Radiobiological evaluation of hypofractionated vs. conventional VMAT for synchronous bilateral breast cancer: a focus on regional nodal irradiation.

PURPOSE: To compare the radiobiological characteristics of hypofractionated radiotherapy (HFRT) and conventional fractionated radiotherapy (CFRT) delivered with volumetric modulated arc therapy (VMAT) for synchronous bilateral breast cancer (SBBC) requiring regional nodal irradiation (RNI).

METHODS

Eighteen patients with SBBC receiving bilateral breast/chest wall irradiation and comprehensive nodal coverage were retrospectively included. For each patient, VMAT plans were generated for CFRT (50 Gy in 25 fractions) and HFRT (40.05 Gy in 15 fractions). Dosimetric parameters of the planning target volume (PTV) were evaluated. Radiobiological assessment included equivalent uniform dose (EUD), tumor control probability (TCP), and normal tissue complication probability (NTCP). TCP was calculated using LQ-Poisson and Niemierko models, while NTCP for the lungs, heart, spinal cord, and esophagus was estimated using Lyman Kutcher Burman and Niemierko models.

RESULTS

Both regimens achieved excellent target coverage, with mean PTV V95% values exceeding 99%. HFRT significantly reduced high-dose regions within the PTV, as reflected by lower V107% values (median: 1.35% vs. 2.60%; IQR: 5.75% vs. 7.85%; P < 0.001). PTV EUD was significantly lower with HFRT than with CFRT (median: 45.69 vs. 51.34 Gy; P < 0.001). Although TCP values were lower with HFRT in both models, tumor control remained high for both schedules. HFRT also significantly reduced EUD and NTCP values for the lungs, heart, and esophagus, whereas no significant differences were observed for the spinal cord.

CONCLUSION

For SBBC patients requiring RNI, HFRT-VMAT achieved target coverage comparable to CFRT while reducing excessive high-dose regions and predicted toxicity to major thoracic organs. Despite slightly lower TCP values, overall tumor control remained favorable, supporting HFRT-VMAT as a feasible treatment option for this patient population.