Photon Beams: Physical Aspects Part 2 PDF

M.Sc. MEDICAL PHYSICS Photon Beams: Physical Aspects Part 2 Dr. KHALID IBRAHIM HUSSEIN  CENTRAL AXIS DEPTH DOSES IN WATER: SSD SETUP ❑The dose at point Q in the patient consists of two components: primary component and scatter component. 𝐷𝑇 = 𝐷𝑃 +...

M.Sc. MEDICAL PHYSICS Photon Beams: Physical Aspects Part 2 Dr. KHALID IBRAHIM HUSSEIN  CENTRAL AXIS DEPTH DOSES IN WATER: SSD SETUP ❑The dose at point Q in the patient consists of two components: primary component and scatter component. 𝐷𝑇 = 𝐷𝑃 + 𝐷𝑆 ❑ The primary dose in a phantom is contribution to the dose at point Q arrives directly from the source, which is represented by the dose in a hypothetical 0 x0 cm2 field size which is obtained by extrapolation of the depth dose versus field size data. ❑ For megavoltage photon beams, it is reasonable to consider collimator scatter as part of primary beam ( defined as effective primary dose in phantom) so that the phantom scatter could be calculated separately. ❑ The scatter component at point Q reflects the relative contribution of the scattered radiation to the dose at point Q the scatter dose is delivered by photons produced through Compton scattering in the patient, machine collimator, flattening filter or air. M.Sc. Medical Physics Dr. Khalid I Hussein  Dosimetry Quantities In terms of energy ❑Dosimetric functions used in the whole photon energy range: ❑ Percentage depth dose (PDD) ❑ Relative dose factor (RDF) ❑Dosimetric functions used at cobalt-60 and below: ❑ Peak scatter factor (PSF) ❑ Collimator factor (CF) ❑ Scatter factor (SF) ❑ Scatter function (S) ❑ Tissue air ratio (TAR) ❑ Scatter air ratio (SAR) ❑Dosimetric functions used at cobalt-60 and above: ❑ Tissue maximum ratio (TMR) ❑ Tissue phantom ratio (TPR) ❑ Scatter maximum ratio (SMR) M.Sc. Medical Physics Dr. Khalid I Hussein  Dosimetry Quantities In terms of setup techniques ❑Source Skin Distance (SSD) ❑ Percentage depth dose (PDD) ❑Source Axial Distance (SAD) ❑ Tissue air ratio (TAR) ❑ Tissue maximum ratio (TMR) ❑ Tissue phantom ratio (TPR) M.Sc. Medical Physics Dr. Khalid I Hussein  Dosimetry Quantities ❑SAD setups are used in treatment of deep seated tumors with multiple beams or with rotational beams. ❑In comparison with constant SSD setup that relies on PDD distributions, the SAD setup is more practical and relies on other dose functions such as: ❑ Tissue-air ratio (TAR) ❑ Tissue-phantom ration (TPR) ❑ Tissue-maximum ratio (TMR) ❑The SSD varies from one beam to another; however, the source-axis distance SAD remains constant.  