166P A mono-institutional experience of field-in-field 3D-CRT planning in breast radiotherapy with FAST and FAST-forward protocols

Abstract

The benefits of hypofractionated radiotherapy with FAST and FAST-forward protocols could be compromised by soft tissue reactions and cardiac toxicity caused by dose inhomogeneity and inaccurate heart sparing. This study aims to evaluate the dosimetric results of whole-breast hypofractionated radiotherapy with the field-in-field 3D conformal radiotherapy (3D-CRT) technique. A mono-institutional consecutive cohort of 122 early-stage invasive breast cancer patients was treated using field-in-field 3D-CRT. Thirty-nine patients were treated with 28.5 Gy in 5 fractions once a week (FAST protocol) and eighty-three patients with 26 Gy in 5 consecutive fractions (FAST-forward protocol). Different parameters were evaluated to assess target coverage, dose conformity and homogeneity and hot spots in unspecified tissues. To assess breast dimensions, nipple-to-pectoral muscle distance (NPD) and maximum mediolateral thickness (MLT) along tangential fields were measured. Evaluated OARs were ipsilateral lung and heart with their respective dose constraints as required by the clinical protocols. The median NPD and MLT were 4.65 cm [1.70-9.30] and 21.55 cm [11.26-32.30] respectively, with a median CTV of 380.50 cm3 [60.29-1255.13]. The median V95% was 99.45% [95.19-100], and the median 105% isodose volume was 0.74 cm3 [0.00-48.02] of whom 0.07 cm3 [0.00-5.22] in the first skin centimetre. This led to a median CI of 0.53 [0.33-1.00] and a median HI of 0.07 [0.03-0.19]. OARs dose-volume constraints were always respected. The median ipsilateral lung V8Gy was 7.32% [0.31-25.15]; as for the heart (left breast only), the median V1.5Gy was 6.45% [0.00-25.12] and median V7Gy was 0.34% [0.00-4.50]. Each plan achieved a clinically acceptable target coverage and homogeneity, with reduced superficial hot spots. These results suggested that field-in-field 3D-CRT can provide good quality FAST and FAST-forward breast RT plans, with the advantages of an efficient and cost-effective delivery technique. The clinical follow-up will give further critical feedback on treatment outcomes and toxicities.