Left-Sided Breast Cancer Locoregional Radiation Therapy With Deep Inspiration Breath Hold (DIBH): Does Volumetric Modulated Arc Therapy (VMAT) Reduce the Cardiac Dose Further When Compared to Tangential Intensity Modulated Radiation Therapy (T-IMRT)?

Abstract

Left-sided breast cancer radiation therapy has been associated with an increased risk of major coronary events. There is evidence demonstrating that the increase is proportional to mean heart dose. DIBH has been shown to decrease the heart dose compared with free breathing for left breast radiation therapy. T-IMRT with DIBH is routinely used in our center for adjuvant loco-regional left breast radiation therapy. This study aims to investigate the potential benefit of utilizing VMAT to reduce heart dose. We compared VMAT to T-IMRT in the treatment of left breast and regional nodes using DIBH. Radiation therapy for 10 patients previously treated with DIBH technique for left-sided breast cancer was replanned with T-IMRT and VMAT using DIBH. A dose of 50 Gy/25# was prescribed to cover the left breast and supraclavicular, axillary and internal mammary nodes. T-IMRT plans were generated using inverse-planned tangentially orientated fields to cover the breast, axillary and internal mammary nodes, and 2 to 4 photon fields to cover the supraclavicular nodes. VMAT treatment plans were generated using 2-3 partial arcs, with an average arc angle of 190 degrees. Plans were optimized to cover at least 95% of PTV by 95% of prescribed dose, while maximally sparing organs at risk (OAR). Dosimetric comparisons were made between the two techniques using the paired t-test. The mean D95 was 47.6 and 48.0 Gy, the mean maximum point dose was 55.2 and 55.8 Gy, and the mean number of monitor units was 533 and 565 for T-IMRT and VMAT respectively. Compared to T-IMRT, VMAT resulted in a significant reduction in volumes receiving moderate to high doses with heart V25 (6.7 vs 2.8 %, p = 0.004), heart V45 (1.5 vs 0.04 %, p = 0.039), lung V20 (18.1 vs 15.2 %, p = 0.003) and lung V30 (16.2 vs 8.5%, p < 0.001). VMAT led to a significant reduction in left anterior descending coronary artery maximum (46.7 vs 35.6 Gy, p = 0.001) and mean dose (27.3 vs 20.1 Gy, p = 0.007). However, VMAT resulted in a trend towards a higher heart mean dose (5.1 vs 6.9 Gy, p = 0.06) and significantly higher heart V5 (16.3 vs 44.7 %, p = 0.002), lung V5 (27.0 vs 60.5 %, p < 0.001), lung mean dose (9.2 vs 10.7 Gy, p = 0.002) and contralateral breast mean dose (1.4 vs 5.1 Gy, p < 0.001). T-IMRT-DIBH should remain one of the standard treatment techniques for loco-regional radiation therapy of left breast cancer. VMAT-DIBH should be adopted into routine practice with caution, given the significant low dose bath to the heart, lung, and contralateral breast resulting in a significantly higher mean lung and contralateral breast dose, and a trend towards higher mean heart dose.