Part VIII:Medical Exposures in Radiotherapy

Part VIII:Medical Exposures in Radiotherapy
IAEA Post Graduate Educational Course on Radiation Protection and Safe Use of Radiation Sources Part VIII:Medical Exposures in Radiotherapy Module 3 Optimization of Protection for Medical Exposure Lecture 5 : Determination of Dose to a Patient-I

Scope- at the end of this lecture the student should
have understood the parameters such as SSD, SAD, Isocentre, field size, penumbra etc be able to appreciate the concept of equivalent square and its use have understood the factors required for calculation of absorbed dose Part VIII.3.5 Determination of Dose to a Patient-I

This lecture will cover…
Definitions and explanations of parameters such SSD, Field Size, SAD, Isocentre, etc Concept of equivalent square and determination of equivalent square for irregular fields Definition and explanations of factors such as PDD, TAR, TMR used for calculating the absorbed dose Part VIII.3.5 Determination of Dose to a Patient-I

Some common terms in External Beam Therapy (EBT)…
Source to Skin Distance (SSD) Source to Axis Distance (SAD) Isocentre Field Size Equivalent square Penumbra The lecturer may introduce the above terms which are required for understanding the dose calculation in EBT. Part VIII.3.5 Determination of Dose to a Patient-I

Part VIII.3.5 Determination of Dose to a Patient-I
What is SSD? SSD is the acronym of Source to Skin distance. It is the distance between the source and the patient skin Machines have Standard SSD at which output and PDD are measured 50 cm for a Cs Unit 80 – 100 cm for Co 60 unit 100 cm for Linear accelerator The lecturer may use this slide to define SSD and impress upon the importance of the factor in the external beam treatment. He may also mention that the SSDs are different for different treatment units. Which range from 50cm to 100cm Part VIII.3.5 Determination of Dose to a Patient-I

Source to Skin Distance
SSD=80cm The above picture may help the student understand the SSD better. This picture gives the SSD for a cobalt unit which is 80 cm. Part VIII.3.5 Determination of Dose to a Patient-I

Why is SSD important? The dose calibration of the External beam unit is at the SSD (or Isocentre – explained later) Any change in this will vary the dose by ‘inverse square’ factor This slide may be used to impress upon the importance of SSD. It may be mentioned that the Radiation beam output is measured and stated at SSD in most cases (or Isocentre). Any change in the SSD will affect the dose by inverse square factor. The term SSD is much used in cases of external beam units which are capable of only vertical motion and also in treatment techniques where SSD is used as reference for setting-up the patient. Part VIII.3.5 Determination of Dose to a Patient-I

What is meant SAD? It is the distance from the source to Isocentre (axis of rotation) It is 80cm – 100 cm for cobalt units 100cm for Linear accelerator IT may be mentioned that the modern external beam units are capable of rotation and have a rotating head. These units rotate about an Axis and the distance between the Source and the Axis of rotation is called SAD. It is usually 80 cm or 100cm in cobalt units and 100cm in case of linear accelerator Part VIII.3.5 Determination of Dose to a Patient-I

What is an isocentre The point in space where the axes of rotation of the Gantry, couch, collimator and the beam axis meet The important term that the student should know in a rotational type of external beam unit is the Isocentre. The lecturer may use this slide to explain the isocentre. In the rotational type of units as discussed earlier the Gantry is capable of rotating about an axis and also the couch and the collimator. The point in space where these there axes meet with the beam central axis is the isocentre. The lecturer may use this figure to explain the isocetre. Isocentre of a treatment unit Part VIII.3.5 Determination of Dose to a Patient-I

What is the significance of Isocentre?
Isocentre is the centre of Couch rotation, Gantry rotation and radiation beam central axis This ensures accurate beam direction if the tumour centre is positioned at the isocentre of the unit At this point the lecturer may mention the significance of isocentre which is used for directing the beam to the tumour. The idea is to position the tumour centre at the isocentre of the machine and this will ensure that what ever the angle of the gantry or the couch or the collimator the radiation beam will be targeted at the tumour. Part VIII.3.5 Determination of Dose to a Patient-I

ISOCENT R IC TREATMENT Click to demonstrate isocentric treatment
The above animation could be used to demonstrate the isocentre rotation of the gantry and that the beam passes through the same point over the entire rotation. Part VIII.3.5 Determination of Dose to a Patient-I

Field Size Field size is the width and length of the radiation beam at SSD or SAD Field size at any depth is usually defined by the 50% width of the profile at that depth The lecturer may use this slide to explain the Field size of a radiation beam. It is usually defined at the SSD or SAD of the treatment unit. It is also the width of 50% width of the radiation beam profile. Part VIII.3.5 Determination of Dose to a Patient-I

Penumbra Penumbra is the unsharp edge of the radiation beam created mainly by the finite source size Penumbra could be classified as Geometric Penumbra which depends on Source Size Source to Diaphragm distance Source to skin distance Radiological Penumbra which is Geometric penumbra + Scatter The lecturer may explain that the radiation beam has an unsharp endue due to the finite source size and scattered radiation and this is Penumbra. Penumbra in a radiation beam could be classified as Geometric penumbra and Radiological penumbra. The factors on which the size of the penumbra depends on could be explained with the help of the above slide. Part VIII.3.5 Determination of Dose to a Patient-I

The Geometric Penumbra
SDD Source to Diaphragm distance SSD Source to skin distance S x (SSD+d-SDD) P = SDD S x f+d-fd P = The lecturer may use this slide to explain how the geometric penumbra of a radiation beam. The size of the geometric penumbra could be calculated using the similar triangle principle. From the figure it could be explained that the size of the geometrical penumbra depends on the 1. Source size 2. Source to Skin (surface) distance 3. Source to Diaphragm It should also be noted that the penumbra size increased with depth which means the penumbra will be larger at depth. It is possible to reduce the size of penumbra by reducing the source size. fd CE - penumbra at surface S x f-fd CE = fd SAD Field size at surface Part VIII.3.5 Determination of Dose to a Patient-I

Physical Penumbra / Radiological penumbra
Radiation beam penumbra Lateral scatter increases the radiation beam penumbra Lower energy, more side scatter and hence larger penumbra Density of the scattering medium affects the penumbra Issues : larger the penumbra, greater the dose to normal tissues In a radiation beam the actual penumbra size is larger than the size of the geometric penumbra which is called physical or Radiological penumbra. This is mainly due to the scatter from the radiation beam which increases the size of the penumbra. The lecturer may also mention about the consequence of larger penumbra size in the treatment of radiation therapy at this point. Part VIII.3.5 Determination of Dose to a Patient-I

Scattered radiation The scatter radiation that contributes to the radiation dose could be termed extra-focal radiation. The above figure could be used to illustrate the effect of scatter on penumbra. Scatter Part VIII.3.5 Determination of Dose to a Patient-I

Physical / Radiological penumbra
In Air In Phantom It must also be mentioned that the scattered radiation increases in the tissue. The figure on the right is the beam profiles obtained in air and hence less scatter. The penumbra is nearly the same as the Geometrical penumbra but for small scatter in air. The figure on the left is the radiation beam profile in phantom which should a larger penumbra due to scatter. Part VIII.3.5 Determination of Dose to a Patient-I

Radiological Penumbra
The actual size of the radiological penumbra could be measured from the cross beam profile. Usually the radiological penumbra is the distance between the 80% and the 20% point on a beam profile. The above figure could be used to illustrate the measurement of radiological penumbra. It may be mentioned at this point that measurement of radiological penumbra is one of important testing during the acceptance and commissioning of a external beam therapy unit. Radiological penumbra is defined as the 80%-20% width of the dose profile Part VIII.3.5 Determination of Dose to a Patient-I

Factors for determination of absorbed dose
Percentage depth dose (PDD) Tissue Air Ratio (TAR) Tissue Maximum Ratio (TMR) Tissue Phantom Ratio (TPR) Scatter Air Ratio (SAR) Scatter Maximum Ratio (SMR) The lecturer may mention that for calculation absorbed dose in the patient during an external beam treatment several factors have been defined. These factors will be described in the following slides. Part VIII.3.5 Determination of Dose to a Patient-I

