ESTRO 36 Abstract Book

S972 ESTRO 36 _______________________________________________________________________________________________

study. Treatment plan is made based on the initial CT scan that is taken post the insertion of the applicator. Then the patient is treated on outpatient basis on 10 fractions of 3.4 Gy over 5 days.The CT scans of these patients, taken before each treatment were separately imported in to the treatment planning system and registered with the initial CT scan respective to the applicator. Three radio channel markers of the applicator are used as reference points to conduct landmark registration on each CT scan. Difference in Max Dose received by skin, ribs and PTV(Planning target volume) during each treatment is recorded. Results Contours of any of the OARs were not exactly similar when CT images were fused on each other. Deduction in volumes of PTV and cavity was noticed. There was always a difference between received doses by the OARs and PTVs between treatments. Variations in received dose by Skin and ribs were statistically significant for 3 treatments and 2 treatments respectively under 5% level. Variations were statistically significant for 4 more fractions for both organs under 10% level. Some data indicates, few times patients received more than 145% of prescribed dose that breach the specific guidelines of APBI. Conclusion The difference recorded in volumes of OARs and iso-doses near the OARs between treatments indicate that the received doses to OARs differ from the prescribed dose in the initial treatment plan. Similarly PTV receiving a lesser dose than the prescribed dose affects the quality of the treatment. It appears that taking a CT scan before each treatment and re-planning is important at this stage to minimize the risk of delivering different doses than the prescribed. EP-1768 What is the proper dose in single fraction HDR brachytherapy as monotherapy for prostate cancer? S. Ruiz Arrebola 1 , A.M. Tornero-López 2 , J.M. De la Vega 3 , P.J. Prada 1 , D. Guirado 3 1 Hospital Universitario Marqués de Valdecilla, Department of Radiation Oncology, Santander, Spain 2 Hospital Universitario Dr. Negrín, Unidad de Radiofísica, Las Palmas de Gran Canaria, Spain 3 Complejo Hospitalario de Granada, Unidad de Radiofísica, Granada, Spain Purpose or Objective High dose rate brachytherapy (HDRBT) as monotherapy for prostate cancer is applied with several fractionation schedules. Usually the linear-quadratic (LQ) model is used to establish their equivalence. However, using the currently accepted value of α/β for prostate cancer, a significant discrepancy between the LQ model predictions and clinical outcomes is found for the only single fraction schedule in use with long-term results [1]. We aim to determine the value of the absorbed dose for an extreme hypofractionation regime of one fraction in HDRBT monotherapy leading to a biochemical failure rate similar to that of most widely used regimes. Material and Methods We used available published data from biochemical control at 5 years, for prostate cancer patients of low and intermediate risk treated with exclusive HDRBT: 7 clinical studies with 9 fractionation schedules, from 1 fraction of 19 Gy to 9 fractions of 6 Gy per fraction. To compare the different schedules, we used the equivalent total dose in 2 Gy fractions, and to describe the biochemical control (BC), we used the LQ model together with the logistics probability function: Electronic Poster: Brachytherapy: Prostate

Results The beam stability is within 3% over 1 hour and the dose homogeneity at a 40 mm diameter breast applicator surface is within ± 5% of the central axis dose. The percentage depth dose at 5 mm is between 35% and 65% depending on the KVp, FSD and applicator diameter. Radiation protection to comply with regulation is easily achieved using a machine mounted shield and the P+ can be used in a room with only 0.3 mm lead equivalent shielding or none if room large (> 6 m X 6 m). The P+ can deliver a high dose to small volume in accessible tumors such as skin cancer (especially facial and eyelid) using applicators between 1 and 4 cm, rectum (endoscopic approach)using applicators 2, 2.5 and 3 cm, breast (IORT)using a dedicated spherical Nice Breast Applicator (3 to 5 cm diameter). In the near future it is proposed that vaginal vault irradiation (stepping source) and other IORT sites will be possible using P+. Conclusion The Papillon + TM is a new multipurpose CXB 50 KVp machine with the advantage of a high dose rate and easy radioprotection. First clinical results achieved in Nice will be presented at time of the meeting. EP-1767 The Dosimetric Consequences Throughout The Treatment Time In APBI With SAVI Applicators. S. Chandrasekara 1 , S. Pella 2 , M. Hyyovain 3 , P. Janeil 2 1 Florda Atlantic University, Physics, Boca raton, USA 2 Florida Atlantic university, Physics, Boca Raton, USA 3 Florida Atlantic University, Physics, Boac Raton, USA Purpose or Objective To analyze the variation in dose received by the organs at risk (OARs), that occur during treatment time ( 10 treatments) of early stage breast cancers with APBI using SAVI applicators. Material and Methods A retrospective analysis of 15 patients treated with SAVI applicators at SFRO Boca Raton were considered for this