ESTRO 2020 Abstract book

S570 ESTRO 2020

lung (V5 Gy), skin strip (V20 Gy, V10 Gy, V3 Gy). Student’s T-test was performed to identify differences between means in 3D-CRT and MRgRT plans. Results 198 patients receiving APBI with 3D-CRT (98 patients, 49.5%) and MRgRT (100 patients, 50.5%) were included. PTV cavity volume was larger with 3D-CRT (mean: 188 cc) compared to MRgRT plans (mean: 98 cc) (p<0.001), inversely correlated with PTV Dmax (mean: 41.9 Gy and 43.3 Gy, p<0.001). Both modalities demonstrated adequate coverage of PTV V95% Rx (mean: 98.2% and 97.4%, p=0.38) and V90% Rx (mean: 99.9% and 99.87%, p=0.39). The percent of ipsilateral breast receiving high dose radiation, 30 Gy, was significantly higher amongst patients receiving 3D-CRT (mean: 32.4% and 19.0%, p<0.001). However, the percent of ipsilateral lung receiving 5 Gy was lower in 3D-CRT patients (mean 27.1% and 32.4%, p=0.01). Additionally, patients undergoing MRgRT had significantly lower volume of skin receiving 20 Gy (mean: 613.9cc and 337.3 cc, p<0.001) and 10 Gy (mean: 994.7cc and 774.7cc, p<0.001), but not 3 Gy (mean: 2576cc and 2301.6cc, p=0.09). There was no significant difference between 3D-CRT and MRgRT plan dosimetrics for: PTV Dmin (mean: 34.1 Gy and 33.8 Gy, p=0.61), percent ipsilateral breast receiving 5 Gy (mean: 63.1% and 60.5%, p=0.18), heart receiving 5 Gy (mean: 3.1% and 2.4%, p=0.42), and contralateral lung receiving 5 Gy (mean: 0.27% and 0.13%, p=0.35). Conclusion For APBI with either 3D-CRT or MRgRT, acceptable PTV coverage was demonstrated with both modalities. MRgRT had greater high-dose sparing of the ipsilateral breast and skin attributable to smaller volumetric expansions from minimizing setup uncertainty, which may result in improved cosmetic outcomes and decreased acute/long term toxicities. PO-0985 Hypofractionated radiotherapy with SIB in advanced incurable breast cancer-HYPORT B study R. Santosham 1 , S. Chatterjee 1 , S. Chakraborty 1 , A. Mahata 1 , S. Mandal 1 , A. Das 1 , A. Kumari 1 , S. Ray 2 , R. Ahmed 3 1 Tata Medical Center, Radiation oncology, Kolkata, India ; 2 Tata Medical Center, Nuclear Medicine, Kolkata, India ; 3 Tata Medical Center, Surgical oncology, Kolkata, India Purpose or Objective To report the early clinical outcome and acute toxicity data of a single arm prospective study evaluating a 5 day hypofractionated external beam radiotherapy schedule with simultaneous integrated boost (SIB) in patients with advanced incurable breast cancer. Material and Methods Patients with locoregionally advanced incurable or metastatic breast cancer requiring palliation of local symptoms were enrolled in this study. Hypofractionated radiotherapy was delivered to a dose of 26Gy in 5 fractions over 1 week to the whole breast and ipsilateral supraclavicular fossa along with 6Gy SIB to the metabolically active tumor. Axillary nodes were irradiated only if involved. Regional PET CT was done before and 3 months after completion of radiotherapy. Acute toxicity was assessed using CTCAE version 4.03 toxicity grading criteria. Clinical response was assessed at 2 weeks and 3 months after completion of radiotherapy. Radiological response was assessed using the PERCIST 1.0 criteria. Results Nineteen out of thirty patients have been enrolled in this study between April 2018 to August 2019. One patient expired prior to the third month evaluation. Hence, we present our analysis on the remaining 18 patients. At presentation, 2 patients had breast related pain score above 5, 3 patients had bleeding from the tumour site and 5 patients had a fungating mass. 4 (22.2%) patients had Gr II skin toxicity at 2 weeks after completion of

