ESTRO 2020 Abstract Book

S524 ESTRO 2020

Purpose or Objective 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 parameters. Conclusion

1 Fondazione Policlinico Gemelli IRCCS, Radioterapia Oncologica, Roma, Italy ; 2 Fondazione Policlinico Gemelli IRCCS, Chirurgia Senologica, Roma, Italy Purpose or Objective In the last years, thanks to diagnostic advantages, implementation in cancer behaviour knowledge and new target therapies, oliogometastatic disease is an even more common presentation. At the same time, oligoprogression is a reality too, that is raising the issue of maintaining systemic therapies that can be considered still efficacy. In literature, some evidences are reported about the role of radiotherapy in oligoprogression with a known driver of tumor progression, for example, gene addicted lung cancer. On the other side, stereotactic body radiotherapy (SBRT) or Stereotactic Radiosurgery (SRS) are emerging as part of oligometastatic disease in many tumours as prostate cancers. In this preliminary study, we analysed the role of radiotherapy in oligoprogressive breast cancer (OPBC) setting. Material and Methods All the OPBC patients (pts) during systemic therapy that underwent a local not antalgic radiotherapy treatment with VMAT, SBRT or SRT technique were included into the study. Data on age, tumor characteristics at diagnosis, systemic therapy, radiotherapy performed were retrospectively collected. Progression free survival-2 (PFS- 2) was defined as the time (months) from end of radiotherapy to new progression without changing systemic therapies. A Kaplan-Meyer analysis was performed for PFS-2 outcome and a subgroup analysis was conducted on histology, subtype, systemic treatment ongoing and parenchyma irradiated. Results From 2013, we retrospectively found 520 breast cancer metastatic pts who underwent RT. Among these, twenty- five pts were enrolled for this study. Median age was 56 (36-86). Systemic therapies ongoing were: 50% anti-Her2 drugs; 16,6% anti-CDK4/6 agents; 22% cytotoxic agents; 11,4% endocrine therapy. Radiotherapy was administered with VMAT or SBRT/SRS FFF technique on brain (72.2% of pts) with a schedule of 50/30 Gy at 5/3Gy (whole brain with a simultaneous integrated boost on lesions) or 25 Gy/8,5Gy (SBRT of single lesions); lung (16,6% of pts) with a schedule of 50 Gy/10Gy; bone with a schedule of 30 Gy/6Gy. Mean PFS-2 was 10.63 months (1-25) [Figure 1]. At the subgroup analysis histology (p=0.37), subtype (p=0.07), systemic treatment ongoing (p=0.18) and parenchyma treated (p=0.97) did not showed a significant correlation with PFS-2, while a treatment that control deeply a driver of cancer progression showed a positive correlation with it (p=0.02 with an advantage for luminal and HER2 respect triple negative breast cancer) [Figure 2]. Conclusion In this generating hypothesis study, preliminary results show that introducing focal radiotherapy treatments in OPBC treatments, can improve PFS-2, prolonging sistemic therapies, especially in good prognosis subtypes with a driver to target with a drug. Further studies are needed on large series to confirm these preliminary results, also with new target therapies ongoing. PO-0983 Mammary gland and implant changes during daily image guided intensity modulated radiotherapy S. Scharm 1 , S. Kampfer 1 , K.J. Borm 1 , S.E. Combs 1 , M.N. Duma 2 1 Klinikum Rechts der Isar -Technical University Munich, Department of Radiation Oncology, Munich, Germany ; 2 Klinikum Rechts der Isar -Technical University Munich and University hospital of Friedrich-Schiller-University Jena, Department of Radiotherapy and Radiation Oncology, Munich- Jena, Germany