Tissue Air Ratio : SAD SETUP ❑ Tissue-air ratio may be defined as the ratio of the dose (Dd) at a given point in the phantom to the dose in free space (Dfs) at the same point. ❑ The term “dose in free space” stands for dose at the center of an equilibrium mass of tissue located in free air (i.e., a spherical tissue mass of radius large enough to provide electronic equilibrium at the center). ❑ TAR is defined as: 𝐷𝑑 𝑇𝐴𝑅 𝑧, 𝐴𝑧 = 𝐷𝑓𝑠 ❑ TAR depends on depth, beam energy, field size, and field shape. Independent of SSD ❑ TARs have traditionally been used for low-energy (up to 60Co)  Tissue Air Ratio : SAD SETUP ❖Relationship between TAR and PDD mathematically: 2 1 𝑓 + 𝑧𝑚 𝑃𝐷𝐷 𝑧, 𝐴, 𝑓 = 𝑇𝐴𝑅(𝑧, 𝐴𝑧 ) × × × 100 𝑃𝑆𝐹(𝐴) 𝑓+𝑧 ❖ The scatter air ratio (SAR) : Represents the scatter component of TAR. It is a useful concept for the dosimetry of irregularly shaped fields (Clarkson technique). ❖ Self-reading : Clarkson Method used for irregular field in Radiotherapy ❖Mathematically SAR is given by: 𝑆𝐴𝑅 𝑧, 𝐴𝑍 = 𝑇𝐴𝑅 𝑧, 𝐴𝑍 − TAR(z, 0) RADIATION TREATMENT PARAMETERS – Backscatter Factor ❑The back scatter factor (BSF) or peak scatter factor (PSF) is special case of TAR at the reference depth of maximum dose on the central axis of the beam. 𝐷𝑃 (𝑧𝑚𝑎𝑥 , 𝐴. 𝑓, ℎ𝑣) 𝑃𝑆𝐹 𝐴, ℎ𝑣 = 𝐷𝑃′ (𝐴, ℎ𝑣) Or 𝑃𝑆𝐹 𝐴, ℎ𝑣 = 𝑇𝐴𝑅(𝑍𝑚 , 𝐴𝑧𝑚 ) ❑ BSF or PSF is a substantial factor for beams in the orthovoltage range of energies. M.Sc. Medical Physics Dr. Khalid I Hussein  RADIATION TREATMENT PARAMETERS DP (zmax ,A,f ,h ) PSF(A,h ) = DP (A,h ) PSF depends upon: Field size A (the larger is the field size,the larger is PSF). Photon energy (except at very low photon energies, PSF decreases with increasing energy). M.Sc. Medical Physics Dr. Khalid I Hussein  Tissue Phantom Ratio: SAD SETUP ❑ For isocentric setups with megavoltage photon energies the concept of tissue-phantom ratio TPR was developed. ❑ TPR is defined as the ratio of the dose rate at a given point at depth in phantom to the dose rate at the same point and distance at reference depth. ❑ Similarly to TAR the TPR depends upon z, AQ , and hv. ❑ TPR is defined as: 𝐷𝑄 𝑇𝑃𝑅 𝑧, 𝐴𝑄 , ℎ𝑣 = 𝐷𝑄𝑟𝑒𝑓  Tissue-phantom ratio TPR and Tissue-maximum ratio TMR ❑Tissue-maximum ratio TMR is a special TPR for zref = zmax. ❑TMR is defined as: 𝐷𝑄 𝑇𝑀𝑅 𝑧, 𝐴𝑄 , ℎ𝑣 = 𝐷𝑚𝑎𝑥 Tissue-phantom ratio TPR and Tissue-maximum ratio TMR ❑ Just like the TAR, the TPR and TMR depend on three parameters: depth, field size, and photon energy and are almost independent of SSD. ❑ The corresponding quantity for the scattered dose is called scatter- phantom ratio (SPR), which is analogous in use to the SAR ❑ The TPR is overcome the limitation of using TAR for high energy photon. ❑ The range of TMR is from 0 for z→ ∞ to 1 for z = zmax. ❑ For constant AQ and hv the TMR decreases with increasing z. ❑ For constant z and hv the TMR increases with increasing AQ.  Tissue-phantom ratio TPR and Tissue-maximum ratio TMR Relationship between TMR and PDD DQ PDD(z,A,f ,h ) = 100 DP DQ TMR(z,AQ ,h ) = DQ max Tissue-phantom ratio TPR and Tissue-maximum ratio TMR ❑Tissue-maximum ratio (TMR) for 10 MV as function of selected field sizes. ❑Field size 0 x 0 cm represent the primary beam and shows the steepest drop-off with depth because of the lack of scatter.  OFF-AXIS RATIOS AND BEAM PROFILES ❑ Dose distributions along the beam central axis are used in conjunction with off-axis beam profiles to deliver an accurate dose description inside the patient. ❑ The off-axis data are usually given with beam profiles measured perpendicularly to the beam central axis at a given depth in a phantom. ❑ The depths of measurement are typically at: ❑ Depths z = zmax and z = 10 cm for verification of machine compliance with machine specifications. ❑ Other depths required by the particular treatment planning system used in the department.  OFF-AXIS RATIOS AND BEAM PROFILES ❑Example of beam profiles measured for two field sizes (10×10 cm2 and 30×30 cm2) of a 10 MV x-ray beam at various depths in water. ❑The central axis profile values are scaled by the appropriate PDD value for the two fields. ❑Combining a central axis dose distribution with off-axis data results in a volume dose matrix that provides 2-D and 3-D information on the dose distribution in the patient. ❑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.  OFF-AXIS RATIOS AND BEAM PROFILES ❑Megavoltage beam profiles consist of three regions: ❑ Central region represents the central portion of the profile extending from the central axis to within 1 cm to 1.5 cm of the geometric field edges of the beam. ❑ Penumbra is the region close to geometric field edges where the dose changes rapidly and depends on field defining collimators, the finite size of the focal spot (source size) and the lateral electronic disequilibrium. ❑ Umbra is the region outside of the radiation field, far removed from the field edges. The dose in this region is low and results from radiation transmitted through the collimator and head shielding.  OFF-AXIS RATIOS AND BEAM PROFILES For each of the three beam profile regions there are specific requirements to optimize the clinical photon beam: The dose profile in the central region should meet flatness and symmetry specifications. The dose profile in the penumbral region should have a rapid falloff with increasing distance from the central axis (narrow penumbra) to optimize beam sharpness at the target edge. The dose profile in the umbral region should be close to zero dose to minimize the dose delivered to tissues outside the target volume.  OFF-AXIS RATIOS AND BEAM PROFILES ❑ Ideal dose profile: Central region: constant dose from target centre to edge of target. Penumbra: zero width. Umbra: zero dose. ❑ Actual dose profile: Central region: profile flat in 80 % of central portion of the field. Penumbra is typically defined as the distance between 80 % and 20 % dose on the beam profile normalized to 100 % at the central axis. Umbra is typically less than 1 % of the dose on the central axis.  OFF-AXIS RATIOS AND BEAM PROFILES ❑Geometric or nominal field size is: ❑ Indicated by the optical light field of the treatment machine. ❑ Usually defined as the separation between the 50 % dose level points on the beam profile measured at the depth of dose maximum zmax.  OFF-AXIS RATIOS AND BEAM PROFILES ❑ The total penumbra is referred to as the physical penumbra and consists of three components: ❑Geometric penumbra : results from the finite source size. ❑Scatter penumbra: results from in-patient photon scatter originating in the open field. ❑Transmission penumbra: results from beam transmitted through the collimation device.  