Coverage of the Subcutaneous Layer of the Breast and Chest Wall: Monte Carlo Calculation at Various Energies

Abstract

Histological studies have determined that breast skin thickness is 3 mm. The subcutaneous layer is at risk of local recurrence for patients with breast cancer; because it lies within the buildup region for megavoltage radiation treatment, placement of bolus for patients receiving chest wall (CW) radiotherapy (RT) is routine at many centers. However, bolus is rarely used for breast (BCS) RT. The risk of Grade 3 acute toxicity is 9.6% with bolus and 1.2% without bolus. We hypothesized that the coverage of the subcutaneous layer was similar for the chest wall and breast through different beam energies.For each beam energy (6, 10 and 15 MV), one patient received CWRT and a second patient received BCSRT. Within the tangent fields, a shell was created using a high-resolution structure from 3 to 5 mm below the skin. Monte Carlo calculation (MC) plans were done for each patient using a 1 mm x 1 mm x 1 mm grid. For each energy, the V90 and V95 were calculated and compared between CW and BCS RESULTS: The V90 and V95 for the 3-5 mm shell was better for CWRT than for BCSRT at each energy. For 3 to 5mm, the V90 CW coverage was 14.0 % better than BCS for 6MV, 28.4% better than BCS for 10 MV and 36.3% better than BCS for 15MV. V95 CW coverage was also better for each energy.This comparison of V90 and V95 coverage of the subcutaneous layer for CWRT and BCSRT demonstrates that CWRT delivers a higher dose at clinically relevant energies. This raises the question of why bolus is deemed to be important for routine chest wall RT when it is not used for routine breast RT.