Relative DosimetryPhotons

What you need to measure!• Required Data (Photon)

• Central Axis Percent Depth Dose• Tissue Maximum Ratio• Scatter Maximum Ratio• Output Factors Sc & Scp ! Sp• Beam profiles• Wedge Factors, Percent Depth Dose• Dynamic Wedge Factors• Block Tray Factors• Block Factor• MLC Transmission Factor• Beam Profiles• Off-Axis factors

Source

f = SSD

zmaxP

A

Q

z

How to Position IC?When properly placed at the surface, the chamber, viewed end-on from beneath the water, will appear to be a complete circle.

PDDpri == 100DQpri

DPpri

FS. 4cm2-40cm2!"5cm,35-40cm depth

PDD

Depthdmax0

Dos

e

Dmax

Ds

Ds

Ds

PDDpri == 100DQpri

DPpri

Small filed size measurementsdmax shiftVoltage bias effect

Pitfalls:

PDD

PDD of MV Photon Beams

100.0

80.0

60.0

40.0

20.0

0.0

PD

D

0 5 10 15 20

SSD = 100 cm10 ¥ 10 cm2

25

Co

46

1018

Depth in water (cm)

PDD Build-up to dmax• Higher energy

beams have more penetrating photons, and create more energetic electrons.

• Thus, the maximum dose occurs at a deeper depth for high energy beams.

Beam Energy dmax (cm)

Ortho 060Co 0.54MV 1.06MV 1.610MV 2.515MV 3.018MV 3.320MV 3.5

Typical dmax

Buildup Region• Interaction of incident photons with

phantom yield secondary electrons.

• Secondary electrons deposit energy downstream. The electron fluence and, thus, absorbed dose increases until reach a maximum (approximate range of electrons)

• Same time -- photon fluence decreases with depth at constant rate. Therefore, fewer photons to eject electrons as increase depth.

• The combination of all these factors yields the dose buildup region

PDD

increase beam energy, increase depth of dmaxHigher energies more skin sparingLower energies less skin sparing

Field Size Dependence 15 MV Photon Beam

16x16

4x4

PDD

Percent Depth Dose• Decreases with depth for all energies

beyond build-up region.

• Increases with energy beyond build-up region.

• If inverse square and scattering are ignored, it follows exponential attenuation.

• Increase with Field Size.

Percent Depth Dose

• Use 4-5 mm diameter ion chamber for depth beyond 1cm.

• Use parallel plate or extrapolation chamber to measure data near the surface.

• Diodes and diamond detectors are appropriate as long as data measured with these detectors is cross-referenced to data measured with an ion chamber.

• Prone to radiation damage and non-linear response.

AAPM TG Report # 106 recommendations:

Is depth ionization data depth dose?

• TCPE exists at the point of measurement.

• The energy spectrum of incident photons does not change with the depth.

• Fluence across the detector remains the same.

• These conditions are met at depths beyond the range of contaminant charged particles

Yes! If:

Is depth ionization data depth dose?

• Results in a variation of ~10% in restricted mass stopping power ratio data for water and air.

• Translates into a spatial uncertainty of less than 1.5 mm in dose in the build up region

However at shallow depth, The contaminants and secondary electrons have energy spectra that change rapidly with depth.

Tissue Maximum RatioRatio of the dose at point Q in the phantom for various depths to the dose at point

Q when the dQ=dmax

TMR ==DQDQmax

Source

(b)

SAD

Q

Aref

AQ

zref

TMR

TMR of MV Photon Beams

Plot of TMR for 10-MV x-ray as a function of depth for a selection of field sizes.

PDD vs. TMRPDD TMR

SSD (Source to surface distance) is fixed, normally 100 cm.

Ion chamber moves in depth. Therefore, SPD (Source to point distance) varies.

Measurements at different points are divided by the measurement at depth dmax.

SSD (Source to surface distance) varies.

Measurement at the same point in space (but different depth in water) is then divided by dose at that point in dmax depth.

Ion chamber does not move in depth. Therefore, SPD (Source to point distance) is fixed (i.e. 100 cm).

1

3

2

1

3

2

Beam Profiles6 MV Photon Beam, Depth of 5.0 cm, Field size of 4x4, 10.4x10.4, and 21x21 cm2.

The flatness of photon beams is extremely sensitive to change in energy of the incident beam. A small change in the penetrative quality of a photon beam results in very large change in beam flatness.

Beam Profiles• Affected by the radially symmetric conical high Z- material flattening filter,

which

• Flattens the beam by differentially absorbing more photons in the center and less in the periphery

• unwanted consequence of flattening the beam is the differential change in beam quality at off-axis points.

• hardens the beam

• Cross beam flatness is defined as:

• One flattening filter for each clinical photon beam results in a compromise of beam flatness characteristics of small and large fields.

• Flattening filters are designed to give a gradually increasing radial intensity. This is referred to as “horns” on a cross-beam profile

F = 100 ! Dmax " DminDmax + Dmin

Beam Profiles

• Cross beam profiles may not be radially symmetric due to non circular focal spot.

