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RADIATION PROTECTION IN RADIOTHERAPY

RADIATION PROTECTION IN RADIOTHERAPY

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RADIATION PROTECTION IN RADIOTHERAPY

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  1. RADIATION PROTECTION IN RADIOTHERAPY IAEA Training Material on 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

  2. Objectives of Part 10 • Be familiar with the ‘design considerations’ as stipulated by appendix II in the BSS • Be able to apply these design considerations in the context of radiotherapy equipment • Be aware of relevant international standards and other documents which provide specification for external beam radiotherapy equipment Part 10, Practical 2

  3. Part 10 : External Beam Radiotherapy IAEA Training Material on Radiation Protection in 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

  4. Contents • Rationale for calibration • Step by step procedure to be followed for calibration of a photon beam from a medical linear accelerator following IAEA TRS 277 • Interpretation of results Part 10, Practical 2

  5. IAEA TRS 277 • Assumes user has a calibration factor for exposure NX or air-KERMA NK for the ion chamber/ electrometer combination in use • Determines absolute dose to water Part 10, Practical 2

  6. What Minimum Equipment is Needed? • Linear accelerator with front pointer • Water phantom, spirit level • Calibrated ionization chamber and electrometer combination • IAEA TRS 277 protocol Part 10, Practical 2

  7. Background • Calibration Chain • Primary Standard Lab: Calibration Cobalt Beam • Secondary Standard Lab: Transfer of calibration factor to the user’s instrument using Cobalt radiation in air • User: Determination of dose in water in user’s beam Part 10, Practical 2

  8. Assume you have a NE 2505/3 3A ion chamber and Farmer electrometer • Chamber volume 0.6cc • Internal radius 3.15mm • Internal length 24mm • Get KERMA factor: • Nk = 9.08 10-3 Gy/div Part 10, Practical 2

  9. ND = NK (1-g) kattkm with g the fraction of brehmsstrahlung generated in water for 60Co = 0.003 katt attenuation in wall correction km material (i.e. non-air) correction for wall and build-up cap If Exposure factor NX is known: NK = NX (W/e) (1 - g)-1 First step: conversion of KERMA (in air) factor from SSDL to Absorbed dose to air chamber factor ND Part 10, Practical 2

  10. TRS 277 work sheet Part 10, Practical 2

  11. Want to calibrate a 6MV X Ray beam • SAD = 100cm • Dmax = 1.5cm Elekta Part 10, Practical 2

  12. Require beam quality • To be specified as TPR2010 = ratio of dose at isocentre with 20cm attenuation to the same with 10cm attenuation Part 10, Practical 2

  13. Want to calibrate a 6MV X Ray beam • SAD = 100cm • Dmax = 1.5cm • TPR2010 = 0.67 Elekta Part 10, Practical 2

  14. Effective point of measurement in chamber • Up stream of the physical centre Part 10, Practical 2

  15. Perform measurement in water phantom • Fill with water to correct depth • Let temperature equilibrate (>1 hour) • Level phantom • Insert chamber • Ensure linac settings and beam orientation correct PTW small water phantom Part 10, Practical 2

  16. Reference conditions Part 10, Practical 2

  17. Want to calibrate a 6MV X Ray beam • SAD = 100cm • Dmax = 1.5cm • TPR2010 = 0.67 • d = 5cm • FS 10x10cm2 • effective point of measurement 0.75r upstream Elekta Part 10, Practical 2

  18. Need correction for • Temperature (the higher the less molecules in chamber) • Pressure (the higher the more molecules in chamber) • PTp = P0/P (T + 273.2)/(T0 + 273.2) • with P and T the measured pressure (in kPa) and temperature (in oC) and P0 = 101.3kPa and T = 20oC as reference conditions Part 10, Practical 2

  19. Need also correction for recombination of ions in the chamber • Effect depends on radiation quality, dose rate and high voltage applied to the chamber • Protocol provides a method to calculate it based on two chamber readings with different high voltages applied: assume here: ps = 1.003 (i.e.we lose 0.3% of the generated ions due to recombination) Part 10, Practical 2

  20. Corrections of electrometer reading Mu = Mraw pTP kh ps with • Mu and Mraw the corrected and the raw reading • pTP and ps the temperature, pressure and recombination correction • kh a humidity correction - in most circumstances kh can be assumed to be 1 Please note that in electron beams also a polarity correction is required Part 10, Practical 2

  21. Calculation of absorbed dose to water, Dw at effective point of measurement Peff Dw (Peff) = Mu ND sw,air pu with Mu ND the corrected reading and the absorbed dose to air factor as discussed before sw,air the stopping power ratio between water and air to correct dose to air to dose to water pu a perturbation correction factor Part 10, Practical 2

  22. Stopping power ratios • From TRS 277 • Energy dependent Part 10, Practical 2

  23. Perturbation correction • From TRS 277 Fig14 • depends on chamber wall material • for 2505/3A material is graphite • pu = 0.993 for TPR2010=0.67 Part 10, Practical 2

  24. Set-up of chamber • Focus Chamber Distance (Peff) = 100cm • Depth = 5cm water • FS 10x10cm2 • TPR2010 = 0.67 • NE 2505/3A chamber • 100 monitor units 95cm chamber 5cm Part 10, Practical 2

  25. Questions: • Where is Peff compared to the geometric centre of the chamber? • What is the stopping power ratio? Part 10, Practical 2

  26. IAEA Worksheet Filled in for 60-Co !!! Part 10, Practical 2

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

  28. Final information • T = 22oC, p = 99.3kPa • Uncorrected readings: 84.5, 84.2, 84.3 and 84.3 Part 10, Practical 2

  29. Questions? Let’s get started... Part 10, Practical 2

  30. Result: 0.858 Gy per 100 monitor units What is your reaction? Shut down the unit? Part 10, Practical 2

  31. Need to find out what the dose normalisation conditions are! • The centre has used as reference conditions a depth of 10cm (as recommended e.g. by several planning systems) • TPR ratio between 10 and 5cm depth: TPR105 = 0.847 • Therefore, the dose at reference point for the centre is 1.013 Gy per 100mu Part 10, Practical 2