Historically, the mammary gland is reported to swell during radiotherapy.This study aims to quantify changes in breast volumes (V) and thereby the impact on dosimetry during daily image guided helical tomotherapy (HT). Material and Methods We retrospectively analyzed a group of 44 consecutive female patients with breast cancer treated in our institute with adjuvant IGRT at the HT Hi Art machine (Accuracy Inc., Madison, WI, USA) from 2014 - 2016. 24 patients were treated using a breast simultaneous integrated boost technique (Group SIB), 7 patients with chest wall irradiation (CWI) after mastectomy (Group CW) and 13 patients received breast irradiation after mastectomy with cosmetic breast implant reconstruction (Group IMPL). 8 patients (3 of Group SIB, 4 of Group IMPL, 1 of Group CW) received an irradiation of the ipsilateral lymphatic drainage (LyD). The PTV (CW, IMPL, breast or lymphatics) dose was for all patients 50.4 Gy/ single doses of 1.8 Gy. The patients in the SIB group were treated with single doses of 2.25 Gy up to total dose of 63 Gy. Actual delivered doses were calculated retrospectively with the PlannedAdaptive software (TomoTherapy Planning Station, version 4, TomoTherapy Inc., Madison, WI, USA). Results The volume of SIB shrinked significantly during RT in 87.5% (21/24) patients. This translated at the end of treatment in an absolute shrinkage of 22.28 cm 3 ±17.54 cm 3 (13.87% ±11.29%) and an increased D mean of 1.32 Gy ±1.11 Gy (p<0.05). Furthermore, the PTV breast also decreased significantly by 31.51 cm 3 ±73.81 cm 3 (2.91% ±6.81%) and the D mean of PTV breast increased by 1.08 Gy ±1.36.The volume/dose to the PTV CW , PTV IMPL and PTV LyD did not change significantly. Further, we analyzed the volume of the V left lung , V right lung , V entire lung , V heart , V contralateral breast (V CB ) , the D mean of lung, heart, CB, V 20 , V 30 , V 40 , V 50 of left, right, entire lung and D max of the contralateral breast. There were no statistical significant differences in these Volumetric shrinkage and dosimetric changes of the breast occur during HT. These changes could be as high as 15% for the SIB regions and translate into an overall dose increase of approx. ½ fraction dose during radiotherapy. Swelling of the PTV did not occur during image guided HT. PO-0984 Dosimetric Analysis of External Beam Modalities for APBI: Comparing 3D-Conformal and MR- Guided RT J. Stanley 1 , W.R. Kennedy 1 , L. Ochoa 1 , O. Green 1 , M. Thomas 1 , I. Zoberi 1 1 Washington University in St Louis, Radiation Oncology, St Louis, USA Purpose or Objective Accelerated partial breast irradiation (APBI) is an alternative to whole breast irradiation as part of breast conserving therapy. One of the most important goals of APBI is to decrease radiation exposure to the clinically uninvolved breast as well as nearby organs-at-risk. Our institution recently began using MR-guided radiation therapy (MRgRT) for APBI as the MR guidance offered the ability to minimize PTV margin beyond the surgical bed plus 1 cm of breast tissue by allowing for volumetric alignment of target tissue prior to each fraction and monitoring for intrafraction motion. The purpose of this report is to quantify the dosimetric gains achieved by MRgRT compared to historical three-dimensional conformal radiation therapy (3D-CRT) APBI. Material and Methods Women with early-stage breast cancer receiving APBI from 2011-2017 at our institution were identified. All patients were treated with external beam radiation to 38.5 Gy in 10 bid fractions. Plans were evaluated for PTV (cavity volume, Dmax, Dmin, V95% Rx, V90% Rx), ipsilateral breast (V30 Gy, V5 Gy), heart (V5 Gy), ipsilateral/contralateral parameters. Conclusion