Abstract P2-13-29: External dosimetry and in vivo measurements improve surgical comprehension of intraoperative radiotherapy using Intrabeam™

Abstract

Abstract Background IORT is an option for patients with limited access to radiotherapy or patients wishing to avoid the prolonged course of External Beam radiotherapy (EBRT). The recent 5-years results of the non inferiority trial TARGIT-A suggest that with a short follow-up the 5-years risk for local recurrence is greater for the IORT arm. Several options can be suggested to improve those results. Objectives We choose to enhance surgical comprehension of IORT with Intrabeam™ by performing in vivo measurements comparing to external dosimetry taking into account tissues heterogeneities using a preoperative CT scan. Material and Methods Nine patients enrolled in french randomised trial RIOP-InCA underwent Intrabeam™ procedure for breast cancer treatment. Each patient had a preoperative CT scan to simulate the treatment on a Monte Carlo platform using an x-ray source model; in order to calculate the dose actually delivered by the Intrabeam™ system which takes into account the tissues heterogeneities. In vivo measurements using thermoluminescent dosimeters are also performed to evaluate the dose to the skin. Comparisons are done between simulated and measured data. Relative depth dose curves are also compared. Results The median age was 68.3 years [58-87]. The mean maximal diameter of lumpectomy was 71.3 mm [50-125]. The median size was 10.8 mm [5-19] and the median margin status was 12.8 mm [2-32]. 2 patients had micro metastatic involvement of sentinel lymph node without axillary clearance. SBR score was 1 for 5 patients and 2 for 2 patients. All histological subtype were ductal, one patient presented associated DCIS. 6 patients presented luminal A phenotype, 2 luminal B and 1 luminal B with HER-2 neu over-expression. 2 patients received supplemental EBRT (HER-2 neu over expression and SLN micro metastatic involvement). 2 patients had adjuvant sequential chemotherapy and all had adjuvant ani-aromatase hormono-therapy. In vivo measurements on the skin using TLDs gives a mean dose of 1.3 Gy ± 1.1 Gy [0.1-4.9 Gy] in comparison with external dosimetry which had a mean dose of 1.5 Gy ± 0.8 Gy [0.3-6.2 Gy] at the same positions. No patient received a dose superior to the prescription of 6 Gy. Relative depth dose curves give a mean deviation of +36.7% ± 24.8% and +50.6% ± 16.8% in the tangential and perpendicular axes respectively from the manufacturer. Using the shielding allow a reduction of the dose by 10 concerning the ribs. Results Relative deviationSDMean deviation dose to the skinIn vivo/external13.3%2.2%Mean deviation doseShielding/ no shielding-60.2%42.6%Depht Dose curveTangential axe36.7%24.8% Perpendicular axe50.6%16.8% Conclusions Results about the dose as a function of depth show clearly that we cannot consider breast as equivalent to water at this energy and may taking in account breast density .It seems to be important to spread the skin correctly from the incision, to recover carefully the applicator and use the shielding in order to avoid secondary effects as skin necrosis and ribs failures. Our X-ray source model allows to have realistic dose distribution wich hepls in better surgical comprehension of IORT particularly in the set up of the applicator. Citation Format: Pierre-Francois Dupré, Dounia Bouzid, Petra Miglierini, Olivier Pradier, Dimitris Visvikis, Julien Bert, Nicolas Boussion. External dosimetry and in vivo measurements improve surgical comprehension of intraoperative radiotherapy using Intrabeam™ [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P2-13-29.