ESTRO 2021 Abstract Book

S287

ESTRO 2021

gratitude extends to the ESTRO family that has provided so much intellectual stimulus and collaboration along the way.

Award lecture: Iridium Award

SP-0392 Merits and challenges for Multicatheter Interstitial Brachytherapy (MCBT) in the treatment of Breast Cancer E. Van Limbergen 1 1 University Hospital Gasthuisberg, Dep. of Radiation Oncology, Leuven, Belgium Abstract Text MCBT has a long clinical track record in the treatment of breast cancer. Nowadays it is in use as a boost in combination with postoperative whole breast irradiation (WBRT) after breast conserving surgery. It is also used as Accelerated Partial Breast Irradiation ( APBI) in carefully selected low (+/- intermediate ) risk breast cancer patients or as a breast conserving appraoch in recurrences in previously irradiated breasts. MCBT is a very versatile technique, able to deliver significant radiation doses very conformally to the Clinical Target Volume, with a very sharp fall off of the dose gradient in the surroundings. Non target breast tissue can be spared, decreasing the Treated Volume and so the risk and severity of late fibrosis. Overlying skin can be maximally spared , so avoiding acute and late skin toxicity. Both facters : less fibrosis and less late skin toxicity will lead to less cosmetic damage after breast conserving treatmentssurgery and radiotherapy Despite these appealing possibilities, MCBT is often not applied for several reason. Availability of the equipment in the department, financial return and is frequently considered as technically more demanding ,although an adequate trainingsprogramme can easily solve this problem . It will take more active time spending of the radiation oncologist in the OR, and certainly would need some manual skills…… For all these reasons new so-called “ simpler” BT techniques have been developed such as Intracavitary device BT (ICBT) like Mammosite Balloon and later multistrut intracavitary devices to improve the dose distribution and dimish skin toxicity. But also intracavitary 50 kV emitting probes have been developed to be placed intraoperatively (IORT-TARGIT). Another IORT technique is with electron beams from a dedicated Linac ( ELIOT)targeted to the resection cavity during surgery. With the increasing experience with APBI in low-risk patients also very conformal external beam techniques have been developed ( 3D conformal , IMRT, IGRT..) in order to obtain comparable outcome results. The advantage of APBI is that due to the small irradiated volumes the needed dose can be delivered in a much smaller time period : In 1 day for IORT techniques to up to 5 days for classic MCBT and ICBT instead of 4 to 6 weeks for classic EBRT. This can have important practical, social and financial advantages for the patients. The early experiences : successes and failures has led tot the developement of recommendations ( GEC-ESTRO and ASTRO) to select candidates based on on their risk profile for local recurrence : age, positive or negative section margins, hormone receptorstatus, the presence or not of EIDC , LVI and lymphnode status). Twelve years follow up with MCBT APBI in Phase II settings have shown that results are similar for local control and survival but with better cosmetic outcome than with WBI, Several Phase III trials have randomized APBI to WBI +/- boost: MCBT was tested in the NIO Hungary trial and the GEC-ESTRO trial both showing non inferiority to WBI but less toxicity. The ELIOT trial showed worse local control rates in APBI arm ( 4,4% vs 0,4%), maybe related to inapproriate patient selection and/or insufficient target covering TARGIT A IORT reported more local failures (3,3% vs 1,3%) but better overal survival due to a lower non breast cancer martality. However 24% of the APBI cases received postop WBI. The NSABP39/RTOG0413 trial randomised in MCBT, ICBT (most of the BTcases) or EBRT APBI 10 x3,85 twice a day vs WBI and showed more local faillures but similar survival outcome The RAPID trial compared EBRT APBI and noted similar local failure rates but increased toxicity and worse cosmesis for the 10 x 3,85 twice daily schedule EBRT Smaller trials in Florence showed similar local failures and improved cosmesis with 5 x6Gy in 5 days APBI versus WBI.The Barcelona trial found no difference in outcome for tumor control and cosmesis in both arms. The data from the RAPID trial and 3 other centers reporting worse cosmetic outcome with the NSABP 10x3,85Gy twice a day protocol, raised doubt on the validity of the radiobiological assumptions made forwhen treating larger volumes with significant shorter interfraction intervals New phase III trials with not accelerated PBI EBRT, delivered in a the same toverall time as WBI.The IMPORT LOW showing similar local failures ( 1,1 and 0,5 % ) and improved cosmesis for PBI versus WBT with a fractionation schedule of 40 Gy in 15 fractions In conclusion PBI is not inferior to WBI in most trials except in the ELIOT setting: key factors is correct selection of low -risk cases and coering the CTV. And volume matters for cosmetic outcome. MCBT has proven to be a very versatile technique to achieve these objectives

Proffered papers: Proffered papers 22: Functional and biological imaging

OC-0393 Intravoxel incoherent motion MRI, a potential alternative for DCE MRI in prostate cancer patients E. Kooreman 1 , P. van Houdt 1 , V. van Pelt 1 , M. Nowee 1 , F. Pos 1 , U. van der Heide 1 1 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands