Comparison of Gonadal Dose Estimation Using Treatment Planning Systems and Monte Carlo Methods in Pelvic Radiotherapy

Unintended irradiation of organs outside the target volume remains a concern in pelvic radiotherapy, particularly for radiosensitive structures such as the gonads. Analytical dose calculation algorithms used in treatment planning systems (TPS) may underestimate out-of-field doses. Monte Carlo (MC) methods provide improved dosimetric accuracy. This study compared gonadal dose estimates from a clinical TPS with those obtained using MC simulations.

A retrospective analysis was performed on 30 patients treated for rectal or gynaecological malignancies using 3D conformal radiotherapy (3DCRT) and volumetric modulated arc therapy (VMAT). Plans were generated in Eclipse using the Anisotropic Analytical Algorithm (AAA). Gonads were retrospectively contoured, and maximum (Dmax) and mean (Dmean) doses were extracted from dose-volume histograms. MC simulations were performed using PRIMO, previously validated against measurements. Differences were assessed using the Wilcoxon signed-rank test.

MC simulations agreed with measurements (<1%). MC-derived Dmax values were higher than TPS estimates, with median values of 47.89 Gy vs 46.58 Gy (right gonad) and 48.37 Gy vs 46.81 Gy (left gonad). Maximum differences reached 5.54 Gy and 5.64 Gy. Discrepancies were greater in low-dose regions, highlighting TPS limitations in out-of-field estimation. No significant differences were observed for Dmean or between techniques.

TPS may underestimate maximum gonadal dose, particularly outside the treatment field. MC simulations provide more accurate estimates, supporting improved fertility risk assessment.

MC-based evaluation may improve the accuracy of gonadal dose assessment, particularly in out-of-field regions. This approach can support more robust plan evaluation, encourage consideration of gonads as organs at risk, and facilitate more individualised patient counselling regarding fertility risks. Increased awareness of unintended gonadal exposure may also promote optimisation strategies to minimise dose.