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

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

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

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

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

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

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

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 = Gy/div

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 = 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

Radiotherapy Part 10, Practical 210 TRS 277 work sheet

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

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

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

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

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

Radiotherapy Part 10, Practical 216 Reference conditions

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

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 )/(T ) 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

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 = (i.e. we lose 0.3% of the generated ions due to recombination)

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

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

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

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 = for TPR =0.67

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 = 0.67 l NE 2505/3A chamber l 100 monitor units chamber 95cm 5cm

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?

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

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

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

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

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

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 = l Therefore, the dose at reference point for the centre is Gy per 100mu