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Ch 10. A System of Dosimetric Calculations. The physics of Radiation Therapy, pp. 200 - 224. Introduction Dose Calculation Parameters Collimator Scatter Factor Phantom Scatter Factor Tissue-Phantom and Tissue-Maximum Ratios Practical Application Accelerator Calculations Irregular Fields
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Ch 10. A System of Dosimetric Calculations The physics of Radiation Therapy, pp. 200 - 224
Introduction • Dose Calculation Parameters • Collimator Scatter Factor • Phantom Scatter Factor • Tissue-Phantom and Tissue-Maximum Ratios • Practical Application • Accelerator Calculations • Irregular Fields • Asymmetric Fields • Other Practical Methods of Calculating Depth Dose Distribution • Irregular Fields • Point Off-Axis • Point Outside the Field
Introduction • Limitation of using TARs, SAR & Percent depth dose for calculating absorbed dose in a patient • Percent depth dose is suitable for SSD treatment technique. • Tissue-air-ratios (TAR) suitable for SAD treatment technique, but limited to energies no higher than Co-60. • Increase the size of chamber build-up cap • Material of build-up cap is different from phantoms • Overcome the limitation of the TAR • Tissue-Phantom Ratio (TPR) • Tissue-Maximum Ratio (TMR)
Introduction • Dose Calculation Parameters • Collimator Scatter Factor • Phantom Scatter Factor • Tissue-Phantom and Tissue-Maximum Ratios • Practical Application • Accelerator Calculations • Irregular Fields • Asymmetric Fields • Other Practical Methods of Calculating Depth Dose Distribution • Irregular Fields • Point Off-Axis • Point Outside the Field
Reference field 1.0 SAD ↓Sc Build-up cap Reference field → Field Size AIR Collimator Scatter Factor (Sc) • The beam output measured in air depends on the field size • Field size ↑; output ↑; collimator scatter ↑ • “Output factor” • Definition • The ratio of the output in air for a given field to that for a reference field (10 x 10 cm) • Direct measurement
Phantom Scatter Factor (Sp) • The change in scatter radiation originating in the phantom reference depth as the field side is change • Definition • The ratio of the dose rate for a given field at a reference depth (e.g. depth of Dmax) to the dose rate at the same depth of the reference field size (10 x 10 cm), with the same collimator opening • Related to the change in the volume of the phantom irradiated
Phantom Scatter Factor (Sp) • Indirect measuring Sp #1 • through backscatter factor (BSF) • BSF can be accurately measured for the photon beam (e.g. 60Co and 4 MV)
1.0 ↓Sc,p Reference field → Field Size Reference field SAD Reference depth PHANTOM Phantom Scatter Factor (Sp) • Indirect measuring Sp #2 • Through total scatter factor (Sp) • Contains both the collimator and phantom scatter
S S d t0 × × rd rd Dd Dt0 Tissue-Phantom and Tissue-Maximum Ratios • Definition of TPR • The ratio of the dose at a given point in phantom to the dose at the same point at a fixed reference depth, usually 5 cm
Tissue-Phantom and Tissue-Maximum Ratios • Definition of TMR • The ratio of the dose at a given point in phantom to the close at the same point at the reference depth of maximum dose • Special case of TPR • Adopted the point of central axis Dmax as a fixed reference depth
Properties of TMR Like TAR, TMR is independent of SSD, increases with energy and field size. TMR 3030 1010 00 Is caused entirely by the primary beam Depth in water TMR data for 10 MV x-ray beams
Tissue-Phantom and Tissue-Maximum Ratios • Relationship between TMR and effective linear attenuation coefficient (μ) • Obtain the effective linear attenuation coefficient (μ) • Plotting μ(determined from TMR data) as a function field size • Extrapolating it back to 0 × 0 field size
Tissue-Phantom and Tissue-Maximum Ratios • Relationship between TMR and percent depth dose (P) • Relationship between TMR and TAR
Scatter-Maximum Ratio (SMR) • Definition • The ratio of the scattered dose at a given point in phantom to the effective primary dose at the same point at thereference depth of maximum dose • For Co-60 γ-rays • SMRs are approximately at the same as SARs • For higher energies • SMRs should be calculated from TMR
Introduction • Dose Calculation Parameters • Collimator Scatter Factor • Phantom Scatter Factor • Tissue-Phantom and Tissue-Maximum Ratios • Practical Application • Accelerator Calculations • Irregular Fields • Asymmetric Fields • Other Practical Methods of Calculating Depth Dose Distribution • Irregular Fields • Point Off-Axis • Point Outside the Field
