ESTRO 2022 - Abstract Book

S1662

Abstract book

ESTRO 2022

On comparison of all dosimetric indices in SRS Rapid Arc® plan, only EFOD was found to increase with tumor volume and rest all parameters were found to be independent of tumor volume. The plan quality matrix other than EFOD did not reveal any correlation from their results.

PO-1876 The robustness planning of SBRT for centrally-located non-small cell lung cancer

H.M. Hung 1 , W.S. Leung 2

1 Pamela Youde Nethersole Eastern Hospital, Clinical Oncology, Chai Wan, Hong Kong (SAR) China; 2 The Hong Kong Polytechnic University, Health Technology and Informatics, Kowloon, Hong Kong (SAR) China Purpose or Objective Stereostatic Body Radiotherapy (SBRT) of lung cancer is usually deliver a high fraction dose to a moving target with heterogeneous density gradient. Strategies in considering setup and dose estimation uncertainties are particularly important. This study aimed to compare the conventional margin-based target volume planning with the robust optimization planning approaches in terms of plan qualities in SBRT of centrally located non-small cell lung cancer (CL-NSCLC). Materials and Methods Twelve CT sets for SBRT of CL-NSCLC were recruited retrospectively. Two SBRT plans will be recomputed by conventional optimization with Planning Target Volume (PTV-O) and the Robust Optimization with the Internal Target Volume (ITV-RO) for each CT set. ITV was created by merging Gross Target Volume (GTV) of 10 motion phases of 4DCT. While 5mm isotropical margin was given to the ITV to form the PTV. The prescribed scheme was 50 Gy in 5 fractions. For comparison purposes, the prescribed dose was normalized to ITV after PTV-O. Planning goals were to cover 95% ITV by varying prescription isodose lines at 70 to 90% in both plans. 5mm isotropic uncertainties with +/- 2% density error (16 scenarios) was introduced to robustness analysis for both optimization approaches. The bandwidth measurement at D95% of ITV from DVH were measured for comparing the robustness. Moreover, worst-case scenarios of dosimetric parameters e.g., ITV coverage, dose spillage (R50%, the ratio of 25Gy isodose volume to ITV) and doses to organs at risk (OARs: lungs, spinal cord, heart, esophagus, trachea and major vessels) were also compared. Results Similar target coverage was observed in all ITV parameters by both approaches but better performance was noted in ITV- RO approach: conformity of ITV (conformity index: 1.13 vs 1.21, p < 0.05), dose spillage (R50% ratio 4.18 vs 4.45, p < 0.05). The worst-case scenarios OARs parameters by ITV-RO were generally lower than that by PTV-O, significant differences were recorded in maximum dose of heart, major vessels and V20Gy (Lungs). The robustness of ITV-RO approach was also assured, the bandwidth of D95% of ITV was 39.6% narrower than that of PTV-O approach ( p < 0.01). Conclusion ITV-RO approach offering advantages in conformity, spillage control and OARs sparing while without sacrificing the target coverage. Moreover, the robustness of ITV-RO also demonstrated superiority of withstanding setup and dose estimation uncertainties. Overall, it has potential clinical benefit to SBRT planning of CL-NSCLC. P. Teles Amaro 1 , L. McDaid 1 , L. Davies 1 , L. Whiteside 1 , A. Clough 1 , C. Faivre-Finn 2,3 , J. Parker 1 , R. Bailey 1 , R. Benson 1 , C. Nelder 1 , E. Pitt 1 , C. Eccles 1,4 , C. Crockett 2 , A. Salem 2 , A. Choudhury 2,5 1 The Christie NHS Foundation Trust, Radiotherapy, Manchester, United Kingdom; 2 The Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom; 3 University of Manchester, Radiation Oncology, Manchester, United Kingdom; 4 University of Manchester, Medicine and health, Manchester, United Kingdom; 5 University of Manchester, Cancer Sciences, Manchester, United Kingdom Purpose or Objective This work reports on our initial experience delivering stereotactic ablative radiotherapy (SABR) to a gluteal metastasis on a 1.5T Elekta Unity MR Linac (MRL) (Elekta AB. Stockholm, Sweden). Materials and Methods A patient with non-small cell lung cancer (NSCLC) was found to have an 8mm right-sided gluteal oligometastasis on staging FDG PET-CT (stage IVa, oligometastatic disease). Discussion at local SABR multi-disciplinary team (MDT) highlighted challenges in localising the sub-centimetre gluteal metastasis on cone-beam CT (CBCT) imaging, therefore a referral was made for treatment on the MRL within the ethically approved, imaging study PRIMER (Clinicaltrials.gov NCT02973828). A preliminary MR scan was performed to ensure the lesion was identifiable. Vendor approved MR sequences (Table 1) were used to determine lesion conspicuity as, ultimately, only these sequences are sanctioned for clinical use on the Unity MRL. A further spectrally selective attenuated inversion recovery (SPAIR) sequence was performed to suppress fat signal and provide further confidence in disease visibility. The patient then underwent CT and MR planning scans (pCT and pMR respectively), positioned supine with arms on thorax and pelvis immobilised with knee and foot step. pCT was performed for the provision of electron density data to inform treatment planning. pCT and pMR image sets were imported into the Monaco treatment planning system (v 5.40.01, Elekta AB. Stockholm, Sweden) for target volume and organ at risk (OAR) delineation and a prescription of 30Gy in 3 fractions was planned. The T2 SPAIR sequence was co-registered with the 6 minute T2 3D Tra sequence to further aid target delineation. PO-1877 Initial experience delivering stereotactic radiotherapy to a gluteal metastasis on a 1.5T MR Linac