ESTRO 2021 Abstract Book

S256

ESTRO 2021

2-Gy fractions (EQD2). Other aspects have to do with the accuracy of dose calculation algorithms used for each course, as B-type algorithms are performing better than A-type ones in heterogeneous media such as the lung, or possible RBE variations in the case of treatment employing various radiation qualities. Less known is the impact of the recovery of damage in normal tissues between treatment courses delivered at long time intervals, as emerging evidence suggests that larger total doses could be tolerated in multiple sessions than from single sessions. Image registration for the treatment of metachronous lesions may set additional requirements since variations between CT images from the various planning instances would influence the accuracy of dose summation. Thus, small variations in setup and fixation between treatment sessions might require the use of rigid registrations approaches. However, the requirements and challenges increase for time- resolved planning, where one would have to account for differences in anatomy between motion phases or for other anatomical changes occurring between treatment courses. From this perspective, the use of deformable image registration might better account for the differences between source and target images at registration. However, the performance of deformable image registration has been questioned in more complex cases that have to account for the disappearance of matter and other approaches have to be developed for this purpose. For example, the disappearance of the target from the previous treatment session poses challenges for the handling of high dose regions between images. Furthermore, normal tissue shrinkage following the first course of treatment could also affect the patient anatomy and consequently the performance of the registration and ultimately dose accumulation strategy. Last, but certainly not least, the quality of the deformable image registration algorithm used has to be carefully tested and validated as it has been shown to influence the calculation accuracy of the accumulated dose based on spatial mapping of the dose points between two images. Accurate accounting for the various sources of uncertainty in image registration and dose summation will improve the analysis of treatment outcome and the deriving of tolerance levels for normal tissue that will ultimately benefit future patients. SP-0352 How to differentiate local tumour progression and recurrence from radiation-induced changes after SBRT M. Dahele 1 1 Amsterdam UMC, Radiation Oncology, Amsterdam, The Netherlands Abstract Text Extra-cranial stereotactic body/ablative radiotherapy/radiosurgery (SBRT/SABR/SRS) is increasingly used in patients with advanced cancer, typically in patients with a relatively limited volume and number of metastases combined with a reasonable estimated prognosis (oligo-metastases), and sometimes in patients with more widespread disease when an increased chance of response is needed [1-3]. When SBRT is used, response evaluation on imaging is sometimes challenging, due to post-radiotherapy effects in normal tissues. These can make it hard to see the treated lesion, and they can cause radiation-induced normal tissue changes that can be difficult to differentiate from tumour persistence/progression. The risks include over-calling disease progression and failing to identify tumour progression [4-7]. Therefore, treating teams, including radiation oncologists, radiologists and other medical specialists, should be familiar with the potential for post- SBRT changes and the challenges they can present when trying to assess treatment response. The radiation oncologist has an important role to play in the multidisciplinary discussion of treatment response and in educating colleagues. In addition to being familiar with what post-SBRT changes can look like, they also need to have an idea of what kind of advice to give about further diagnostics or follow-up. The advice may sometimes need to be pragmatic, recognising the natural history and time-course of post-SBRT changes, and the high probability of local control with most SBRT treatments. This talk will include examples from lung, bone and liver SBRT and emphasize some general principles. [1] Chalkidou A, et al. Stereotactic ablative body radiotherapy in patients with oligometastatic cancers: a prospective, registry-based, single-arm, observational, evaluation study. Lancet Oncol. 2021;22(1):98-106. [2] Poon I, et al. Evaluation of Definitive Stereotactic Body Radiotherapy and Outcomes in Adults With Extracranial Oligometastasis. JAMA Netw Open. 2020;3(11):e2026312. [3] Chmura S, et al. Evaluation of Safety of Stereotactic Body Radiotherapy for the Treatment of Patients With Multiple Metastases: Findings From the NRG-BR001 Phase 1 Trial. JAMA Oncol. 2021. doi: 10.1001/jamaoncol.2021.0687. Online ahead of print. [4] Huang K, et al. Radiographic changes after lung stereotactic ablative radiotherapy (SABR)--can we distinguish recurrence from fibrosis? A systematic review of the literature. Radiother Oncol. 2012;102(3):335- 42. [5] Ronden MI, et al. Brief Report on Radiological Changes following Stereotactic Ablative Radiotherapy (SABR) for Early-Stage Lung Tumors: A Pictorial Essay. J Thorac Oncol. 2018;13(6):855-862. [6] Soliman M, et al. Anatomic and functional imaging in the diagnosis of spine metastases and response assessment after spine radiosurgery. Neurosurg Focus. 2017;42(1):E5. [7] Haddad MM, et al. Stereotactic body radiation therapy of liver tumors: post-treatment appearances and evaluation of treatment response: a pictorial review. Abdom Radiol (NY). 2016;41(10):2061-77.

Symposium: Update in advanced breast cancer

SP-0353 Axillary management after primary systemic treatment L. Boersma 1 1 Maasticht University Medical Centre +, Radiation Oncology (Maastro), GROW School for Oncology and Developmental Biology, Maastricht, The Netherlands Abstract Text Until recent, an axillary lymph node dissection (ALND) was considered standard treatment in breast patients with cN+ disease, who have been treated with primary systemic treatment (PST). Since an ALND can results in