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Sonja Dieterich, PhD, DABRStanford University Hospital
Ellen Wilcox, PhDSt Francis Hospital, CT
Carlo Cavedon, D.S.Ospedale San Bartolo, Vicenza, Italy
With slides from Remy Durand, Huaying Ji, Joseph Novotny
Conflict of Interest Sonja Dieterich has a consulting agreement with Cyberheart Inc.
Learning Objectives Describe the basic concepts of small field reference dosimetry
Know which detectors are appropriate to use
Learn the right questions to ask when you see a plan
Be aware of whole body dose
90% of attention goes to:90% of attention goes to: Spatial Accuracy
Robot pointing accuracy (calibration verification): 0.2 mm – 0.3 mm
Closer to clinical reality: End‐to‐end (E2E)
Manufacturer E2E spec: <0.95 mm
My clinic: 0.3 mm – 0.7 mm I start to investigate if E2E > 0.7 mm Older paper by Yu et al (E2E = 1.1 mm) Older paper by Yu et al (E2E 1.1 mm)
had 2 mm slice thickness
Known dependency of spatial accuracy vs. slice thickness
1 mm or 1.25 mm is currently standard slice thickness
Think of dosimetric accuracy as y“shifting isodose lines”
Why is Dosimetry important? In the brain:
Pre‐SRS AVM embolization with Onyx or glue Near cavities or superficial tumors
SRS has moved outside the brain:L Lung
T‐spine Our Small Fields are getting smaller Our Small Fields are getting smaller What is the WB dose for 85 (!) lesions?
T bl f CTable of Contents
Reference Dosimetry What is a small field? The new IAEA framework for SRS fields
Relative (Patient) Dosimetry Relative (Patient) Dosimetry Dose calculation algorithms In‐vivo dosimetryv vo dos et y Whole‐body dose
What is a “Small Field”? “Small” changes with time:
No lateral e‐ equilibrium (quick: what is that for 6MV?) < 10 mm < 4 mm?
Ch ll Challenges: How to build a collimator Focal spot size large compared to collimator Focal spot size large compared to collimator Inverse square law breaking down Detector resolution …
Why Microbeams? Functional targets:
Epilepsy, Parkinson’s (for non‐DBS candidates))
Neurology: Facet blocks, hyperhydrosis
Intractable pain (palliative, Intractable pain (palliative, capsulotomy)
Obsessive‐Compulsive Disorder Intractable severe depression Intractable, severe depression (fMRI: hyperactivity of Brodmann25)
Small animal applications Small animal applications
R f d f d d fi ldReference dose for non‐standard fields
TG‐51 presentations: “Must have 10 cm x 10 cm field”
Gammaknife? Cyberknife? Tomotherapy? y py
These fields are not flat over detector volume
IAEA and AAPM task groups on small field dosimetry
The IAEA conceptThe IAEA concept
(TG 51) 60 mm cone for CK Standard beam data acquisition (TPR, OCR) for CK: 80 cm SAD How to obtain 100 cm SSD ? Easy: move robot up (by how much?)
How to obtain kQ?N fl tt i filt bl ( DWO R ) No flattening filter – no problem (see DWO Rogers)
How to handle gradient corrections?
TG 51 output = 1.0153 cGy/MU at dmaxTG 21 output = 1.0162 cGy/MU at dmax same dayp y/ max y
15
Effects of Detector Size onReference Dosimetry
1.2
0.8
1
0.4
0.6plan
BeamData
0
0.2
30 -20 -10 0 10 20 30
• No flattening filter: round edges• Length of Farmer ~ 2 cm
Practical Procedure (1) Move robot up to extend SSD to 100 cm. Eqv square = 0.9 Eqv circle *q q 9 q
Eqv square = 0.9 6 100/80 = 6.75 cm @ 100 cm SSD
%dd( ) h b f Measure %dd(10,6.75,100) with CyberKnife. Compare %dd(10,6.75,100) with local or reference data (e g BJR sup 25†)(e.g. BJR sup 25†)
* Day MJ & Aird EGA in BJR sup 25, 1996, p138-153†
17
† Jordan TJ in in BJR sup 25, 1996, p62 - 110
Practical Procedure (2) Take the corresponding %dd(10,10,100) from the reference data‐set.
Obtain kQ with %dd(10,10,100).
Obtain P (10 10 100) from reference dataObtain Pgr(10,10,100) from reference data.
Change calibration condition to CyberKnife calibration condition (SSD setup to SAD setup)condition (SSD setup to SAD setup).
18
Practical Procedure (3) Perform measurement based upon standard TG 51 procedure.