Percentage Depth Dose (PDD)
Field Size (s)=AB Percentage depth dose commonly know as PDD could be defined as the percentage of dose to a small mass of tissue at depth ‘d’ to the dose to the same mass of tissue at depth dmax . The lecturer may use the above figure to explain how the PDD. It may also be mentioned that PDD is a function of field size, depth, energy and SSD. The percentage depth dose increases with field size due to increased scatter Increases with energy as penetration is more Decreases with depth due to attenuation and inverse square factor. Increases with SSD due to inverse square factor. PDD (S,Q,f,d) = D Dmax x 100 Part VIII.3.5 Determination of Dose to a Patient-I

Percentage Depth Dose 60Co
This figure may be used to explain how the PDD decreases with depth. It may be pointed out that it is nearly exponential. This is the PDD vs depth graph for Cobalt 60 beam. It should also be noted here that the dose increases initially, reaches a maximum (100%) and then decreases. This initial increase is termed build-up. The depth at which it reaches a maximum is the maximum range of the photo electron which is 0.5cm for cobalt beam. Part VIII.3.5 Determination of Dose to a Patient-I

Depth Dose Points to Remember
In case of depth dose measurements the field size is defined at surface of phantom Increases with SSD, Beam quality (energy), Field size. Decreases with depth Value is normalized to 100 % at depth of dmax for all field sizes It may be mentioned that the filed size for which the depth dose is measured is mentioned at the surface of the phantom. With increase in SSD due to decrease in inverse square law factor for larger distance the PDD increases. The PDD at Dmax will be 100% Part VIII.3.5 Determination of Dose to a Patient-I

Depth dose - point to ponder
Depth dose increases with SSD - Why? Ans: Larger the SSD, smaller is the decrease in dose due to inverse square law At this point the lecturer may pose this question to the students and also explain the reason for increase in PDD with increase in SSD. Part VIII.3.5 Determination of Dose to a Patient-I

Convert PDD from One SSD to another...
PSF [S/F] PDD(s,f2,Q,d) = PDD(s,f1,q,d) Fs2 Where PDD(s,f2,Q,d) = PDD(s,f1,q,d) PSF [S] f1 +d f2 f1 +d f2+ dm F = Fs = f f2 +d f1 +dm f2+ d It may be explained that most often in Radiation therapy it becomes necessary to treat with different SSD than the one for which the percentage depth dose has been measured. In such cases it is possible to correct the PDD for the SSD to be used. This basically involves correction for increase in Field size and consequently increase in scattered dose with increase in SSD and increase in inverse square factor correction for the depth. (f1 +d)2 (f2+ dm)2 PSF [S/F] (f1 +dm)2 (f2+ d)2 PSF [S] Part VIII.3.5 Determination of Dose to a Patient-I

Tissue Air Ratio Field Size(S)=AB TAR(s,d,Q)= Dd /Dair In case of rotational unit and SAD type of treatments where the Source to Skin distance changes for each field, TAR which is independent of SSD was formulated. It is the ratio of dose to a small mass of tissue at depth ‘d’ in phantom to the dose to the same mass of tissue in air with both measurements obtained without the position of the probe being moved. This TAR increases with Field size due to scatter, increases with depth below dmax depth and then decreases with increase in depth, and increases with energy of the radiation. Part VIII.3.5 Determination of Dose to a Patient-I

Variation of TAR with depth
The above figure could be used to illustrate the variation of TAR with depth in phantom. Part VIII.3.5 Determination of Dose to a Patient-I

Variation of TAR with field size
This figure could be used to illustrate the increase in TAR Values with increase in field size. This is due to increased scatter with increase in field size. Part VIII.3.5 Determination of Dose to a Patient-I

TARs - Points to Remember
Field Size is defined at SAD TAR is the ratio of phantom dose to dose “in air” at Dmax Independent of SSD Depends on beam quality and depth Value is 1.00 at depth of d max for FS --> 0 Position of the detector not altered between measurements Increase the depth by adding more water above the point It must be noted that in case of use of TAR the field size is defined at SAD which is the isocentre of the treatment unit. It should also be mentioned that the main advantage of using TAR is that it is independent of SSD. Part VIII.3.5 Determination of Dose to a Patient-I

