Pre-installation empirical testing of room shielding for high dose rate remote afterloaders

Abstract

Purpose : Many facilities are acquiring high dose rate remote afterloading units. It is economical that these units be placed in existing shielded teletherapy rooms. Scatter-radiation barriers marginally protect uncontrolled areas from a high dose rate source especially in a room that houses a non-dynamic Cobalt-60 unit. In addition the exact thickness and material composition of the barriers are unknown and therefore, a calculation technique may give misleading results. Also, it would be impossible to evaluate an entire wall barrier by taking isolated core samples in order to assist in the calculations. A quick and inexpensive measurement of dose equivalent using a rented high activity 192Ir source evaluates the barriers and locates shielding deficiencies. Methods and Materials : We performed transmission calculations for primary and scattered radiation based on National Council on Radiation Protection and Measurements Reports 49 and 51, respectively. We then rented a high activity 21.7 Ci (8.03 × 10 11Bq) Ir-192 source to assess our existing teletherapy room shielding for adequacy and voids. This source was placed at the proposed location for clinical high dose rate treatment and measurements were performed. Results : No deficiencies were found in controlled areas surrounding the room, but large differences were found between the calculated and measured values. Our survey located a region in the uncontrolled area above the room requiring augmented shielding which was not predicted by the calculations. A canopy shield was designed to potentially augment the shielding in the ceiling direction. Conclusion : Pre-installation testing by measurement is an invaluable method for locating shielding deficiencies and avoiding unnecessary enhancement of shielding particularly when there is lack of information of the inherent shielding.