Photon Dosimetry concepts and Calculations

1. Photon Dosimetry concepts and Calculations Chapter 21 W/L

2. Dose Calculation • The treatment planning team has to quantify the overall prescribed dose of radiation and determine how much dose will be delivered over the time frame outlined. • There are many parameters of photon beam calculation.

3. Monitor Units • Monitor Units (MU): a measure of output for linear accelerators. • The dose rate varies slightly from one moment to the next • Normally the dose rate for the linear accelerator is 1.0 cGy/MU for a 10 x 10 field size defined at the isocenter.

4. Radiation Therapy Prescription • Radiation Therapy Prescription: • Legal document defines the treatment volume • Intended tumor dose (TD) • Number of treatments • Dose per treatment • Frequency of treatment • States the type and energy of radiation • Beam-shaping devices such as wedges and compensators • Any other appropriate factors. • Clear, precise and complete

5. Isodose Plan • Isodose Plan: part of the prescription • May show field sizes • Machine angels • Doses • Beam weighting • Wedges compensators or blocks

6. Treatment Time • Treatment time: length of time a unit is physically left on to deliver a measured dose. • Factors considered: • Beam energy • Distance from the source of radiation • Field size

7. Dose • Dose (absorbed dose): measured at a specific point in a medium and refers to the energy deposited at that point. • Measured in gray (Gy), which is defined so that 1 Gy equals 1J/kg.

8. Depth • Depth: distance beneath the skin surface where the prescribed dose is to be delivered. • Opposed fields: patient’s midplane is used • Multiple field arrangements: isocenter used • Depth affects measurements of dose attenuation.

9. Separation • Separation: a measurement of the patients thickness from the point of beam entry to the point of beam exit. • Normally measured along the beams central axis. • Calipers • ODI readings

10. Source Distance • Source to Skin Distance (SSD): the distance from the source or target of the treatment machine to the surface of the patient. • Source-axis Distance: the distance from the source of photons to the isocenter.

11. Setup • SSD: Isocenter established at the patients skin surface • When the gantry rotates around the patient, the SSD will continually change. • Dose calculations often at DMAX. (Given dose) • SAD: Isocenter established within the patient • The SAD and the isocenter are at a fixed distance and therefore do not change.

12. Isocenter • Isocenter: the intersection of the axis of rotation of the gantry and the axis of rotation of the collimator for the treatment unit. • Cobalt 60: SAD of 80 cm • Linear Accelerators: SAD of 100 cm

13. Field Size • Field Size: the physical size set on the collimator of the therapy unit that determines the size of the treatment field at a reference distance (defined at the machine’s isocenter) • SAD: the field size set inside the patient (size measured on patients skin will be smaller) • SSD: field size set on the collimator will be the same measured at the patients skin

14. Scatter • Backscatter: radiation that is deflected back toward the patient • Most of the absorbed dose received by the patient results from the collisions of the scattered electrons produced when the primary photon interacts with the collimator.

15. DMAX • DMAX: the depth at which electronic equilibrium occurs for photon beams; the point where dose reaches its maximum value. • Mainly depends on the energy of the beam • The depth of maximum ionization increases as the energy of the beam increases. • Factors such as field size and distance may also influence the depth

17. DMAX • DMAX dose occurs at the same depth for a given energy regardless of field size or distance from the source. • The actual reading differs for different field sizes.

18. Output • Output: the dose rate of the machine, the amount of radiation exposure produced by a treatment machine or source as specified at a reference field size and at a specified reference distance. • Changing the field size, distance, or attenuating medium will change the dose rate. • Increases with field size: primary component the same, increased scatter adds to the output • If the distance increases, dose rate decreases due to ISL

19. Cobalt-60 Output • Dose rate for Co-60 machine in cGy/min • Dose rate due to the radioactive decay of the isotope Co-60 • Can be assumed constant over short periods of time • The Time that a machine is left On is adjusted by 1% increase every month TO2 = TO1 x 1.01

20. Linac Output • Dose rate for Linac in cGy/MU • Dose rate varies from one moment to the next • Ionization chamber shuts down machine after predetermined dose has been given

21. Output Factor • Output Factor: the ratio of the dose rate of a given field size to the dose rate of the reference field size • Allows for the change in scatter as the collimator setting changes • Relates the dose rate of a given collimator setting to the dose rate of the reference field size

22. Dose Rate • Commonly measured at the isocenter of the treatment machine. • The dose rate of the beam is inversely proportional to the square of the distance • Dose rate(Given f.s.) = Dose rate(Reference f.s.) x Output factor(Given f.s.) • Dose Rate: • Output (of machine) • Output factor (C.S. field size) • Scatter Ratio BSF or PSF (EFS/CS) • PDD(EFS)/100 • Tray factor • Inverse square correction

23. Equivalent Squares • Equivalent Squares: rectangular field size that demonstrate the same measurable scattering and attenuation characteristics of a square field size. • Used to find the output, output factor, and tissue absorption factors. • 4 (A/P)

24. Effective Field Size • Effective Field Size (EFS): the equivalent rectangular field dimension of the open or treated area within the collimator field dimensions when blocks are used to customize the shape of the treatment area. • The EFS equivalent square is normally used to determine the tissue absorption factors.

