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RADIATION PROTECTION IN RADIOTHERAPY Part 10: Optimization of protection in External Beam Radiotherapy PRACTICAL EXERCISE IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources IAEA Training Material on Radiation Protection in Radiotherapy

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Radiotherapy Part 10, Practical 22 Objectives of Part 10 l Be familiar with the ‘design considerations’ as stipulated by appendix II in the BSS l Be able to apply these design considerations in the context of radiotherapy equipment l Be aware of relevant international standards and other documents which provide specification for external beam radiotherapy equipment

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Part 10 : External Beam Radiotherapy Practical 2: Calibration of a megavoltage photon beam using TRS 277 IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources IAEA Training Material on Radiation Protection in Radiotherapy

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Radiotherapy Part 10, Practical 24 Contents l Rationale for calibration l Step by step procedure to be followed for calibration of a photon beam from a medical linear accelerator following IAEA TRS 277 l Interpretation of results

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Radiotherapy Part 10, Practical 25 IAEA TRS 277 l Assumes user has a calibration factor for exposure N X or air- KERMA N K for the ion chamber/ electrometer combination in use l Determines absolute dose to water

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Radiotherapy Part 10, Practical 26 What Minimum Equipment is Needed? l Linear accelerator with front pointer l Water phantom, spirit level l Calibrated ionization chamber and electrometer combination l IAEA TRS 277 protocol

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Radiotherapy Part 10, Practical 27 Background l Calibration Chain n Primary Standard Lab: Calibration Cobalt Beam n Secondary Standard Lab: Transfer of calibration factor to the user’s instrument using Cobalt radiation in air n User: Determination of dose in water in user’s beam

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Radiotherapy Part 10, Practical 28 Assume you have a NE 2505/3 3A ion chamber and Farmer electrometer l Chamber volume 0.6cc l Internal radius 3.15mm l Internal length 24mm l Get KERMA factor: l N k = 9.08 10 -3 Gy/div

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Radiotherapy Part 10, Practical 29 First step: conversion of KERMA (in air) factor from SSDL to Absorbed dose to air chamber factor N D l N D = N K (1-g) k att k m with n g the fraction of brehmsstrahlung generated in water for 60 Co = 0.003 n k att attenuation in wall correction n k m material (i.e. non-air) correction for wall and build-up cap l If Exposure factor N X is known: N K = N X (W/e) (1 - g) -1

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Radiotherapy Part 10, Practical 210 TRS 277 work sheet

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Radiotherapy Part 10, Practical 211 Want to calibrate a 6MV X Ray beam l SAD = 100cm l D max = 1.5cm Elekta

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Radiotherapy Part 10, Practical 212 Require beam quality l To be specified as TPR 20 10 = ratio of dose at isocentre with 20cm attenuation to the same with 10cm attenuation

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Radiotherapy Part 10, Practical 213 Want to calibrate a 6MV X Ray beam l SAD = 100cm l D max = 1.5cm l TPR 20 10 = 0.67 Elekta

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Radiotherapy Part 10, Practical 214 Effective point of measurement in chamber l Up stream of the physical centre

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Radiotherapy Part 10, Practical 215 Perform measurement in water phantom l Fill with water to correct depth l Let temperature equilibrate (>1 hour) l Level phantom l Insert chamber l Ensure linac settings and beam orientation correct PTW small water phantom

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Radiotherapy Part 10, Practical 216 Reference conditions

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Radiotherapy Part 10, Practical 217 Want to calibrate a 6MV X Ray beam l SAD = 100cm l D max = 1.5cm l TPR 20 10 = 0.67 l d = 5cm l FS 10x10cm 2 l effective point of measurement 0.75r upstream Elekta

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Radiotherapy Part 10, Practical 218 Need correction for l Temperature (the higher the less molecules in chamber) l Pressure (the higher the more molecules in chamber) l P Tp = P 0 /P (T + 273.2)/(T 0 + 273.2) n with P and T the measured pressure (in kPa) and temperature (in o C) and P 0 = 101.3kPa and T = 20 o C as reference conditions

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Radiotherapy Part 10, Practical 219 Need also correction for recombination of ions in the chamber l Effect depends on radiation quality, dose rate and high voltage applied to the chamber l Protocol provides a method to calculate it based on two chamber readings with different high voltages applied: assume here: p s = 1.003 (i.e. we lose 0.3% of the generated ions due to recombination)

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Radiotherapy Part 10, Practical 220 Corrections of electrometer reading M u = M raw p TP k h p s with n M u and M raw the corrected and the raw reading n p TP and p s the temperature, pressure and recombination correction n k h a humidity correction - in most circumstances k h can be assumed to be 1 Please note that in electron beams also a polarity correction is required

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Radiotherapy Part 10, Practical 221 Calculation of absorbed dose to water, D w at effective point of measurement P eff D w (P eff ) = M u N D s w,air p u with M u N D the corrected reading and the absorbed dose to air factor as discussed before s w,air the stopping power ratio between water and air to correct dose to air to dose to water p u a perturbation correction factor

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Radiotherapy Part 10, Practical 222 Stopping power ratios l From TRS 277 l Energy dependent

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Radiotherapy Part 10, Practical 223 Perturbation correction l From TRS 277 Fig14 l depends on chamber wall material l for 2505/3A material is graphite l p u = 0.993 for TPR 20 10 =0.67

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Radiotherapy Part 10, Practical 224 Set-up of chamber l Focus Chamber Distance (P eff ) = 100cm l Depth = 5cm water l FS 10x10cm 2 l TPR 20 10 = 0.67 l NE 2505/3A chamber l 100 monitor units chamber 95cm 5cm

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Radiotherapy Part 10, Practical 225 Questions: l Where is P eff compared to the geometric centre of the chamber? l What is the stopping power ratio?

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Radiotherapy Part 10, Practical 226 IAEA Worksheet Filled in for 60-Co !!!

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Please fill in the same sheet for ‘your’ 6MV linac Conditions and readings on the next page...

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Radiotherapy Part 10, Practical 228 Final information l T = 22 o C, p = 99.3kPa l Uncorrected readings: 84.5, 84.2, 84.3 and 84.3

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Radiotherapy Part 10, Practical 229 Questions? get started... Let’s get started...

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Radiotherapy Part 10, Practical 230 Result: 0.858 Gy per 100 monitor units What is your reaction? Shut down the unit?

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Radiotherapy Part 10, Practical 231 Need to find out what the dose normalisation conditions are! l The centre has used as reference conditions a depth of 10cm (as recommended e.g. by several planning systems) l TPR ratio between 10 and 5cm depth: TPR 10 5 = 0.847 l Therefore, the dose at reference point for the centre is 1.013 Gy per 100mu

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