Measure P (CK) at the measurement point Measure Pgr(CK) at the measurement point. Calculate dose to dmax:
Q
TPRorddNM
DQ
wDQw %
,
PMM raw )100,10,10()(60
,,gr
grQ
CowD
QwD P
CKPkNN
19
Example SSD 6 %dd( 6 ) 6 %1. 100 cm SSD 60 mm cone, %dd(10,6.75,100) = 64.02%.
2. %dd(10,10,100) = 65.4% *.3 With this %dd(10 10 100) k = 0 99283. With this %dd(10,10,100), kQ= 0.9928.4. Pgr(10,10,100) = 0.9897*. 5. 78.5 cm SSD, 60 mm cone, %dd(10,5.3,78.5) = 59.43%.5. 78.5 c SS , 60 co e, %dd( 0,5.3,78.5) 59.43%.6. Pgr(10,5.3,78.5) = 0.98787. CyberKnife new kQ = 0.9909Q
8. TG 51 output = 1.0153 cGy/MU at dmax
9. TG 21 output = 1.0162 cGy/MU at dmax same day
20
* Calculated from BJR data.
Back to: The IAEA conceptBack to: The IAEA concept
Example for a device: GKExample for a device: GK
Measuring each field not possibleMeasuring each field not possible Even with Perfexion, 8 segments Plan‐class: Isocentric, all beams, defined , ,collimator
GK Plan‐Class Specific Reference pCalibration
Verification of Dose Calculation
Suitable Detectors ? Plus Plus: BANG gels Diamond Diamond OLD …
Uncertainties for Output Factors• Error bars getting larger below 20 mm
MC f b•MC of beam
•MC of detectors
Fi ld D SiField vs. Detector Size1.000
1 5cm depth Wid h f 0.800
1.5cm depth
5c 7.5c
10c 12.5c
Width of a PTW60012 diode compared to small
0.600
15c site 5c
site 7.5c site 10c
site 12.5c site 15c
compared to small collimator OARs
0.400
0 000
0.200
0.0000.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
Al ith i ( ld ) C i l SRSAlgorithms in (older) Cranial SRS A head is very similar to a ysphere (surface corrections!)
Fairly homogeneous Fairly homogeneous Nasopharynx AVM embolizations …
Density can be i t d b H O approximated by H2O
(hence no CT used on Gamma Knife)
Need for better algorithms in SRS Most systems used path‐length correction Narrow beams:
field dimensions smaller then maximum range of secondary electrons
Steep dose gradients Steep dose gradients Gets exacerbated by tissue heterogeneity
Better alternative:Better alternative: Collapsed cone convolution‐superposition Monte Carlo
Example:Example: 6 MV Photon Monte Carlo
Use MC in all lung cases, T‐spine, Head & Neck
Brain: supra‐orbital, pituitary, <1.5 cm to skin, embolizedAVM
Re‐calculation or re‐optimization
Older (e.g. Ray‐tracing) algorithms: +8%‐14% off for RPC lung phantom (RTOG‐0236)
D diff b h hi h f ll l i ( t %!) Dose difference may be much higher for small lesions (up to 40%!) Dose difference of varying % known issue for all older Tx planning
algorithms in combination with small beams, not limited to C b k ifCyberknife
Example I: Dosimetry for SBRTa p e : os et y o S
Example II: Dosimetry for SBRTa p e : os et y o S
Independent verification of MCIndependent verification of MC
Recently got approved for RTOG‐0618 RTOG‐0618 = RTOG‐0236 + inhomogeneity3 g ycorrections
Done on a motion platform
June 11, 2009 9th ISRS Congress, Seoul, Korea 2009
Why In‐Vivo Dosimetry?Why? Challenges? Frameless SRS Field size: no exit field for
di d l SBRT: Gating/ABC Motion‐adaptive with
diode placement Imaging: No space for EPID
(Gamma Knife)Motion adaptive with Synchrony
Motion‐adaptive with moving MLC
Non‐isocentricity(Cyberknife)
C Field matching for
retreatments
Just one idea for in‐vivo Antenna is very similar to a gold seedAntenna is very similar to a gold seed Invasive procedure to implant marker
Implantable Dosimeter
1 25 mm CT slice thickness• 1.25 mm CT slice thickness• Looks like two closely placed
fiducials!fiducials!• Not uncommon in patients …
01/26/2007 6th Annual Cyberknife Users’ Meeting 39
Why is it important? We are treating benign & functional cases
Long survival Pediatrics (protons!) Pediatrics (protons!) MU per delivered dose
The issues: What is the error bar on secondary cancer risk? Absolute vs relative risk Absolute vs. relative risk Delayed radiation response Risk of treating vs. no treatment/other treatmentsDiff i i k l ( R dO N ) Difference in risk tolerance (e.g. RadOnc vs. Neurosurgeon)
GK: Extracranial Dose for SingleGK: Extracranial Dose for Single Isocenter
Travel home safely!Travel home safely!