OFF-AXIS RATIOS AND BEAM PROFILES ❑ Beam flatness F is assessed by finding the maximum Dmax and minimum Dmin dose point values on the beam profile within the central 80 % of the beam width. Beam flatness F is defined as: Dmax − Dmin F = 100  Dmax + Dmin ❑ Standard linac specifications require that F ≤ 3% when measured in a water phantom at a depth z = 10 cm with SSD = 100 cm for the largest field size available (typically 40×40 cm2).  OFF-AXIS RATIOS AND BEAM PROFILES ❑Beam symmetry S is usually determined at zmax to achieve maximum sensitivity. ❑The areas under the zmax profile can often be determined using an automatic software option on the water tank scanning device (3-D isodose plotter). and should be less than 2 %. ❑Any two dose points on a beam profile, equidistant from the central axis point, should be within 2 % of each other. ISODOSE DISTRIBUTIONS IN WATER PHANTOMS ❑Physical characteristics of radiation beams are usually measured in phantoms under standard conditions: ❑ Homogeneous, unit density phantom ❑ Flat phantom surface ❑ Perpendicular beam incidence ❑Central axis depth dose data in conjunction with dose profiles contain complete 2-D and 3-D information about the radiation beam. ❑Planar and volumetric dose distributions are usually displayed with isodose curves and isodose surfaces, which connect points of equal dose in a volume of interest. ❑The isodose curves and surfaces are usually drawn at regular intervals of absorbed dose and are expressed as a percentage of the dose at a specific reference point.  ISODOSE DISTRIBUTIONS IN WATER PHANTOMS ❑ An isodose chart for a given single beam consists of a family of isodose curves usually drawn at regular increments of PDD. ❑ Two normalization conventions are in use: ❑ For SSD set-ups, all isodose values are normalized to 100 % at point P on the central beam axis (point of dose maximum). ❑ For SAD set-ups, the isodose values are normalized to 100 % at the isocentre. ❑ The isodose charts for an SSD set-up are thus plots of PDD values; isodose charts for an SAD set-up are plots of either TAR or TMR. ISODOSE DISTRIBUTIONS IN WATER PHANTOMS ❑For SSD set-ups, all isodose values are For SAD set-ups, the isodose values normalized to 100 % at point P on the are normalized to 100 % at the central beam axis (point of dose maximum isocentre. at depth zmax).  ISODOSE DISTRIBUTIONS IN WATER PHANTOMS Isodose distributions for various photon radiation beams: orthovoltage x rays, cobalt-60 gamma rays, 4 MV x rays, 10 MV x rays Parameters that affect the single beam isodose distribution are: Beam quality, Source size, Beam collimation, Field size, Source-skin distance, Source-collimator distance Absolute and relative dose measurement with ionization ❑ The dose parameters for radiotherapy treatment are most commonly measured with ionization chambers