TAR - Point to ponder TAR does not depend on SSD - Why? Ans: The distance between the Source to the detector is not altered between measurements and hence no ‘inverse square law’ influence At this point the lecturer may ask this question and allow a discussion before coming out with answer, which is animated. Part VIII.3.5 Determination of Dose to a Patient-I

Scatter air ratio SAR (S,Q,d)= TAR(S,Q,d) - TAR(S,Q,0)
Field Size(S)=AB The lecturer may mention that as discussed earlier the tissue air ratio has decrease in dose due to attenuation with depth and increase in dose due to the scatter. The tissue air ration has two components, one the primary beam and the scattered radiation. A term Scatter Air ratio (SAR) has been defined to determine the dose in complicated shapes. This is the ratio of scattered dose to a small mass of tissue at depth ‘d’ to the dose to same mass of tissue at Dmax in air. The scatter air ratio is obtained by subtracting the zero area tissue air ration (the primary only) from the tissue air ratio of the entire filed. SAR (S,Q,d)= TAR(S,Q,d) - TAR(S,Q,0) Part VIII.3.5 Determination of Dose to a Patient-I

SAR’s Points to Remember
Increases with beam quality and Field size, decreases with depth Obtained by extrapolation (TAR0) and subtraction SAR value is the phantom scatter dose per unit dose “in air” Sector SAR used to predict dose for complex field shapes (e.g. IRREG) inside open area, behind shields, outside of field It may be mentioned that the scatter air ration changes with Beam quality, field size and depth the same was as TAR. One needs to determine only the primary beam in order to obtain the SAR. The SAR is very useful to predict dose for complex irregular field shapes which will be discussed. Part VIII.3.5 Determination of Dose to a Patient-I

Determination of dose for Irregular field
This slide may be used to explain the contribution of scatter dose from various angle to the point of measurement. Scatter contribution for irregular field determined by summing SAR for various angles - sector integral

Peak scatter factor Field Size(S)=AB Earlier this term peak scatter factor was called Back scatter factor as it was used to refer to the radiation scattered back to the point of measurement. However with high energy beams where there could be forward scatter also to the Depth of maximum ionisation point this is referred to as Peak Scatter factor. This is the ratio of dose to a small mass of tissue at Dmax in phantom to the same mass of tissue at Dmax in air. PSF (S,Q) = Dmax/ Dair Part VIII.3.5 Determination of Dose to a Patient-I

PSF or BSF Points to Remember
BSF(Back scatter factor) is an older term than PSF, used for lower energies PSF or BSF is actually a TAR value for a depth of dmax Useful for conversion of dose in air to dose in tissue and vise versa The lecturer may remind here about the equation for TAR and the TAR at dmax is the Peak Scatter factors. It should be also noted that the Peak scatter factor will always be greater than one. Part VIII.3.5 Determination of Dose to a Patient-I

Tissue Phantom Ratio Field Size(S)=AB The lecturer may explain that for high energy radiation, measurements in air is not practical as it would be necessary to supply the dosimeter with a large build-up that it would not be fully irradiated by small area beams. Hence as an alternative to TAR, Tissue Phantom Ratio(TPR) is used. Tissue phantom ratio is defined as the ratio of dose to a small mass of tissue at depth ‘d’ to the same mass of tissue at the reference depth ‘dref’ TPR(S,Q,d) = Dd / Dref Part VIII.3.5 Determination of Dose to a Patient-I

Tissue Maximum Ratio Field Size(S)=AB Tissue Maximum Ratio is the same as the Tissue phantom ratio except that the reference depth is the depth of maximum ionisation. If the radiation output is specified at Dmax then TMR is used for calculation, if the radiation output is specified at some other reference depth say 10cm then TPR measured with reference depth of 10cm is used. TMR(S,Q,d) = Dd / Dmax Part VIII.3.5 Determination of Dose to a Patient-I

TMR Vs Depth Both TPR and TMR increases with depth initially till the buildup point is reached and then decreases with depth. Part VIII.3.5 Determination of Dose to a Patient-I

Variation of TMR with Field Size
This slide may be used to explain that TMR and TPR increase with the increase in field size. Part VIII.3.5 Determination of Dose to a Patient-I

