Out-of-field Organs Dose Measurement and Estimation of Associated Risk of Second Cancer after Radiotherapy Management of Paediatric Patients: A Case Study of a Nigeria Hospital

This study addresses the gap in data on out-of-field organ doses and the associated second cancer risk in Nigerian paediatric radiotherapy patients by measuring these doses during external beam Cobalt-60 treatments for the spine, thigh, head and neck, and abdomen at the Radiotherapy Department of University College Hospital (UCH), Ibadan. Calibrated thermoluminescent dosimeters chips were used to measure doses in organs outside the primary field. Gonads, kidneys and thyroid were monitored for spine fields; umbilicus and gonads for thigh fields; and gonads plus thyroid for head-and-neck and abdomen fields. Excess Relative Risk (ERR) and Excess Absolute Risk (EAR) for radiation-induced malignancy were estimated with BEIR VII models. During spine, thigh and abdomen treatments, the gonads received the highest mean out-of-field doses—87.4 cGy, 11.7 cGy and 2.95 cGy, respectively. In head-and-neck treatments, the thyroid absorbed the largest mean dose (42.2 cGy). Reflecting these dose distributions, the greatest mean excess relative risk (ERR) was found in the gonads for spine (0.674), thigh (0.056) and abdomen (0.031) exposures, whereas the thyroid showed the highest ERR during head-and-neck irradiation (3.254). Excess absolute risk (EAR) followed a similar pattern, with peak values in the gonads—8.772, 1.567 and 0.447 (per 10 000 person-years) for spine, thigh and abdomen treatments, respectively. Although stray doses from Cobalt-60 therapy are modest, the projected secondary-cancer risks to highly radiosensitive tissues—particularly the gonads and thyroid—remain clinically significant. Survivorship protocols for Nigerian children treated to these sites should include lifelong surveillance of the affected organs. Further dose reduction is feasible through rigorous field-size optimisation, appropriate shielding and continual refinement of treatment techniques. Keywords: Out-of-field organ; Dose, risk of second cancer, Paediatric; Radiotherapy; ERR and EAR