that come in many sizes and geometrical shapes. ❑ Usually each task of dose determination is carried out with ionization chambers designed for the specific task at hand. ❑ In many situations the measured chamber signal must be corrected with correction factors that depend on influence quantities, such as chamber air temperature and pressure, chamber polarity and applied voltage, and photon beam energy. ❑ The relative dose measurement (PDD, dose profile, and isodose charts) are most commonly measured with solid state detectors such as diodes. ❑ In addition to direct measurements, isodose charts may also be generated by calculations using various algorithms for treatment planning, most commonly with commercially available treatment planning systems (TPSs). Absolute and relative dose measurement with ionization Absolute and relative dose measurement with ionization ❑ Doses and dose rates at reference points in a phantom for megavoltage photon beams are measured with relatively large volume (0.6 cm3) cylindrical ionization chambers in order to obtain a reasonable signal and good signal to noise ratio. Absolute and relative dose measurement with ionization Relative dose distributions for photon beams beyond zmax are usually measured with small volume (0.1 cm3) ionization chambers in order to obtain good spatial resolution.  Assignment The dose measurements are normally measured using flat surface phantom, perpendicular beam incidence, and homogeneous unit density phantom, but in clinical situations are usually more complex. Write a report on the most correction methods preformed for radiotherapy dose measurements, including; contour irregularities, oblique beam incidence, tissue inhomogeneities, the use of wedges, compensators and bolus.  DELIVERY OF DOSE WITH A SINGLE EXTERNAL BEAM ❑ Outputs for x ray machines and radionuclide teletherapy units are usually given in centigray per minute (cGy/min) at zmax in a phantom at a nominal source- surface distance SSD. ❑ Outputs for linacs are usually given in centigray per monitor unit (cGy/MU) at zmax in a phantom at a nominal source- surface distance SSD..  DELIVERY OF DOSE WITH A SINGLE EXTERNAL BEAM ❑Transmission ionization chambers in linacs are usually adjusted such that the beam output (dose rate) corresponds to: ❑ 1 cGy/MU ❑ at zmax in phantom (point P) ❑ for a 10×10 cm2 field ❑ at SSD = 100 cm. DP ( zmax ,10,100, h ) = 1 cGy/MU  DELIVERY OF DOSE WITH A SINGLE EXTERNAL BEAM ❑ 𝐷𝑃ሶ 𝑧𝑚𝑎𝑥 , 𝐴, 100, ℎ𝑣 , the dose rate at point P for an SSD of 100 cm for an arbitrary field size A is obtained by multiplying 𝐷𝑃ሶ 𝑧𝑚𝑎𝑥 , 10,100, ℎ𝑣 = 1cGy/MU with the relative dose factor RDF(A,hv). 𝐷𝑃ሶ 𝑧𝑚𝑎𝑥 , 𝐴, 100, ℎ𝑣 = 𝐷𝑃ሶ 𝑧𝑚𝑎𝑥 , 10,100, ℎ𝑣 × 𝑅𝐷𝐹(𝐴, ℎ𝑣)  DELIVERY OF DOSE WITH A SINGLE EXTERNAL BEAM ❑The PDD Formalism ❑ The number of monitor units (MUs) required to deliver a tumor dose TD at point Q using a single SSD field, SSD of 100 cm, and field size A is: 𝑇𝐷 𝑀𝑈 = 𝑇𝐷ሶ 𝑇𝐷 = 𝐷ሶ 𝑃 (𝑧𝑚𝑎𝑥 , 10,100, ℎ𝑣) × 𝑅𝐷𝐹(𝐴, ℎ𝑣) × 𝐹 × 𝑃𝐷𝐷(𝑧, 𝐴, 𝑓, ℎ𝑣) × 𝑊𝐹(𝐴, 𝑧, , 𝑥) × 𝑇𝐹 × 𝑂𝐴𝑅(𝑧, 𝑥) × 𝐼𝑆𝑄 ሶ stands for tumor dose rate. 