TPRs and TMRs Points to Remember
Field size defined at SAD Increases with beam quality and field size Cousins of TAR, but with calibration “in phantom” TPR (or TMR) value is 1.00 at dref for all field sizes TMR is a special case of TPR where reference depth is dmax depth i.e d ref = d max It should be noted that the Field size is mentioned at SAD or Isocentre forTPR and TMR. TMR does not change with SSD as the chamber position is not altered during measurements. The TPR/TMR value at reference depth has to be 1. Part VIII.3.5 Determination of Dose to a Patient-I

Relative dose factor (Output factor)
The lecturer may mention here that the output of a radiation beam is measured for a reference field size (viz 10 x 10 cm2) at the reference depht. However with the change in the field size is changed the output is will reduce due to less scattered radiation from the tissue and if the field size in increased the output will increase. To account for this change in output, Relative Dose factors are measured for various field sizes. This is the ratio of dose at reference depth for the field size in question to the dose at same depth for reference field size. Scatter from reference field Additional Scatter from larger field RDF(S) = D(dref,S) / D(dref, S ref) Part VIII.3.5 Determination of Dose to a Patient-I

RDF Vs Field Size The output factor increases with field size due to increased scattered radiation. Part VIII.3.5 Determination of Dose to a Patient-I

Relative Dose factor - points to remember
The beam output depends on field size The output increases with increase in field size due to increased collimator scatter; characterised by collimator scatter factor Sc Increase in scatter within the phantom; characterised by phantom scatter factor Sp Part VIII.3.5 Determination of Dose to a Patient-I

Relative Dose Factor Points to Remember
The field size is varied, relative to a reference field size (usually a 10 x 10 cm2) RDF measured “in phantom” only Increases with filed size due to increased scatter Used to convert dose from reference field size to a specific field size Part VIII.3.5 Determination of Dose to a Patient-I

Square Field to Rectangular fields..
10cm D Measured data for this But may treat with this 20cm 5cm Square field Rectangular field The lecturer may point out that the measured parameters such as the PDD, TAR, TPR and SAR are measured for square field sizes. However the treatment field could be rectangular or circular or irregular in shape. In order to use the data measured for square field sizes for rectangular fields the equivalent square concept was introduced. Part VIII.3.5 Determination of Dose to a Patient-I

Equivalent square field
The lecturer may point out using the above figure that even when the geometrical area of the square and the rectangle shown above are same (both 100cm2) the equivalent square field for the rectangular field of 20cm x 5cm will be 8cm2 as for as the radiation output at point D is concerned. This is because of reduced scatter reached from the elongated field. As a rule of thumb the equivalent square field for any rectangular field could be obtained using the formula 2 ab/(a+b) where a and b are the width and length of the fields. Part VIII.3.5 Determination of Dose to a Patient-I

Equivalent square Table
A table is also provided for obtaining the exact equivalent square field for any rectangular field. The lecturer may explain the use of the above table in determining the equivalent square. Part VIII.3.5 Determination of Dose to a Patient-I

Wedge Factor Dose at reference point with wedge
Wedge factor Wf= = Dw/Dref Dose at reference point without wedge Part VIII.3.5 Determination of Dose to a Patient-I

Shielding tray factor Dose at reference point with tray
Shielding Tray factor Ws= = Ds/Dref Dose at reference point without tray Part VIII.3.5 Determination of Dose to a Patient-I

Summary – you learnt about …
The parameters such as SSD, SAD, Field Size, Penumbra, Equivalent Square and their importance Method of obtaining equivalent square for irregular fields Factors that are required to determine the absorbed dose in patient Part VIII.3.5 Determination of Dose to a Patient-I

Try these questions Define Isocentre and explain the advantages of isocentric treatment Explain the factors that affect the radiation penumbra What is scatter air ratio? And explain how this factor is useful Calculate the correction factor to be applied to PDD for an increase in SSD from 80cm to 110 cm Part VIII.3.5 Determination of Dose to a Patient-I

References The Physics of Radiology H.E Johns & J R Cunningham The Physics of Radio Therapy Faiz M Khan The Modern Technology of Radiation Oncology, edited by J. Van Dyk Part VIII.3.5 Determination of Dose to a Patient-I

Part VIII:Medical Exposures in Radiotherapy

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