Lect 05 Photon beams Part 1 PDF

M.Sc. MEDICAL PHYSICS Principles of Physics in Radiation Oncology - RAD 6135 Photon Beams: Physical Aspects Part 1 Dr. KHALID IBRAHIM HUSSEIN  PHOTON SOURCES FOR EXTERNAL BEAM THERAPY Photon sources with regard to type of photons: Gamma ray sources X-ra...

M.Sc. MEDICAL PHYSICS Principles of Physics in Radiation Oncology - RAD 6135 Photon Beams: Physical Aspects Part 1 Dr. KHALID IBRAHIM HUSSEIN  PHOTON SOURCES FOR EXTERNAL BEAM THERAPY Photon sources with regard to type of photons: Gamma ray sources X-ray sources Photon sources with regard to photon energies: Monoenergetic sources Heterogeneous sources Photon sources with regard to intensity distribution: Isotropic Non-isotropic M.Sc. Medical Physics Dr. Khalid I Hussein  PHOTON SOURCES FOR EXTERNAL BEAM THERAPY ❑For a given photon source, a plot of number of photons per energy interval versus photon energy is referred to as photon spectrum. ❑All photons in a monoenergetic photon beam have the same energy h. M.Sc. Medical Physics Dr. Khalid I Hussein  PHOTON SOURCES FOR EXTERNAL BEAM THERAPY Gamma ray sources are usually isotropic and produce monoenergetic photon beams. X-ray targets are non-isotropic sources and produce heterogeneous photon spectra. In the superficial and orthovoltage energy region the x-ray emission occurs predominantly at 90o to the direction of the electron beam striking the x-ray target. In the megavoltage energy region the x-ray emission in the target occurs predominantly in the direction of the electron beam striking the target (forward direction). M.Sc. Medical Physics Dr. Khalid I Hussein  INVERSE SQUARE LAW ❑In external beam radiotherapy: ❑ Photon sources are often assumed to be point sources. ❑ Beams produced by photon sources are assumed to be divergent. 2 𝐷𝐴 𝑓𝑏 = 𝐷𝐵 𝑓𝑎  PENETRATION OF PHOTON BEAMS INTO PATIENT A photon beam propagating through air or vacuum is governed by the inverse square law. A photon beam propagating through a phantom or patient is affected not only by the inverse square law but also by the attenuation and scattering of the photon beam inside the phantom or patient. The three effects make the dose deposition in a phantom or patient a complicated process and its determination a complex task. M.Sc. Medical Physics Dr. Khalid I Hussein  PENETRATION OF PHOTON BEAMS INTO PATIENT ❑ For a successful outcome of patient radiation treatment it is imperative that the dose distribution in the target volume and surrounding tissues is known precisely and accurately. ❑ This is usually achieved through the use of several empirical functions that link the dose at any arbitrary point inside the patient to the known dose at the beam calibration (or reference) point in a phantom. ❑ Dosimetric functions are usually measured with suitable radiation detectors in tissue equivalent phantoms. ❑ Dose or dose rate at the reference point is determined for, or in, water phantoms for a specific set of reference conditions, such as: Depth in phantom z Field size A Source-surface distance (SSD). M.Sc. Medical Physics Dr. Khalid I Hussein  PENETRATION OF PHOTON BEAMS INTO PATIENT ❑ Typical dose distribution for an external photon beam follows a known general pattern: ❑ The beam enters the patient on the surface where it delivers a certain surface dose Ds. ❑ Beneath the surface the dose first rises rapidly, reaches a maximum value at a depth zmax, and then decreases almost exponentially until it reaches a value Dex at the patient’s exit point. M.Sc. Medical Physics Dr. Khalid I Hussein  PENETRATION OF PHOTON BEAMS INTO PATIENT ❑Surface dose: ❑ For megavoltage x-ray beams the surface dose is generally much lower (skin sparing effect) than the maximum dose at zmax. ❑ For superficial and orthovoltage beams zmax = 0 and the surface dose equals the maximum dose. ❑ The surface dose is measured with parallel- plate ionization chambers for both chamber polarities, with the average reading between the two polarities taken as the correct surface dose value. M.Sc. Medical Physics Dr. Khalid I Hussein  PENETRATION OF PHOTON BEAMS INTO PATIENT ❑Contributors to surface dose Ds: ❑ Photons scattered from the collimators, flattening filter and air. ❑ Photons backscattered from the patient. ❑ High energy electrons produced by photon interactions in air and any shielding structures in the neighborhood of the patient. ❑Typical values of surface dose: ❑ 100 % superficial and orthovoltage ❑ 30 % cobalt-60 gamma rays ❑ 15 % 6 MV x-ray beams ❑ 10 % 18 MV x-ray beams M.Sc. Medical Physics Dr. Khalid I Hussein  PENETRATION OF PHOTON BEAMS INTO PATIENT ❑ Buildup dose region: ❑ The region between the surface (z = 0) and depth z = zmax in megavoltage photon beams is called the dose buildup region. ❑ The dose buildup results from the relatively long range of secondary charged particles that first are released in the patient by photon interactions and then deposit their kinetic energy in the patient through Coulomb interactions. M.Sc. Medical Physics Dr. Khalid I Hussein  PENETRATION OF PHOTON BEAMS INTO PATIENT Depth of dose maximum zmax depends upon: Photon beam energy (main effect) Field size (secondary effect) For a given field size: zmax increases with photon beam energy. For 5x5 cm2 fields, the nominal values of zmax are: Energy 100 kVp 350 kVp Co-60 4 MV 6 MV 10 MV 18 MV zmax(cm) 0 0 0.5 1.0 1.5 2.5 3.5 M.Sc. Medical Physics Dr. Khalid I Hussein  PENETRATION OF PHOTON BEAMS INTO PATIENT At a given beam energy: For fields smaller than 5×5 cm2, zmax increases with increasing field size because of in-phantom scatter. For field 5×5 cm2, zmax reaches its nominal value. For fields larger than 5×5 cm2, zmax decreases with increasing field size because of collimator and flattening filter scatter. M.Sc. Medical Physics Dr. Khalid I Hussein  PENETRATION OF PHOTON BEAMS INTO PATIENT ❑ Exit dose : The dose delivered to the patient at the beam exit point is called the exit dose. ❑ Close to the beam exit point the dose distribution curves slightly downwards from the dose curve obtained for a infinitely thick phantom as a result of missing scatter contribution for points beyond the dose exit point. ❑ The effect is small and generally ignored. M.Sc. Medical Physics Dr. Khalid I Hussein  RADIATION TREATMENT PARAMETERS The main parameters in external beam dose delivery with photon beams are: Depth of treatment z Fields size A Source-skin distance (SSD) in SSD setups Source-axis distance (SAD) in SAD setups Photon beam energy Number of beams used in dose delivery to the patient Treatment time for orthovoltage and teletherapy machines Number of monitor units (MUs) for linacs M.Sc. Medical Physics Dr. Khalid I Hussein  RADIATION TREATMENT PARAMETERS ❑Point P is at zmax on central axis. ❑Point Q is arbitrary point at depth z on the central axis. ❑Field size A is defined on patient’s surface. ❑AQ is the field size at point Q. ❑SSD = source-skin distance. ❑SCD = source-collimator distance M.Sc. Medical Physics Dr. Khalid I Hussein  RADIATION TREATMENT PARAMETERS ❑ Several functions are in use for linking the dose at a reference point in a water phantom to the dose at arbitrary points inside the patient. ❑ Some of these functions can be used in the whole energy range of interest in radiotherapy from superficial through orthovoltage and cobalt-60 to megavoltage ❑ Others are only applicable at energies of cobalt-60 and below. ❑ Or are used at cobalt-60 energy and above. ❑ Cobalt-60 serves as a transition point linking various dosimetry techniques.  