Practical Applications (accelerator calculations) • Machine are usually calibrate to deliver • 1 rad per MU • At reference depth (t0) • For a reference field size (10 × 10 cm) • Source to calibration point distance of SCD
Dose / MU =K dose rate under calibration conditions field size changed Sc (rc) Sp(r) (SSD factor) distance (SSD) changed PDD(d,r)/100 depth changed Practical Applications (accelerator calculations) SSD technique: MU = TD / (Dose / MU)
Practical Applications (accelerator calculations ) example 1 SSD technique: Machine: 4 MV photons Calibration conditions: SSD = 100 cm, dmax = 1 cm, field size = 10 10 cm2. Calibration dose rate = 1 cGy / MU Treatment conditions: SSD = 100 cm, d = 10 cm, field size = 15 15 cm2, Sc(1515)=1.020, Sp(1515)=1.010, %DD=65.1, TD = 200 cGy. Dose/MU = 1 1.02 1.01 65.1 × 1 = 67.07 MU = 200 100 / 67.07 = 298
Practical Applications (accelerator calculations) example 2 SSD technique: Machine: 4 MV photons Calibration conditions: SSD = 100 cm, dmax = 1 cm, field size = 10 10 cm2. Calibration dose rate = 1 cGy / MU Treatment conditions: SSD = 120 cm, d = 10 cm, field size = 15 15 cm2, Sc(12.512.5)=1.010, Sp(1515)=1.010, %DD=66.7, TD = 200 cGy. Dose/MU = 1 1.01 1.01 [(100+1)/(120+1)]2 66.7 = 47.4 MU = 200 100 / 47.4 = 422
Dose / MU =K dose rate under calibration conditions field size changed Sc (rc) Sp(rd) (SAD factor) distance (SSD) changed TMR(d,rd) depth changed Practical Applications (accelerator calculations) SAD technique: MU = ID / (Dose / MU)
Practical Applications (accelerator calculation) example 3 SAD technique: Machine: 4 MV photons Calibration conditions: SCD = 100 cm, dmax = 1 cm, field size = 10 10 cm2. Calibration dose rate = 1 cGy / MU Treatment conditions: SAD = 100 cm, d = 8 cm, field size = 6 6 cm2, Sc(66)=0.970, Sp(66)=0.990, TMR(8, 66)=0.787, TD = 200 cGy. Dose/MU = 1 0.970 0.990 0.787 × 1 = 0.756 MU = 200 / 0.756 = 265
Irregular fields • Calculation of percent depth dose for an irregular field • Obtain average SMR by using Clarkson type integration • SMR(d,rd) is then converted to TMR(d,rd) • TMR(d,rd) may be converted into percent depth dose • Final expression
Computer Program • Data permanently stored in this computer program • A table of SMRs as functions of radii of circular fields • The off-axis ratios (Kp) • The following data are provided for a particular patient • Contour point • outline of the irregular field • The coordinate (x,y) of the point of calculation • Reference point • Patient measurements • Patient thickness at various points of interest • SSDs • Source to film distance
As soon as a given area reaches its prescribed dose, it is shielded during subsequent treatments
Asymmetric Fields • Independent jaw • Field center positioned away from the true central axis of the beam • Parameters changes after a field is collimated asymmetrically • Collimator scatter • Phantom scatter • Off-axis beam quality • Beam-flattening filters – greater beam hardening close to the central axis can be assumed equal to symmetrical field
SAD technique: SSD technique: Dose / MU =K Dose / MU =K Sc (rc) Sp(rd) Sc (rc) Sp(r) (SAD factor) (SSD factor) TMR(d,rd) PDD(d,r)/100 OARd(x) OARd(x) MU = ID / (Dose / MU) MU = TD / (Dose / MU) Practical Applications (asymmetric fields)
Asymmetric Fields • OARd(x) • off-axis ratio at depth d • data are derived from cross-beam profiles • measured at a number of depths in a phantom • for the largest field available (e.g. 40 ×40 cm)
Introduction • Dose Calculation Parameters • Collimator Scatter Factor • Phantom Scatter Factor • Tissue-Phantom and Tissue-Maximum Ratios • Practical Application • Accelerator Calculations • Irregular Fields • Asymmetric Fields • Other Practical Methods of Calculating Depth Dose Distribution • Irregular Fields • Point Off-Axis • Point Outside the Field • Point under a block
Irregular Fields • Clarkson’s technique is not practical for routine manual or computerized calculation • Time consuming • Considerable amount of input data
×1 ×1 ×2 ×2 ×1 ×1 Irregular Fields • Approximate method • Reasonably accurate calculations for most blocked field • Effective field • blocked field • Sc • Collimator field • unblocked field, defined by the collimator • Percent depth dose, TMR, Sp
a d b Q + c P Point Off-Axis • Clarkson’s technique is also not practical for manual calculation • Day’s method • PPD can be calculated at any point within the medium using the central axis data
2a 2d 2b 2b a d b Q 2a 2d + c P 2c 2c Point Off-Axis • The dose at depth d along the axis through Q
c c a b a c b ● Point Outside the Field
c a ● b ‘t’ is the block transmission factor. Points under a block