Out of Field Photon and Photoneutron Measurements in Modern Medical Linear Accelerators

Abstract

ABSTRACT newlineThe overall objective of this thesis is to investigate the out of field photon and photo neutron doses from radiotherapy with photons. During cancer radiotherapy, patients receive undesired dose from out of field photons and photo neutrons produced in collimators and in the patient. The out of field photon is studied extensively from commercially available treatment planning system and compared with the ionization chamber and thermoluminescence detectors. Clinical relevance was studied from 10 ca. prostate patients for 3-dimensional conformal radiation therapy and intensity modulated radiation therapy modality treatments. Photo neutrons were studied using various detectors like BF3 counter for survey, track etching detector and TLD for various geometrical parameters and activation foils for absorbed neutron dose measurements. The out of field photon doismetry study was carried to compare the measured data with the treatment planning system and found to differ significantly. Our measurements using ionization and the comparison with respect to thermoluminescence detectors were adequate for the entire objective of 10 clinical cases. The treatment planning system underestimates the out of field dose by an average around of 40 % (over a range of 1 to 5 cm from the field edge) which increases as distance from field edge increases. Out of field dose measurements in all critical organs for prostate were studied in detail for 3-dimensional conformal and intensity modulated radiation therapy for all the clinical cases. Individual components of out of field photon production study shows the poor out of field dose calculations in TPS, with respect to that obtained from TLD and ionization measurements. Underestimation of dose increases from 10 % to 80 %, while moving far from radiation field edge. Complex IMRT plans resemble this underestimation of TPS calculations in greater extend and need to be taken into account for any secondary cancer calculations and comparisons. newlineDuring photon radiotherapy, neutrons are produc