RADIATION TREATMENT PARAMETERS ❑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  RADIATION TREATMENT PARAMETERS ❑Radiation beam field size ❑Four general groups of field shape are used in radiotherapy ❑ Square (produced with collimators installed in therapy machine) ❑ Rectangular (produced with collimators installed in therapy machine) ❑ Circular (produced with special collimators attached to treatment machine) ❑ Irregular (produced with custom made shielding blocks or with multileaf collimators) ❑For any arbitrary radiation field and equivalent square field or equivalent circular field may be found. The equivalent field will be characterized with similar beam parameters and functions as the arbitrary radiation field.  RADIATION TREATMENT PARAMETERS Radiation beam field size ❑ Radiation fields are divided into two categories: Geometric and dosimetric (physical). ❑ According to the ICRU, the geometric field size is defined as “the projection of the distal end of the machine collimator onto a plane perpendicular to the central axis of the radiation beam as seen from the front center of the source.” ❑ The dosimetric field size (also called the physical field size) is defined by the intercept of a given isodose surface (usually 50 % but can also be up to 80 %) with a plane perpendicular to the central axis of the radiation beam at a defined distance from the source. M.Sc. Medical Physics Dr. Khalid I Hussein  RADIATION TREATMENT PARAMETERS Radiation beam field size ❑Equivalent square for rectangular field: ❑An arbitrary rectangular field with sides 2ab a and b will be approximately equal to a aeq = square field with side aeq when both a+b fields have the same area ❑Equivalent circle for square field: ❑An arbitrary square field with side a will be equivalent to a circular field with radius req when both fields have the same area. a req =  M.Sc. Medical Physics Dr. Khalid I Hussein  RADIATION TREATMENT PARAMETERS Collimator factor  ❑ Exposure in air and dose to small mass of medium in air Dmed contain two components: ❑ Primary component is the major component. It originates in the source, comes directly from the source, and does not depend on field size. ❑ Scatter component is a minor, yet non-negligible, component. It represents the scatter from the collimator, air and flattening filter (in linacs) and depends on the field size A. M.Sc. Medical Physics Dr. Khalid I Hussein  RADIATION TREATMENT PARAMETERS Collimator factor or collimator scatter factor or relative exposure factor (REF) is defined as: X ( A, h ) ( K air ( A, h ))air D( A, h ) CF( A, h ) = Sc ( A, h ) = REF( A, h ) = = = X (10, h ) ( K air (10, h ))air D(10, h ) ❑ CF is normalized to 1 for the nominal field of 10×10 cm2 at the nominal SSD for the treatment machine. ❑ CF > 1 for fields A exceeding 10×10 cm2. ❑ CF = 1 for 10×10 cm2 field. ❑ CF < 1 for fields A smaller than 10×10 cm2. M.Sc. Medical Physics Dr. Khalid I Hussein  RADIATION TREATMENT PARAMETERS Relative dose factor tottal scatter factor T For a given photon beam with energy ℎ𝑣 at a given SSD, the dose at point P (at depth zmax) depends on field size A; the larger is the field size the larger is the dose. The ratio of the dose at point P for field size A to the dose at point P for field size 10×10 cm2 is called the relative dose factor RDF or total scatter factor Sc,p in Khan’s notation or machine output factor OF: DP ( zmax , A, f , h ) RDF( A, h ) = Sc,p ( A, h ) = DP ( zmax ,10, f , h ) For A = 10×10 cm2 RDF( A, h ) = 1 M.Sc. Medical Physics Dr. Khalid I Hussein  RADIATION TREATMENT PARAMETERS Relative dose factor (RDF) (Total scatter factor SC,P ) ❑ RDF is defined as the dose rate for a given field size at a reference depth in a phantom to the dose rate at the same point and depth for