Second cancers following radiotherapy: a suggested common dosimetry framework for therapeutic and concomitant exposures.

Radiotherapy has always involved the unavoidable irradiation of organs and tissues outside the target volume. Consequently, delivery of the prescribed target dose whilst minimizing the dose to surrounding critical organs is a key objective of treatment planning. One of the detrimental effects of unwanted irradiation of critical organs is the probability of induction of a second cancer. This commentary is intended to draw attention to the difficulty of estimating second cancer risk following radiotherapy, including the contribution from concomitant exposures arising from treatment simulation and verification. It is suggested that effective dose is an unsatisfactory quantity in these circumstances and that estimates of probability of subsequent cancer would be more appropriate. In view of the large uncertainties in absolute risk estimation, it is suggested that concomitant organ doses and risks are assessed relative to the corresponding doses and risks from the therapeutic exposure. Because concomitant exposures are an essential and integral part of the overall treatment regimen, it is also suggested that separate specification and justification of individual concomitant exposures, for the purposes of compliance with the UK IR(ME)R 2000 Regulations [1] is unnecessary, provided that a dose ‘‘envelope’’ for the entire treatment regimen has been justified prior to treatment.