Critical Appraisal of the Risk of Secondary Cancer Induction From Breast Radiation Therapy With Volumetric Modulated Arc Therapy Relative to 3D Conformal Therapy.

PURPOSE: To evaluate the excess absolute risk (EAR) comparing volumetric modulated arc therapy (VMAT) and 3-dimensional (3D) conformal radiation therapy (CRT) in breast cancer radiation therapy treatment.

METHODS AND MATERIALS: Two VMAT arrangements (VMAT_tang and VMAT_full, i.e. partial arcs with and without a sector of 0 Monitor Unit, respectively) and a 3D CRT (field-in-field [FinF]) plan were calculated with an accurate dose calculation algorithm, Acuros, in 20 patients presenting with early-stage breast cancer. The dose prescription was 40.05 Gy in 15 fractions. The planning aim was to maximize the dose reduction in the lungs, contralateral breast, heart, and coronary artery. EAR was estimated using different models: linear, linear-exponential, plateau, and full model, which better uses a carcinogenesis model and epidemiologic data for carcinoma induction and which accounts for cell repopulation or repair during the radiation therapy dose fractionation. EAR was computed for contralateral structures-breast and lung-as well as the ipsilateral lung. Normal tissue complication probability (NTCP) was computed to estimate the ipsilateral lung, heart, and skin toxicity, to balance with respect to second cancer induction.

RESULTS: The planning objectives were fulfilled with all the planning techniques. EAR for contralateral breast carcinoma induction, estimated with the most accurate model, was 1.7, 2.4, and 8.5 (per 10,000 patients per year) with FinF, VMAT_tang, and VMAT_full, respectively. For the contralateral lung, these figures were 1.5, 1.6, and 7.3 (per 10,000 patients per year), respectively. NTCP for all the analyzed endpoints was significantly higher with FinF relative to both VMAT settings, with VMAT_full presenting the lowest toxicity risk.

CONCLUSIONS: VMAT, in particular with the VMAT_tang setting, could have the same risk of second cancer induction as 3D CRT delivered with the FinF setting for the contralateral organs while reducing acute and late NTCP for the ipsilateral organs. VMAT might be considered a safe technique for breast cancer treatment for those aspects.