Session Item

3D-CRT is a widely used standard technique for breast radiotherapy, but it is technically difficult to cover the target with sufficient dose while sparing the organs at risk, especially for the locally advanced left-sided breast cancer due to the heart. We hypothesized that VMAT could improve target coverage and decrease the dose to the organs at risk. We aimed to compare the dosimetric parameters between 3D-CRT and VMAT for left-sided breast radiotherapy with regional nodal irradiation (RNI) and clarify the clinical factors to select ideal patient population for VMAT.

We made both 3D-CRT and VMAT plans using actual planning CT scans in 24 left-sided breast cancer patients who received adjuvant radiotherapy with RNI. The target volume was the chest wall or breast, and supraclavicular region with or without internal mammary nodes (IMN). Tangential photon beams technique with matching fields was used for 3D-CRT. Two partial arcs with 6MV were used for VMAT. A dose of 50 Gy in 25 fractions was prescribed to the reference point for 3D-CRT, and to cover 75% of the PTV for VMAT. VMAT plans were made for actual clinical treatment. 3D-CRT plans were made for the study purpose. Dosimetric parameters including D95%, D75%, D2% of PTV and CTV, homogeneity index (HI), conformity index (CI), IMN D95%, V20Gy, V5Gy and mean and maximal doses of lung and V40Gy, V35Gy, V25Gy and mean and maximal doses of heart and LAD were compared between 3D-CRT and VMAT. The Wilcoxon signed-rank test was used for dosimetric comparison. Multiple regression analysis was performed to evaluate the factors associated with a higher reduction rate of heart V40Gy and total lung V20Gy dose by VMAT.

16 patients (67%) received IMN irradiation. Mean PTV and CTV coverage (D95%, D75%) was significantly higher for VMAT (PTV: 48 Gy, 50 Gy and CTV: 41 Gy, 49 Gy) compared to 3D-CRT (PTV: 32 Gy, 48 Gy and CTV: 32 Gy, 46 Gy), respectively (all P < 0.01). Mean PTV HI and CI were better for VMAT (1.1 and 1.1 in VMAT, 1.7 and 2.7 in 3D-CRT, P < 0.01). Mean total and ipsilateral lung V20 Gy was lower for VMAT versus 3D-CRT (11.4% and 25.5% versus 14.3% and 31.8%, P < 0.01), whereas mean total and ipsilateral lung V5 Gy were higher for VMAT (30.9% and 60.3% versus 21.2% and 46.2%, all P < 0.01). Mean V40Gy, V35Gy, V25Gy of heart and LAD were significantly lower in VMAT (heart: 0.7%, 1.4%, 2.7% in VMAT, 3.4%, 4.3%, 5.8% in 3D-CRT, all P < 0.01, and LAD: 19.6%, 27.2%, 41.7% in VMAT, 36%, 41.8%, 50.5% in 3D-CRT, P = 0.02 (V25Gy), P < 0.01 (others)). In multivariate regression analysis, IMN irradiation and lower BMI were associated with increased reduction rate of lung V 20 Gy and heart V40Gy by VMAT (P = 0.01, < 0.05).

VMAT for left-sided breast radiotherapy with RNI had better target coverage and lowered lung V20 Gy and heart V40 Gy. Patients with IMN irradiation and lower BMI may be the optimal candidates for VMAT.