ESTRO 2021 Abstract Book

S1503

ESTRO 2021

unstable patients when no tracking capabilities are available. Also, margins of this last group are the most affected when images are skipped.

Conclusion Intra-fraction tracking is able to reduce uncertainties of patient motion to a minimum. In case tracking is not available, most of the patients could receive an appropriate treatment. Nevertheless, attention should be paid to unstable patients for which tracking might be mandatory. Also care should be taken when choosing imaging frequency in this group of patients.

Digital Poster: Quantitative functional and biological imaging

PO-1776 SPECT-CT sentinel lymph nodes atlas for breast cancer: implications for lymph nodes contouring. S. Novikov 1 , P. Krzhivitskiy 2 , A. Valitova 2 , Y. Melnik 1 , J. Bryantceva 1 , I. Akulova 1 , P. Krovorotko 3 , S. Kanaev 1 1 N.N. Petrov Research Cancer Center, Radiotherapy, St Petersburg, Russian Federation; 2 N.N. Petrov Research Cancer Center, Nuclear Medicine, St Petersburg, Russian Federation; 3 N.N. Petrov Research Cancer Center, Breast surgery, St Petersburg, Russian Federation Purpose or Objective Considering the prevalence of sentinel lymph node (SLN) invasion by the tumor we propose that the atlas of SLNs can help to validate the most widely used guidelines for contouring regional lymph nodes in patients with an early (T1-2N1) breast cancer, in particular, with positive SLNs that would receive postoperative nodes irradiation instead of an axillary dissection. Materials and Methods A total of 246 single photon emission computed tomography- computed tomography (SPECT-CT) datasets obtained after after intra-/peri- tumoral injection of 99mTc-radiocolloids were screened retrospectively. All SPECT-CT images were non-rigidly fused with the reference simulation CT. Further, comprehensive three- dimensional atlas was created and used for evaluation of lymph nodes clinical target volume (CTV-LN) defined by ESTRO, RTOG and RADCOMP contouring guidelines. Results SPECT-CT visualized 518 SLNs. The majority of SLNs detected in the axillary level I (67%); they were covered by ESTRO and RTOG contours in 84% and 85% cases, respectively. The majority of out-of- contours SLNs were located in the lateral subgroup detected in 19% women. Interesting that lateral axillary LNs usually receive high therapeutic dose during the tangential irradiation of the breast. SPECT-CT data indicate that posterior border of Level I axillary contours can be safely shifted anteriorly (to the imaginary line connecting anterior- lateral surface of the II-V ribs with anterior surface of m. latissimus dorsi) that would help to reduce absorbed doses in the lung and heart. Sentinel and second echelon nodes with radiocolloids uptake were visualized in the level II, III, and supraclavicular regions in 16%, 7%, 1% patients. They were nearly always (97-99%) inside the recommended CTV-LN. Subpectoral SLNs (48 cases) were usually localized very close to the thoracic vault. This fact underlines the necessity to cover the anterior surface of the I-III ribs carefully. Internal mammary SLNs were visualized in 33 (9%) women, in all cases located below IV rib and were out of ESTRO and RTOG contours in 3 and 6 cases, respectively. 7mm margins around internal mammary vessels would help to cover internal mammary SLNs in all cases. Conclusion SPECT-CT atlas of SLNs demonstrated that in most cases ESTRO, RTOG and RADCOMP guidelines correctly represented CTV-LNs with the exception of SLNs located in the lateral axillary and internal mammary regions. Proposed modification of level I axillary contours can reduce the dose to normal tissues. PO-1777 68Ga-fibroblast activation protein inhibitor PET/CT on GTV delineation of oesophageal cancer L. Zhao 1 , H. Chen 2 , Q. Lin 1 1 The First Affiliated Hospital of Xiamen University, Department of Radiation Oncology, Xiamen, China; 2 The First Affiliated Hospital of Xiamen University, Department of Nuclear Medicine & Minnan PET Center, Xiamen, China Purpose or Objective To compare 68 Ga-fibroblast activation protein inhibitor (FAPI ) and 18 F-FDG PET/CT in imaging locally advanced oesophageal cancer, and evaluate the potential usefulness of 68 Ga-FAPI PET/CT on gross target volume (GTV) delineation aimed at radiotherapy planning for oesophageal cancer as compared with contrast-enhanced