• Therefore, cross-beam data is characterized by a set of two orthogonal dose profiles measured perpendicular to the beam’s central axis at a given depth in a phantom

Note:

Beam Profiles6 MV Photon Beam, Field Size of 10.4x10.4 cm2, Depths of 1.5, 5.0, 10.0, 15.0 and 25 cm

Isodose Distributions (20 X 20 Cm2)

18 MV 6 MV

Note contaminant electrons contribute to dose outside the field at shallow depths. The magnitude and extent of dose outside the geometric edge of a field at shallow depths increases with beam energy.

Cross Beam Measurements

!"#$%&'#()#'%*#&+*,'#-.#/*'*,'-0#)(1*2###3"#45,-00*,'#6*&)70*6*5'#-.#8*57690*:(-5##

Diode CC04 CC13

0.8x0.8 mm2

4 mm 6 mm Diameter

Penumbra 20%~80%

4.0 mm 6.1 mm 7.2 mm

Output factorsA linear accelerator is calibrated by measuring ionization per monitor unit at the reference depth. This measurement is converted to dose and divided by appropriate PDD to determine the dose at dmax for a standard field size and SSD (i.e, 10cm2 and 100cm respectively). As the field size changes the radiation output changes. This change is quantified by measuring output at dmax on CAX for each FS and dividing by output at dmax on CAX for 10x10 cm2 field)

Output factors

• OF(r) or Scp: output factor at dmax for field size rxr.

• Sc(r): collimator scatter correction factor measured in air for field size rxr.

• Sp(r): phantom scatter correction for filed size rxr.

• Scp should be measured for square field sizes from 2x2 to 40x40 cm2

OF(r) == SP (r) !! SC (r)"" SP (r) ==SCP (r)SC (r)

Measuring Sc & Sc,p

Measuring Sc • Mini-phantom

• Water-equivalent materials.

• 4g/cm2 diameter and 10g/cm2 depth to maintain lateral CPE and eliminate contaminant electron.

• For small segment fields(c

How to get from PDD to TMR

TMR(d , rd ) ==

PDD(d , r, f )100

!!""##

$$%%&&ii

f ++ df ++ dmax

!!

""##

$$

%%&&2

iiSp (rdmax )Sp (rd )

!!

""####

$$

%%&&&&

• f: SSD

• r: field size at the surface

• rd: field size at depth d; rd=r(f+d)/f

• rdmax: field size at dmax; rdmax=r(f+dmax)/f

• Sp: Phantom Scatter correction factor

Scatter-Maximum ratios

• SMR is the ratio of scattered dose at a point to the effective primary dose at the same point at dmax.

• TMR can be divided into two parts

• Primary component, TMR (d,0)

• Scatter component, SMR

• Sp(0): phantom scatter correction for fs=0 (extrapolated)

SMR(d , rd ) == TMR(d , rd )ii

Sp (rd )Sp (0)

!!

""####

$$

%%&&&& ''TMR(d ,0)

•Physical wedges are made of lead, brass or steel. When placed in a radiation beam, they cause a progressive decrease in the intensity across the beam and a tilt of isodose curves under normal beam incidence.

•Dynamic wedges provide the wedge effect on isodose curves through a closing motion of a collimator jaw during irradiation.

Wedges

The wedge angle is defined as the angle through which an isodose curve at a given depth in water (usually 10 cm) is tilted at the central beam axis under the condition of normal beam incidence.

Wedge/Open Comparison FS = 10 x 10 cm2

15 MV (W/O)

6 MV (W/O)

Wedges

Wedge factors• Measurment condition similar to that of Scp.

• Measurements for different wedged field sizes @ dmax divided by measurement for 10x10 none-wedged field.

• The IC should be placed on the CAX with its axis parallel to a constant thickness of the wedge.

• Two sets of reading is required with wedge-position rotated 180o between them.

• Don’t forget wedge PDD and the dmax shift.

Other factors

• Block Tray Factor

• MLC Transmission Factor

• Off-Axis factor, OAF

OAF

–30 –20 –10 0 10 20 30

120

100

80

60

40

20

0

Rel

ativ

e d

ose

Depth (cm)2.5

30

20

10

5

Distance from central axis axis (cm)

The off-axis ratio (OAR) is usually defined as the ratio of dose at an off-axis point to the dose on the central beam axis at the same depth in a phantom.

Approaches to Dose Computation Algorithms

Data measured in water and in air

Parameterize water data

“Model” based methods

“Correction” based methods

Calculate dose directlybased on beam and phantom configurations

Reconstitute water data

Calculate inhomogeneity corrections to water data

Correction vs. Model Based Methods “Model” based

methods“Correction” based methods

• Measured data used as basis for Dose Computation

• Require measurements with buildup cap in air or in a mini-phantom

• Require lots of data. Generating functions used to reduce size of data set for convenient clinical use (i.e. less storage space).

• Patient dose distribution obtained by first computing Dose in water from generating function, then correcting for tissue heterogeneity, patient contour, and beam modifiers.

• Measured data used to setup description of treatment beam.

• Require a parameter to estimate size of photon source at target.

• Require more time for tuning of model parameters.

• computing beam and beam transport (i.e. beam interactions in treatment head and in patient) directly.