Abstract P4-11-11: Preoperative Radiotherapy Increases Eligibility for Partial Breast Irradiation by Significantly Reducing Normal Tissue Exposure

Introduction: External-beam accelerated partial breast irradiation (EB-APBI) is the most common technique used on NSABP B-39 primarily due to the non-invasive nature of the treatment. Many patients thought to be eligible for EB-APBI become ineligible at the time of planning due to inability to meet dose-volumetric constraints. EB-APBI in the preoperative setting will reduce the volume of normal tissue treated potentially increasing the number of patients eligible for APBI. This study tested the hypothesis that pre-operative EB-APBI will not only decrease target volumes but will decrease normal tissue exposure significantly increasing eligibility for APBI. Materials and Methods: Forty patients with 41 previously treated early stage breast cancers (tumors ≥4 cm) were retrospectively analyzed from a prospective cohort. Imaging studies (MRI, US and mammogram) were utilized to create a spherical pre-op tumor volume using the largest reported dimension centered within the previously contoured lumpectomy cavity (LPC). Plans were created and optimized for each patient using the pre-operative tumor volume (pre-op) and LPC (post-op) using NSABP B-39 guidelines. Dose-volumetric constraints were analyzed between the cohorts using a t-test analysis. The primary end-point was to evaluate for differences in patient eligibility and normal tissue exposure. Results: The median tumor volume was 93 cc (range 24-570 cc) and 250 cc (range 46-879 cc) in the pre-and post-operative setting respectively. This reduction in tumor volume translated into an increase in patient eligibility for EB-APBI with 35/41 (85%) cases being eligible for EB-APBI in the preop setting versus 18/41 (44%) cases in the post-op setting (p=0.0002). In the pre-op setting 6 cases were ineligible due to violation of one constraint by 5% and no case violated multiple constraints. In the post-op setting, 12 cases had 1 and 11 cases multiple reasons for ineligibility due to exceeding dose constraints by 5%. The most common reason for ineligibility in both groups was > 60% of the ipsilateral breast volume receiving 50% of the dose. The mean volume of ipsilateral breast receiving 50% of the dose was 42% and 63% in the pre-and post-op groups respectively. The mean contralateral breast dose and ipsilateral lung V20 in the pre-and post-op groups were 1 versus 4% and 3 versus 9%. All DVH criteria were statistically significantly improved in the pre-op setting including heart V5 and V40, ipsilateral breast V5, V20, V50 and V80, contralateral breast dose, chest wall V5, V10 and V20; ipsilateral lung V5, V10, V20 and volume of skin receiving 50% of the dose. Contralateral lung dose and thyroid max dose were not significantly different between plans. Conclusions: Administration of EB-APBI in the pre-op setting decreases the size of the target volume which significantly increases the utility of APBI nearly doubling the eligibility for APBI in this cohort. The largest benefit is seen by reducing the volume of breast receiving 50% of the dose. This decreased dose to normal tissues will potentially result in decreased morbidity and improved cosmesis. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P4-11-11.