ESTRO 2023 - Abstract Book

S610

Monday 15 May 2023

ESTRO 2023

Purpose or Objective Treating lesions in the pancreas with stereotactic body radiotherapy (SBRT) is challenging due to proximity to highly radiosensitive and movable organs. Daily re-optimization of the original plan according to the changing anatomy is one of the innovative features of MR-guided radiotherapy that gives the opportunity to avoid toxity while not compromising target coverage. In this analyzis we aimed to determine the dosimetric benefit of using SMART for both organ at risk (OAR) and PTV coverage in 25 patients treated for lesions in the pancreas by determining the frequency of patients who would have theoretically suffered from a toxicity or coverage loss if the plan had not been adapted. Materials and Methods A total of 125 delivered fractions were analyzed. For each fraction the reason for re-optimization (insufficient coverage- COV, OAR constraint violation-OAR, unacceptable heterogenity-hot-spot) had been noted prospectively. Doses prescribed in the original plan were compared to the doses that PTV or OAR would be subjected to if the original plan was applied to the the anatomy of the day (predicted plan) and the dose that was actually delivered (re-optimized plan). Results The predicted plan fulfilled coverage and OAR constraints in 8 fractions (6.4%) in which re-optimized plan was not generated and in 6 patients (4.8%) re-optimized plan was considered less suitable than the predicted plan. Among the 111 re-optimized fractions (88.8%) the reason for adaptation were: 28 OAR (22.4%), 28 OAR + COV (22.4%), 48 COV (38.4%), 1 OAR+hot-spot (0.8%), and 6 hot spot (4.8%). In the 57 (45.6%) fractions when OAR constraints were violated the mean of duodenum hard constraints (0.5cc<36Gy and 1cc<33Gy) increased from 26.7 Gy and 25.4 Gy in the original to 30.1 and 27.6 Gy in the predicted plan, than decreased to 26.5 Gy and 24.8 Gy in the re-optimized plan. In the 76 (60.8%) fractions when coverage was predicted to be insufficient, the dose recieved by 95% of PTV decreased from 35.8 Gy in the original plan to 34.2 Gy in the predicted plan, than increased to 35.9 Gy in the re-optimized plan. When individual patients were considered, in 6 patients (24%) the predicted dose to the duodenum violated the constraints in all 5 fractions (mean of predicted dose of 39 Gy decreased to 31.8 Gy in the adapted plan) and in 3 patients (12%) the prescribed dose of all 5 fractions was predicted to cover less than 90% of PTV (86% coverage in the predicted plan increased to 97.5% in the adapted plan) Conclusion Plan adaptation in SMART for pancreatic lesions improves dosimetric profile by decreasing OAR doses in 45.6% of fractions, increasing coverage in 60.8% of fractions to at least original plan levels. Decreased duodenum toxicity in 24% and increase coverage in 12 % of patients can theoretically be achieved with SMART use. PD-0746 Dosimetric consequence of intra-fractional motion for stereotactic treatment of central lung lesions S.N. Thomsen 1,2 , D.S. Møller 3,4 , M. Knap 5 , T.B. Nyeng 4 , L. Hoffmann 3,4 1 Faculty of Health, Aarhus university, Department of Clinical Medicine, Aarhus, Denmark; 2 Aarhus University Hospital, Department of Oncology, Medical Physics, Aarhus , Denmark; 3 Faculty of Health Sciences, Aarhus University, Department of Clinical Medicine, Aarhus, Denmark; 4 Aarhus University Hospital, Department of Oncology, Medical Physics, Aarhus, Denmark; 5 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective Stereotactic radiotherapy (SRT) for small peripheral lung lesions results in high local control rates. However, former SRT trials on centrally located lesions have shown high toxicity levels including grade 5. In the STRICT-lung trial (NCT05354596), central lung lesions are treated with SRT, using an inhomogeneous dose distribution to the target, always favoring rigorous constraints to organs at risk (OAR). The mean dose to the GTV D(GTVmean) is escalated up to 85Gy in 8 fractions, but is limited by the OAR constraints. Due to the proximity to the OAR, this results in a steep dose gradient, very sensitive to intra-fractional shifts. In this study, we present the dosimetric consequences of intra-fractional motion observed for STRICT- lung patients (pts). Materials and Methods Eighteen pts have been treated in/ad modum the STRICT-lung trial. The PTV margin was 4mm. The pts were set up based on daily cone beam CT (CBCT) soft tissue target match to the planning CT (pCT). After treatment delivery, a second CBCT image was obtained for investigation of intra-fractional target shifts. Retrospectively, the two CBCTs were matched to the pCT on the target and the intra-fractional 3D target shift was calculated. For each fraction, contours delineated on the pCT were deformably transferred to both CBCTs and the dose was calculated based on stoichiometric calibration curves. The difference in D(GTVmean) and dose D(0.5cc) to the OAR closest to the lesion was calculated for all eight fractions and plotted as box plots. Results The median [range] target shift was 2.9mm [0.1, 14.2]. A boxplot of the shifts for each pt is shown in Fig 1A, together with the PTV margin. The target shifts were primarily in the cranial and dorsal directions. The median change in D(GTVmean) was 0.44Gy [-14.11, 5.61], meaning that some of the pts received far less target dose than planned due to intra-fractional target shifts (Fig 1B). For most of the pts, the OAR in the closest proximity to the lesion was shifted towards the high-dose region resulting in an increased dose (Fig 1C). Both target shifts and how they influenced the dose to the target and OARs were highly pt -specific and depended on both target location and planned dose distribution. Fig 2 illustrates the dosimetric and geometric shifts for pts 7 and 11. The direction of the shifts seen in these pts is similar to what is observed in most of the other pts. For pt 7 the bronchi move into the high-dose region, while for pt 11 the lesion moves out of the high-dose region resulting in a drop of the D(GTVmean) from 77Gy to 64Gy.