ESTRO 2021 Abstract Book

S1593

ESTRO 2021

Conclusion Allowing more beams having distal edge in the brainstem significantly reduced the mean dose to Brain-CTV, while significantly increasing the biological dose to the brainstem. LET or RBE-based optimisation methods have the potential of regaining the low dose to the normal brain tissue, while keeping the biological dose to the brainstem low. Until such methods are clinically available simple planning guidelines, like avoiding the distal edge in more than one of minimum three fields, are warranted. PO-1870 PRETREATMENT SELECTION FOR LAD SPARING IN PERSONALIZED BREATH HOLD BREAST RADIOTHERAPY E. Ippolito 1 , A. Di Donato 1 , M. Marrocco 1 , B. Floreno 1 , F. Giannetti 1 , G. D'Auria 2 , T. Gamucci 2 , S. Ramella 1 1 Campus Bio-Medico University, Radiation Oncology, Rome, Italy; 2 Sandro Pertini Hospital, Medical Oncology, Rome, Italy Purpose or Objective To identify a set of pretreatment anatomic or planning characteristics correlated with left descending artery (LAD) dose and therefore provide guidance in the selection of patients with left-breast cancer treated with deep inspiration breath hold radiotherapy (DIBH-RT). Materials and Methods We retrospectively identified patients with left-sided breast cancer who underwent whole breast radiotherapy in DIBH. All patients had both plans in DIBH and free-breathing (FB). The following anatomical and treatment parameters were obtained from FB CT scans: lung volume, heart volume, breast separation, minimum distance from LAD to tangent open field. Receiving operating characteristics was performed to define the cut-off point of parameters to predict LAD maximum dose >10 Gy and LAD mean dose > 4 Gy. Areas under the curve (AUCs) were calculated for all variables. Post-test probability has been calculated to evaluate advantage for parameters combination. Results One hundred ninety-seven patients were identified. A combination of pretreatment anatomical parameters was identified. Adding consecutively all parameters allowed to reach a positive predictive value (PPV) to identify patients at risk of higher dose to LAD > 90%. The strongest predictor at FB CT scan of LAD maximum dose > 10 Gy and a LAD mean dose > 4 Gy was the minimum distance of LAD from tangent open fields. Other parameters were lung volume and heart volume for LAD Dmax > 10 Gy and lung volume, heart volume and breast separation for LAD Dmean > 4 Gy. Conclusion The dosimetric benefit of DIBH is valid for all patients and DIBH should be preferred for all left sided patients; however, evaluating a set of pretreatment parameters on FB simulation CT scan allows to early identify patients at risk of higher dose to LAD. PO-1871 Prospective study on the feasibility of single-fraction SABR for bone and lymph node metastases C. Mercier 1,2 , M. Claessens 1,2 , G. De Kerf 1 , C. Billiet 1,2 , I. Joye 1,2 , D. Verellen 1,2 , P. Ost 1,3 , P. Dirix 1,2 1 Iridium Netwerk, Radiation Oncology, Antwerp, Belgium; 2 University of Antwerp, Integrated Personalized and Precision Oncology Network (IPPON), Antwerp, Belgium; 3 University of Ghent, Faculty of Medicine and Health Sciences, Ghent, Belgium Purpose or Objective Single-fraction (SF) stereotactic ablative body radiotherapy (SABR) is a potential option for non-spine bone (NSB) and lymph node (LN) oligometastatic disease (OMD), but might be underused in clinical practice. To report on the fractionation practice patterns of all SABR cases treated in a prospective observational cohort. Second, to recalculate all multifraction (MF) plans to a SF plan estimating theoretical incorrect fractionation allocation. Materials and Methods All patients with NSB and/or LN OMD referred for SABR were preferably treated with a SF to 20.0 Gy. Primary endpoint was the clinical feasibility of delivering a SF while respecting the dose constraints of the report of the American Association of Medical Physicists task group 101 for the organs at risk (OAR). Results Between June 2019 and January 2020, 45 patients received SABR to a total of 60 NSB (25/60) and LN (35/60) metastases.