Estimation of risk of radiation-induced carcinogenesis in adolescents with nasopharyngeal cancer treated using sliding window IMRT.

Purpose: To estimate the risk of radiation-induced carcinogenesis based on whole-body dose measurement on adolescent patients undergoing intensity-modulated radiotherapy (IMRT).

Methods And Materials: Ten adolescent patients with nasopharyngeal cancer were planed and treated to a dose of 70.2 Gy using sliding window IMRT. Peripheral dose (PD) was measured using thermoluminescent dosimeters kept at anterior, lateral and posterior positions of each axial plane at the level of xiphoid process, umbilicus and gonads of every patient. The associated risk of radiation-induced carcinogenesis was estimated based on the measured whole-body dose and using age- and sex-specific ICRP-60 nominal probability coefficient of 7.5% (boys) and 9.5% (girls) per Sv.

Results: In all patients, measured PD per monitor unit (MU) decreases almost exponentially with out-of-field distance and varies with gantry angle. Highest whole-body dose equivalent ranged from 0.5318 to 0.9867 Sv (mean=0.8141 Sv, SD=0.138) which was measured posteriorly at the level of xiphoid process. Whole-body dose was represented by the average dose at xiphoid process and all measurement positions ranged from 0.3661 to 0.8766 Sv (mean=0.658 Sv, SD=0.16) and 0.2267 to 0.5277 Sv (mean=0.3859 Sv, SD=0.09), respectively. The associated mean risk of radiation-induced carcinogenesis estimated based on different representation of mean whole-body dose was 6.57%, 5.3% and 3.11%, respectively. Higher mean risk of 7.32% was estimated among girls as compared to 6.25% for boys.

Conclusions: Knowledge of risk of secondary malignancy is particularly important in adolescents and should be considered when choosing the optimal treatment technique and delivery system.