fractionation techniques, possible controversy regarding selection criteria for APBI, and external forces including reimbursement rates, which generally favor external beam RT approaches. PP40 Presentation Time: 9:27 AM A Quantitative Evaluation of Pd-103 Permanent Breast Seed Implant Mark-Up Procedure Danielle Anderson, PhD 1 , Deidre Batchelar, PhD 1 , Michelle Hilts, PhD 1 , Marie-Pierre Milette, PhD 1 , Juanita Crook, MD 2 . 1 Physics, BCCA - SAH Centre for the Southern Interior, Kelowna, BC, Canada; 2 Radiation Oncology, BCCA - SAH Centre for the Southern Interior, Kelowna, BC, Canada. Purpose: Permanent breast seed implant (PBSI) is an attractive adjuvant treatment for early stage breast cancer following lumpectomy. However, because target localization is heavily dependent on ultrasound guidance and treatment team experience, widespread implementation remains challenging. The purpose of this study is: (1) to investigate the stability of a series of external landmarks in locating the seroma centroid, and (2) to evaluate the impact on the dose distribution of using these external landmarks to guide the implant. Materials and Methods: Fifteen consecutive PBSI patients treated during 2016 were included in the study. Two to four weeks before the implant, a CT simulation is acquired with the patient positioned on a breast board, and planning performed using MIM Symphony (Cleveland, OH, USA). On the day of the implant, to help guide implant set-up in the operating room, the patient undergoes a mark-up procedure at the CT simulator. The fiducial needle (FN) entry point and the projection of the implant center are temporarily marked on skin based on room-coordinate moves from external landmarks - a nipple marker and/or lateral tattoo. All marks (medial and lateral tattoos, nipple marker, FN entry, and implant center) have radiopaque markers placed on them and a pre-implant CT is acquired. The original (planning CT) seroma contour is deformed to the pre-implant CT using MIM Maestro, and is adjusted by an expert reviewer if necessary. The distance between external landmarks and seroma centroid was measured and compared for the two CT images. The potential influence of choice of primary guiding external landmark on dosimetry was simulated by evaluating and comparing the dose distributions resulting from positioning the treatment plan on the pre-implant CT using each of four different markers (lateral tattoo, nipple marker, FN entry point, and implant center). The CTV (seroma) and an evaluative target volume (ETV; 0.5 cm margin on CTV) coverage was scored. Results: The change (mean  SD) in landmark position with respect to seroma centroid between the planning CT and pre-implant CT in each orthogonal direction, and the total displacement, are plotted in figure 1. The mean change in displacement between all external landmarks and the seroma centroid was less than 5 mm in each orthogonal direction. The five landmarks all had similar consistency in position with respect to the seroma centroid, except the medial tattoo was significantly better at locating the seroma centroid in the medial-lateral direction. None of the landmarks chosen to guide plan placement resulted in a significant difference in dosimetry. The 60 plans created had a median ETV V90% of 97.2% (range 52.7 - 100%), and 80% of the plans resulted in a CTV V100 O 98%. Conclusions: Reproducible patient positioning on a breast board and careful definition of key external landmarks pre-implant are valuable in providing an approximate location of the internal seroma. Localizing the treatment plan using external landmarks achieved acceptable seroma coverage in the majority of cases. A pre-implant mark-up procedure is thus recommended to complement and facilitate an accurate implant, but ultrasound guidance remains invaluable to verify the target location and avoid instances of inadequate target coverage. PP41 Presentation Time: 9:36 AM Seed Distribution Stability in Permanent Breast Seed Implant Brachytherapy Michelle Hilts, PhD 1,2,3 , Cassidy Northway, BSc 1,2 , Deidre Batchelar, PhD 1,2 , Daniel Morton, MSc 1,3 , Marie-Pierre Milette, PhD 1,2 , Juanita Crook, MD 1 . 1 BC Cancer Agency - Southern Interior, Kelowna, BC, Canada; 2 University of British Columbia - Okanagan, Kelowna, BC, Canada; 3 University of Victoria, Victoria, BC, Canada. Purpose: Permanent Breast Seed Implant (PBSI) is completed in a single outpatient session and is an attractive treatment option for patients with early stage breast cancer. However concern has been raised over seeds potentially shifting over time from their implanted positions. In this study we evaluate the stability of the implanted seed distribution in PBSI over the month immediately following implant. Materials and Methods: 31 consecutive patients who received PBSI were eligible for this study. Two patients were excluded as not all post-implant CT scans were available. For the remaining 29 patients, post-implant CTs obtained on day of implant (Day0) and approximately one month later (Day30) were retrospectively analyzed. All post-implant seromas were defined from the seroma contoured on planning CTs using deformable image registration and adaptive contouring, then evaluated and edited by the radiation oncologist as required. A medical physicist identified implanted seeds on all CT images. Rigid registration of Day0 and Day30 CTs using seroma as region of interest was performed to assess the spatial correspondence of the centre of mass (COM) of the 90% isodose volumes. The following were recorded from Day0 and Day30 CTs: 90% contiguous isodose cloud size (volume, dimensions - ML, AP, SI) and position (COM coordinates), the maximum (max) spread between all implanted seeds in each direction (ML, AP, SI), the distance of each seed from its distribution COM, and number of seeds located within the seroma. Paired t-tests were used to test for significant differences between Day0 and Day30. Results: There was an average of 3311 days between Day0 and Day30 scans. A total of 2107 of 2108 implanted seeds were identified, meanSD S53 Abstracts / Brachytherapy 16 (2017) S14eS118