𝐷𝑃ሶ 𝑧𝑚𝑎𝑥 , 10,100, ℎ𝑣 = 1cGy/MU ❑ Note:𝑇𝐷 The correction need for TMR calculation tech  DELIVERY OF DOSE WITH A SINGLE EXTERNAL BEAM The TMR (Isocentric) Formalism ❑The number of monitor units (MUs) required to deliver a tumor dose TD at point Q using a single SAD field, SAD of 100 cm, and field size AQ is: 𝑇𝐷 𝑀𝑈 = ሶ 𝑇𝐷 𝑇𝐷 = 𝐷ሶ 𝑄𝑟𝑒𝑓 (𝑧𝑟𝑒𝑓 , 𝐴𝑄 , 100𝑆𝐴𝐷 , ℎ𝑣) × 𝐹 × 𝑇𝑀𝑅(𝑧, 𝐴𝑄 , ℎ𝑣) × 𝑊𝐹(𝐴, 𝑧, , 𝑥) × 𝑇𝐹 × 𝑂𝐴𝑅(𝑧, 𝑥) ❑Note: DQref ( zref , AQ ,100SAD , h )  2  f + zref   DP ( zmax ,10,100SSD , h )  RDF( A, h )     f   DELIVERY OF DOSE WITH A SINGLE EXTERNAL BEAM DP ( zmax ,10,100, h ) = 1 cGy/MU DP ( zmax , A,100, h ) = = DP ( zmax ,10,100, h )   RDF( A, h ) For zref = zmax , TPR = TMR and DQref = DQmax DQref ( zref , AQ ,100SAD , h )  2  f + zref   DP ( zmax ,10,100SSD , h )  RDF( A, h )     f  TMR of 6 MV EXAMPLE OF DOSE CALCULATION EXAMPLE OF DOSE CALCULATION  EXAMPLE OF DOSE CALCULATION 𝑇𝐷 𝑀𝑈 = 𝐷ሶ 𝑟𝑒𝑓 𝑃𝐷𝐷(𝑆𝑆𝐷, 𝑟1 , 𝑑) × 𝑆𝐶 (𝑟𝑐 ) × 𝑆𝑃 (𝑟1 ) × 𝑇𝐹 × 𝑂𝐴𝑅(𝑧, 𝑥) 𝑇𝐷 𝑀𝑈 = 𝐷ሶ 𝑟𝑒𝑓 𝑃𝐷𝐷(100,10,10) × 𝑆𝐶 (12) × 𝑆𝑃 (10) × 𝑇𝐹 × 𝑂𝐴𝑅(10,3) 40 Y/o patient diagnosis as prostate cancer stage 2. Patient to be treat with 200cGy using 6MV photon beam, SSD =100 cm. The field parameters as obtained from the treatment planning shown in figure below calculate the MU setting for this patient? The calibration dose = 1 cGy/MU. Anterior field: 8×8 cm2 open field weight W = 1.0 Depth of 6cm Right posterior field: Left posterior field: 6×6 6×6 cm2 wedge field cm2 wedge field weight weight W = 0.8 W = 0.8 wedge angle 60ͦͦ wedge angle 60ͦͦ Depth of 10ͦͦcm Depth of 10ͦͦcm  𝑃𝑟𝑒𝑠𝑐𝑟𝑖𝑏𝑒𝑑 𝑑𝑜𝑠𝑒 𝑀𝑜𝑛𝑖𝑡𝑜𝑟 𝑢𝑛𝑖𝑡 = ሶ max , Aref ,f ,E) × 𝑃𝐷𝐷 𝑧, 𝐴, 𝑓, 𝐸 × 𝑅𝐷𝐹 𝐴, 𝐸 × 𝐼𝑆𝐹 × 𝑂𝐴𝐹 × 𝑇𝐹 𝐷(z 100 i The prescribed dose for the individual beam can be calculated as follows: 𝑇𝑢𝑚𝑜𝑟 𝑑𝑜𝑠𝑒 𝑝𝑟𝑒𝑠𝑐𝑟𝑖𝑏𝑒𝑑 𝑑𝑜𝑠𝑒 ×𝐵𝑒𝑎𝑚 𝑊𝑒𝑖𝑔ℎ𝑡 (𝑊) Prescribed dose = 𝑡𝑜𝑡𝑎𝑙 𝑤𝑒𝑖𝑔ℎ𝑡 200×1 Prescribed dose of the Anterior field = = 77 𝑐𝐺𝑦 2.6 200×𝑜.8 Prescribed dose of the Right posterior field= = 61.5 𝑐𝐺𝑦 2.6 200×𝑜.8 Prescribed dose of the Left posterior field = = 61.5 𝑐𝐺𝑦 2.6  Field Di(cGy) ሶ 𝑫(cGy/MU) PDD RDF WF MU Anterior 77 1 82.4 0.985 1 ? Left post. 61.5 1 65.1 0.962 0.795 ? Right post. 61.5 1 65.1 0.962 0.795 ? Class work: Calculate the MU for the above treatment fields  EXAMPLE OF DOSE CALCULATION ❑Problem: For a 6Mv beam, what is the dose to the depth of 5cm when the dose at 3cm is 200cGy? Given PDD value at D3 =.951 and PDD value at D5 =.871. ❑180cGy is prescribed to a depth of 12cm at a point 5cm off of the central axis. Compute the required monitor units for a 6MV beam delivered 105SSD with a field size of 12x12cm2. Here %DD = 75.3, Sc = 1.008, Sp = 1.009, TMR = 0.82, and OAF = 1.023. ❑An accelerator is calibrated to give 1cGy/MU at an SSD of 100cm, field size of 10 x 10 cm2 at depth of dmax. Calculate the dose using the isocentric formalism given a treatment depth of 10cm, collimator field size 10x15 cm2, 100MU, and an SSD 100cm. (Given RDF = 1.019 and TMR(13.2,10) = 0.79082)