2047: Partial Breast Irradiation With Helical Tomotherapy Using RTOG 0413 Planning Constraints

Abstract

To investigate the ability to meet RTOG protocol 0413 dose volume constrains for partial breast treatments using helical tomotherapy. CT studies from 10 breast cancer patients (5 right, 5 left sided) in the supine position were selected. The tumor bed was contoured and RTOG Protocol 0413 guidelines were used to expand the gross target volume, and generate clinical target, planning target, and PTV_EVAL volumes. PTV_EVAL is defined as the planning target volume excluding the part outside the ipsilateral breast, the first 5 mm of tissue under the skin, and any expansion beyond the posterior extent of the breast tissue (chest wall, pectoralis and lung). A dose of 38.5 Gy to the PTV_EVAL was planned in 10 fractions. A 5 cm jaw length and a pitch of 0.215 were used for planning. The doses to the ipsilateral breast, contralateral breast, heart, and both lungs were restricted during plan optimization to meet protocol guidelines. The treatment time and PTV_EVAL doses are reported. Table 1 lists the average (range) of volumes that received the allowable dose according to protocol. For various structures, the RTOG volume limits are based on the percentage of the prescription dose (PD) that is allowed to a percentage volume. The second row lists the allowable volume limit. The third row lists the average and range of volumes that received the dose limit. In all cases, the maximum dose to the contralateral lung was <3% of the prescription dose. The average (range) treatment time was 10 (7–14) minutes. In 9 of the 10 cases, all PTV_EVAL and sensitive structure volume limits were acceptable (i.e. within 5% of specified value). In one right-sided case, the contralateral lung volume to 5% of the PD was 40% and the respective heart volume was 60%. This PTV_EVAL was located medially and dose limits to the contra lateral breast likely compromised the delivery. A solution with helical tomotherapy could be theoretically found with this particular situation, however, treatment times exceeded practical limits.Table 1Contralat. Lung, 5% of PDIpsilat. Lung, 30% of PDHeart, 5% of PDNormal Breast, 50% PDPTV_Eval, Max. DosePTV_Eval, 95% of PDRTOG vol. limit15%15%R: 5%, L: 40%60%120% of PD—Median13%10%R: 5%, L: 37%40%108%99%Average14%10%R: 15%, L: 33%41%108%99%Range0-40%8-17%R:0-60%, L:17-38%28-65%103-120%95-100% Open table in a new tab In the large majority of patients, partial breast treatment helical tomotherapy plans were acceptable by RTOG Protocol 0413 guidelines. Although not the focus of the present study, megavoltage CT scanning prior to each fraction would also add the benefit of daily visualization and localization of the seroma, i.e. the tumor bed (Langen et al, IJROBP, 63S, p. S179, 2005), thus potentially allowing a decrease in treatment margins. Partial breast accelerated irradiation is feasible with helical tomotherapy delivery.