Treatment Planning I Isodose Distributions

ISODOSE DISTRIBUTIONS

• Isodose Chart• Measurements of isodose curves• Parameters of isodose curves

– Beam quality– Source size, SSD and SDD – the penumbra

effect– Collimation and flattening filter– Field size– Wedge Filters

ISODOSE DISTRIBUTIONS

• Combination of radiation fields– Parallel opposed fields– Multiple fields

• Isocentric techniques– Stationary beams– Rotation therapy

• Wedge Fields Techniques• Tumor dose specification

Isodose Chart

SSD SAD

Isodose Chart

Isodose Chart

Measurements of isodose curves

• 3D water phantoms

Parameters of isodose curves

200 kvp 60Co

4 MV 10 MV


• Source SizeS

SDD

SSD

dP

A B

C

D E

Homework: Derive the penumbra (DE) as a function of other parameters


• Flattening Filter• Flatness defined at 10

cm depth• Horns at dmax• Below 10 cm,

rounding off of the profiles

• Energy off axis is lower than central axis


• Field size– SSD technique: the field size is defined at the

surface– SAD technique: the field size is defined at the

depth– Field size defined as the 50% line, normalized

to the central axis value at the depth.

Wedge FiltersNormalized at dmax A: with wedge B: without wedge

Wedge Filters

Wedge Angle

• Refers to the complement of the angle the isodose line is tilted in relation to the central axis, for a given depth.

• This depth is normally defined as 10 cm (ICRU 24).

θ

Wedge Transmission Factor• Measured at some depth beyond the dmax, usually 10

cm.• Old 60Co isodose curves were normalized to the dmax

without the wedge. The isodose curves already included the wedge factor.

• With advent of TPS, we normally input the isodosecurves normalized to the dmax with the wedge, and introduce a wedge factor to account for the transmission factor of the wedge.

• Wedge factor depends on the depth and field size.

Wedge Systems

• Wedge for selected field sizes, were used for 60Co units because of limited output of the units.

Design of Wedge Filters

Universal Wedges• Elekta provide this technique in their linacs.• It is a 60 wedge which is mounted in a carrousel

that goes in and out of the photon beam.• If you need a different wedge angle, the linac

automatically weight treatment to provide for the angle needed (e.g., 30 wedge is done by assigning 50% of the treatment with an open beam and 50% of the treatment with the 60 wedge)

Dynamic or Virtual Wedges

• To be discussed separately

Combination of Radiation Fields

Different weights

Parallel Opposed Fields

Multiple Fields

N -> ∞ = Rotation Therapy

Stationary Beams

Rotation Therapy

100 degrees

180 degrees

360 degrees

4 MVField Size = 7x12 cm

Wedge Fields Technique

90 / 2oθ φ= −

Wedge Fields Technique

ICRU 50 AND 62

Treatment Planning ProcessTreatment Planning Process

•Gross Tumor Volume (GTV)palpable or imaged

•Clinical Target Volume (CTV)GTV + subclinicalmultiple CTV’s possible

•Planning Target Volume (PTV)CTV + margin (internal+setup)


•Gross Tumor Volume (GTV)GTV is the gross demonstrable extent andlocation of the malignant growthThe GTV consists of primary tumor (GTVPrimary) and possibly metastatic lympha-denopathy (GTV nodal) or other metastases(GTV M).


•Clinical Target Volume (CTV)CTV is a tissue volume that contains demons-trable GTV and/or subclinical malignantdiseases that must be eliminated. This volumehas to be treated adequately in order to achievethe aim of radical therapy.


•Clinical Target Volume (CTV)•Description of treatment of subclinical

extension adjacent to a GTVClinical experience indicates that outside theGTV there is generally subclinical involve-ment that can not be detected by the stagingprocedures. This CTV is then usually denotedby CTV 1.


•Clinical Target Volume (CTV)•Description of treatment of subclinical

extension at a distance from a GTVAdditional volumes (CTV’s) with presumedsubclinical spread (e.g. Regional lymph nodes)may be considered for therapy. They are thendesignated by CTV II, CTV III, etc.


•Internal Target Volume (CTV)•ICRU 62 recommends that an internal margin(IM) be added to CTV to account for internalphysiological movements and variation in size,shape, and position of the CTV during therapyin relation to na internal reference point and itscorresponding coordinate system.


•Planning Target Volume (PTV)The volume that includes CTV with an IMAs well as a set-up margin (SM) for patientMovement and set-up uncertainties is calledThe planning target volume (PTV). To Delineate PTV, IM and SM are not addedLinearly but are combined subjectively.


GTVGTVCTVCTV

PTVPTV

TreatmentTreatment IncludedIncluded

GTVGTVCTVCTV

PTVPTVIrradiatedVolume

TreatedVolume


•Planning Organ at Risk Volume (PRV)The organ at risk (OR) needs adequate protectionjust as CTV needs adequate treatments.Once the OR is identified, margins need to beadded to account for its movements, internalas well as set-up.


• Treated VolumeAdditional margins must be provided around thetarget volume to allow for limitations of thetreatment technique. Thus the minimum targetdose should be represented by an isodose surfacewhich adequately covers the PTV to providethat margin. The volume enclosed by this isodosesurface is called the treated volume.


• Irradiated VolumeThe volume of tissue receiving a significant dose (e.g., >= 50% of the specified target dose) is calledthe irradiated volume.

Specifications of Target Dose

Maximum target dose

(larger than 2 cm2)

Minimum target dose

ICRU Volumes and Margins

Specification of Target Dose

• The ICRU Reference Point– The point should be selected so that the dose at this

point is clinically relevant and representative of thedose throughout the PTV

– The point should be easy to define in a clear andunambiguous way

– The point should be selected where the dose can beaccurately calculated

– The point should not lie in the penumbra region orwhere there is a steep dose gradient


• Stationary Photon Beams– For single beam, the TD should be specified in the central

axis of the beam placed within the PTV– For parallel opposed, equally weighted beams, the point of

TD specification should be on the central axis midwaybetween the beam entrances

– For parallel opposed, unequally weigthed beams, the TD should be specified in the central axis placed within thePTV

– For any other arrangement of two or more intersecting beams, the point of TD specification should be at the intersection of the central axes of the beams placed within the PTV


• Rotation Therapy– For full rotation or arcs of at least 270 degrees, the TD

should be specified at the center of the rotation in theprincipal plane.

– For smaller arcs, the TD should be stated in the principal plane, first at the center of rotation and, second, at thecenter of the target volume. This dual point specification is required because in small arc therapy, past pointtechniques are used that give maximum absorbed dose close to the center of the target area.


• Additional Information– The specification of TD is only meaningful if sufficient

information is provided regarding the irradiation technique.– Radiation quality, SSD or SAD, field sizes, beam

modification devices, beam weighting, correction for inhomogeneities, dose fractionation and patient positioningshould be included as well.

Lab - 6 MV photon beam• Output in air (Sc)

– Field sizes at 100 cm 4,6,8,10,12,14,17,20,25,30,40– Distance to chamber = 100 cm– Copper buildup cap

• Output in phantom (Sc,p)– Field sizes at 100 cm 4,6,8,10,12,14,17,20,25,30,40– Distance to chamber = 100 cm– At dmax depth (1.5 cm)

• PDD for a 10x10 cm 100cm SSD– At dmax, 5, 10 and 15 cm depth

• TAR for a 10x10 cm– At dmax, 5, 10 and 15 cm depth

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Treatment Planning I Isodose Distributions