25. Tissue Absorption Factors • Tissue absorption factors: different methods for measuring the attenuation of the beam as it travels through matter. • Percent Depth Dose (PDD): • works best with SSD setups • Tissue Air Ratio (TAR) • Tissue Phantom Ratio (TPR) • Tissue Maximum Ratio (TMR)

26. Percent Depth Dose (PDD) • Percent Depth Dose (PDD): the ratio expressed as a percentage, of the absorbed dose at a given depth to the absorbed dose at a fixed reference depth usually DMAX. • Calculated from two measurements at two different points in space. • Requires SSD be constant. • Written as PDD(d,s,SSD)= • Dependant on: • ↑ Energy- more penetrating- ↑ PDD • ↑ Depth- ↓ PDD due to attenuation through matter • ↑ Field size- more scatter- ↑ PDD • ↑ SSD- ↑ PDD- ISL

27. Percent Depth Dose (PDD) PDD = Dose @ d . Dose @ Dmax

28. Tissue Air Ratio (TAR) • Tissue Air Ratio (TAR): the ratio of the absorbed dose at a given depth in tissue to the absorbed dose at the same point in air. • Dependant of: • ↑Energy- ↑TAR • ↑Field size- ↑ TAR • ↑Depth- ↓ TAR • Independent of SSD

29. Tissue Air Ratio (TAR) • Calculated using two measurements at the same point in space • When the depth in tissue corresponds to the level of DMAX, the TAR is known as the backscatter factor. • Build-up cap: device made of acrylic or other phantom material that is placed over an ionization chamber to produce conditions of electronic equilibrium.

30. Tissue Air Ratio (TAR) TAR = Dose in tissue Dose in Air *Normally used to perform calcs for SAD treatments of low energy machines. *There is no patient backscatter in the “in Air” measurements.

31. Backscatter Factor (BSF) • When blocks are used. • Backscatter Factor (BSF): the ratio of the dose rate with a scattering medium to the dose rate at the same point without a scattering medium at the level of maximum equilibrium. (TAR at the level of DMAX) • PSF for megavoltage units.

32. Backscatter Factor (BSF) The % change in dose due to a change in field size. Readings are made at Dmax and compared to the dose in air. ↑FS  ↑Backscatter  ↑BSF

33. Dose to a Point • The dose to any point in a medium is made up of two parts: • Primary radiation: the photons that originate in, and fan out from the source. • Scatter radiation: other parts in the medium interact with the primary photons.

34. Scatter Air Ratio (SAR) • Scatter Air Ratio (SAR): the difference between the TAR for a field of definite area and that for a zero area. • The primary part of the total absorbed dose is represented by the zero area TAR. • A measure of the contribution from scattered radiation.

35. Tissue Phantom Ratio (TPR) • Tissue Phantom Ratio (TPR): the absorbed dose at a given depth in phantom to the absorbed dose at the same point at a reference depth in phantom. • The deeper the reference depth the greater the TPR.

36. Tissue Maximum Ratio (TMR) *Tissue Maximum Ratio (TMR): TPR at DMAX *TAR = TMR x BSF

37. Transmission Factors • Transmission Factors: the ratio of the radiation dose with the device to the radiation dose without the device and accounts for the material in the beams path. • Tray transmission factor • Wedge factor • Compensator factor

38. Other Factors • Tray transmission factor: defines how much of the radiation is transmitted through a block tray. • Tray factors vary with beam energy- as energy increases, the effect of the material in the beams path is lessened because of the greater penetrating power of the higher energy. • Wedge: attenuates the radiation beam progressively across a field

39. Given Dose • Given dose: the dose at DMAX • AKA: applied dose, entrance dose, peak absorbed dose, or DMAX dose • Point where PDD is 100% Given dose = TD/ PDD x 100 TD: tumor dose, dose at depth • Direct relationship: DoseA/PDDA = DoseB/PDDB

40. Exit Dose • Exit dose: the dose absorbed by a point that is located at the depth of DMAX at the exit of the beam.

41. Calculations • Field size variations, energy changes, and modifiers in the beam’s path can alter the amount of radiation received by the patient. Time setting = Dose at a point/Dose rate at that point • Dose at a point: the prescribed dose as determined by the doctor • Dose rate at that point: represents the dose rate of the treatment unit at the point of calculation

42. Calculations • When calculating dose to a given depth, all other points in the radiation field will be exposed to radiation for the same amount of time. • The dose to these points is dependant on their depth.

43. SSD Calculations • Output or dose rate of the machine should be expressed at the depth of DMAX • Field size: defined at the skin surface • Dose rate: measured in tissue at the depth of DMAX • Find equivalent square of the collimator setting (used for output factor) • Find equivalent square of the EFS (used for PDD) • Determine the PDD • Determine prescribed dose • Determine the treatment unit setting

44. Extended Distance • Extended Distance: patient is set up at a distance beyond the isocenter or reference distance. • PDD is used because its nonisocentric • When MUs are calculated for setup at distances greater than the standard, ISL is used to account for the decrease in dose rates beyond the isocenter.

45. Inverse Square Law • Inverse Square Law: describes the change in beam intensity caused by the divergence of the beam. I1 / I2 = D22 / D12 I1 =original dose rate I2 = new dose rate D22 = original distance D12 = new distance

46. Mayneord’s Factor • Mayneord’s Factor: a special application of the inverse square law. • Does not account for changes in scatter because of a change in beam divergence. New PDD = PDD x (SSD1 + d / SSD1 + DMAX)2 x (SSD2 + DMAX / SSD1 + d)2 • Inverse Square Factor: = (Reference source calibration distance Treatment SSD + DMAX)2 • SSD: reference point typically at DMAX • SAD: reference point at isocenter

47. TMR/TPR SAD • COF(C.S.) (collimator output factor): used to determine scatter, measured in air, from the collimators • The increase in the collimator opening, the more collimator surface area the photons will have to interact with. • PSF(EFS) (phantom scatter factor): used to determine scatter from the patient.