ambient
temperature and pressure, source autoradiographs, IMRT planar dose distributions, setup SSD's,
and CT number data. Patient outcomes, safety, and a host of other issues often rely on the
quality and interpretation of that data.
One thing that sets medical physicists apart from most everyone else in the clinical environment
and is crucial to performing our jobs well is having a strong grasp on the theory of measurement.
Key concepts include: the statistical nature of data, the role of calibration, instrument
resolution, and uncertainty and error propagation.
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15/16
Posted by Roy at 12:30 AM 2 comments:
Labels: measurement, statistics
While we may not determine strict error levels for every measurement we make, it's important
to have a grasp of these concepts while collecting and interpreting data.
Why does my IMRT data taken with a planar detector array pass, even though it
"looks" totally different than the TPS generated data? Probably because the detector
array has relatively low resolution and the dose shape you see is the result of
interpolation by software.
Is my data skewed due to systematic or random errors?
Is my new outlier subject to regression to the meanor the start of a new trend?
Are my data groupings producing a Simpson's paradox?
What are your thoughts?
27 April 2012
Posted by Roy at 11:26 PM No comments:
Labels: medical physics
What is medical physics?
...or this blog has to start somewhere.
If you are reading this blog, you are likely familiar with medical physics, but for my first officialpost I'm going to talk about what medical physics is.
Medical physics is a field of applied science and engineering, in which physicsbased techniques
form the basis of diagnostic and therapeutic medical technologies. The most well known of these
technologies are xray imaging, CT, MRI, ultrasound, and radiation therapy. Many medical
physics technologies utilize ionizing radiation. The potential hazard of ionizing radiation is
arguably the reason why medical physicists exist as clinical personnel, versus solely as
researchers and developers, as is the case with biomedical engineers.
While most broadly medical physics is the application of physics techniques across all of
medicine, the term "medical physics" is generally used to refer to three primary areas: diagnostic
imaging, nuclear medicine (radionuclide based imaging and therapies), and radiation therapy.
The majority of clinical medical physicists work in radiation therapy.
Historically medical physics arose from the application of physics discoveries and technologies to
medicine, most importantly the xray for imaging starting in 1896. As these technologies were
more broadly adopted by hospitals, more physicists and knowledgeable personnel were needed in
clinical settings. Eventually, medical physics specific training emerged and technologies, such as
medical electron linear accelerators, were developed from the ground up with medicine in mind.
Today medical physicists work as clinicians, academic researchers, industry experts, and
educators, or spend their time as any combination of those.
Further reading:
Wikipedia entry on Medical Physics
What do Medical Physicists Do?(AAPM)
365 Days of Medical Physics blogging
Welcome to my new blog!I'm intending to make short posts (nearly) every day for the next year
covering a broad range of medical physics topics.
The idea behind this blog is largely to help motivate me to consistently push my medical physics
knowledge, but also to foster discussion online relevant to others learning medical physics.
I am a medical physicist involved in radiation therapy, so this blog will center on issues related
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Posted by Roy at 10:56 PM 2 comments:
Labels: blog, intro
to clinical radiation therapy physics, but I will also venture into some diagnostic and research
territory. For topics more closely related to my research on particle therapy and computational
medical physics, I will probably post on Will Work for Science, the blog I coauthor will Herr Dr.
Niels Bassler.
I hope you enjoy the posts. Please comment, tell me how awesome this is, correct me, and/or
make topic suggestions!
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