reference field size (10 x 10 cm2). ❑For a given photon beam with energy ℎ𝑣 at a given SSD, the dose at point P (at depth zmax) depends on field size A; the larger is the field size the larger is the dose. ❑The ratio of the dose at point P for field size A to the dose at point P for field size 10×10 cm2 is called the relative dose factor RDF or total scatter factor Sc,p in Khan’s notation or machine output factor OF: DP ( zmax , A, f , h ) RDF( A, h ) = Sc,p ( A, h ) = DP ( zmax ,10, f , h ) For A = 10×10 cm2 RDF( A, h ) = 1 M.Sc. Medical Physics Dr. Khalid I Hussein  RADIATION TREATMENT PARAMETERS When extra shielding is used on an accessory tray or a multileaf collimator (MLC) is used to shape the radiation field on the patient’s surface into an irregular field B, then the RDF(B,h ) is in the first approximation given as: RDF(B, h ) = CF( A, h )  SF(B, h ) Field A represents the field set by the machine collimator. Field B represents the actual irregular field on the patient’s surface. M.Sc. Medical Physics Dr. Khalid I Hussein  CENTRAL AXIS DEPTH DOSES IN WATER: SSD SETUP ❑ Central axis dose distributions inside the patient are usually normalized to Dmax = 100 % at the depth of dose maximum zmax and then referred to as percentage depth dose (PDD) distributions. The depth of maximum dose is measured at a pointon central axis for small field (e.g, 5 x5 or less)> ❑ Percentage depth dose depends on four parameters: ❑ Depth in phantom z ❑ Field size A on patient’s surface ❑ Source-surface distance f = SSD ❑ Photon beam energy h ❑ Beam collimation system 𝐷 ❑ 𝑃𝐷𝐷 𝑧, 𝐴. 𝑓, ℎ𝑣 = 100 𝐷𝑄 𝑃 ❑ PDD ranges in value from ❑ 0 at z →  ❑ To 100 at z = zmax M.Sc. Medical Physics Dr. Khalid I Hussein  CENTRAL AXIS DEPTH DOSES IN WATER: SSD SETUP ❑Properties of the PDD: ❑ After the initial build, PDD decreases depth almost exponentially with depth. ❑ The depth of maximum dose increases with increase in in beam quality. ❑ The higher the beam energy. The more gradual is the fall off of the depth dose curve beyond the depth of maximum dose. ❑ PDD has two dose components: a) the primary dose contributed by the primary photons- the photons that have traversed the overlying medium without interacting; and b) the scattered dose contributed by the scattered photons. ❑ PDD increases with increase in the field size due to increase in the scattered component of the dose. ❑ The primary component of dose is independent of the field size (provided the field dimensions are not less than the range of laterally scattered secondary electrons). ❑ Field dimensions as well as shape (e.g. square, rectangular, or irregular) affect the PDD because of changes in the scatter contribution. M.Sc. Medical Physics Dr. Khalid I Hussein  CENTRAL AXIS DEPTH DOSES IN WATER: SSD SETUP For a constant A, f, and hv. PDD Dependence of high energy For a constant z, A. The PDD first increases from the surface photon beams on field size increases with increasing f to z = zmax (buildup region), because of a decreasing effect and then decreases with z of depth z on the inverse M.Sc. Medical Physics Dr. Khalid I Hussein square factor. CENTRAL AXIS DEPTH DOSES IN WATER: SSD SETUP ❑Mayneord F factor: ❑ PDD for standard SSD (e.g. 100 cm) can be converted to PDD for another SSD by multiplying it by an approximate factor, known as the Mayneord F factor. ❑ Mayneord F factor is based on a strict application of the inverse square law, without considering changes in scatter, as the SSD is changed. ❑ The Mayneord F factor is given by: 2 2 𝑓2 + 𝑧𝑚 𝑓1 + 𝑧 𝐹= 𝑓1 + 𝑧𝑚 𝑓2 + 𝑧 M.Sc. Medical Physics Dr. Khalid I Hussein  Class activity (Group 6 presentation) ❑ Advantages and disadvantages of: ❑ Film dosimeter ❑ Ionization dosimeter ❑ Diode dosimeter M.Sc. Medical Physics Dr. Khalid I Hussein