Relative Dosimetry. Photons

Transcription

1 Relative Dosimetry Photons

2 What you need to measure! Required Data (Photon) Central Axis Percent Depth Dose Tissue Maximum Ratio Scatter Maximum Ratio Output Factors S c & S cp! S p Beam profiles Wedge Factors, Percent Depth Dose Dynamic Wedge Factors Block Tray Factors Block Factor MLC Transmission Factor Beam Profiles Off-Axis factors

3 PDD Source PDD pri = 100 D pri Q pri D P f = SSD 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. P Q A z max FS. 4cm 2-40cm 2!"5cm,35-40cm depth z

4 PDD PDD pri = 100 D pri Q pri D P Pitfalls: D max Small filed size measurements d max shift Voltage bias effect Dose D s D s D s 0 d max Depth

5 PDD of MV Photon Beams PDD Co SSD = 100 cm cm Depth in water (cm)

6 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. Typical dmax Beam Energy Ortho 0 60 Co 0.5 4MV 1.0 6MV MV MV MV MV 3.5 dmax (cm)

7 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

8 PDD increase beam energy, increase depth of d max Higher energies more skin sparing Lower energies less skin sparing

9 PDD Field Size Dependence 15 MV Photon Beam 4x4 16x16

10 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.

11 Percent Depth Dose AAPM TG Report # 106 recommendations: 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 crossreferenced to data measured with an ion chamber. Prone to radiation damage and non-linear response.

12 Is depth ionization data depth dose? Yes! If: 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

13 Is depth ionization data depth dose? However at shallow depth, The contaminants and secondary electrons have energy spectra that change rapidly with depth. 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

14 TMR TMR = D Q Source D Q max (b) Tissue Maximum Ratio Ratio of the dose at point Q in the phantom for various depths to the dose at point Q when the d Q =d max SAD Q A ref A Q z ref

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

16 PDD vs. TMR PDD TMR 1 SSD (Source to surface distance) is fixed, normally 100 cm. 1 SSD (Source to surface distance) varies. 2 Ion chamber moves in depth. Therefore, SPD (Source to point distance) varies. 2 Ion chamber does not move in depth. Therefore, SPD (Source to point distance) is fixed (i.e. 100 cm). 3 Measurements at different points are divided by the measurement at depth dmax. 3 Measurement at the same point in space (but different depth in water) is then divided by dose at that point in dmax depth.

17 Beam Profiles 6 MV Photon Beam, Depth of 5.0 cm, Field size of 4x4, 10.4x10.4, and 21x21 cm 2. 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.

18 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: F = 100! D max " D min D max + D min 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

19 Beam Profiles Note: 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

20 Beam Profiles 6 MV Photon Beam, Field Size of 10.4x10.4 cm2, Depths of 1.5, 5.0, 10.0, 15.0 and 25 cm

21 Isodose Distributions (20 X 20 Cm 2 ) 6 MV 18 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.

22 Cross Beam Measurements!"#$%&'#()#'%*#&+*,'#-.# /*'*,'-0#)(1*2## # 3"#45,-00*,'#6*&)70*6*5'# -.#8*57690&#0*:(-5## Diode CC04 CC13 Diameter Penumbra 20%~80% 0.8x0.8 mm 2 4 mm 6 mm 4.0 mm 6.1 mm 7.2 mm

23 Output factors A 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 d max for a standard field size and SSD (i.e, 10cm 2 and 100cm respectively). As the field size changes the radiation output changes. This change is quantified by measuring output at d max on CAX for each FS and dividing by output at d max on CAX for 10x10 cm 2 field)

24 Output factors OF(r) = S P (r)! S C (r) " S P (r) = S CP(r) S C (r) OF(r) or S cp : output factor at d max for field size rxr. S c (r): collimator scatter correction factor measured in air for field size rxr. S p (r): phantom scatter correction for filed size rxr. S cp should be measured for square field sizes from 2x2 to 40x40 cm 2

25 Measuring Sc & Sc,p

26 Measuring S c 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<4cm),high Z material (Brass etc.) should be used. Corrections for energy absorption coefficients and energy spectra change are needed. TG 74 recommendations

27 How to get from PDD to TMR! TMR(d, r d ) = PDD(d,r, f ) $! # &i# " 100 % " f: SSD f + d f + d max $ & % 2! i S p(r d max ) $ # " S p (r d ) & % r: field size at the surface r d : field size at depth d; r d =r(f+d)/f r dmax : field size at d max ; r d max=r(f+d max )/f S p : Phantom Scatter correction factor

28 Scatter-Maximum ratios! SMR(d, r d ) = TMR(d, r d )i S (r ) $ p d # " S p (0) & ' TMR(d,0) % SMR is the ratio of scattered dose at a point to the effective primary dose at the same point at d max. TMR can be divided into two parts Primary component, TMR (d,0) Scatter component, SMR S p (0): phantom scatter correction for fs=0 (extrapolated)

29 Wedges 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. 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.

30 Wedges Wedge/Open Comparison FS = 10 x 10 cm2 15 MV (W/O) 6 MV (W/O)

31 Wedge factors Measurment condition similar to that of S cp. Measurements for different wedged field d max divided by measurement for 10x10 nonewedged 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 wedgeposition rotated 180 o between them. Don t forget wedge PDD and the d max shift.

32 Other factors Block Tray Factor MLC Transmission Factor Off-Axis factor, OAF

33 OAF The off-axis ratio (OAR) is usually defined as the ratio of dose at an offaxis point to the dose on the central beam axis at the same depth in a phantom. Relative dose Depth (cm) Distance from central axis axis (cm)

34 Approaches to Dose Computation Algorithms Data measured in water and in air Parameterize water data Correction based methods Model based methods Reconstitute water data Calculate inhomogeneity corrections to water data Calculate dose directly based on beam and phantom configurations

35 Correction vs. 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). Model based methods 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. Patient dose distribution obtained by first computing Dose in water from generating function, then correcting for tissue heterogeneity, patient contour, and beam modifiers. computing beam and beam transport (i.e. beam interactions in treatment head and in patient) directly.