ESTRO 2020 Abstract Book

S82 ESTRO 2020

Results There was negative correlation between ADC and SUVmax in the GTV group but it was insignificant. In the lymph node (LN) group, there was statistically significant negative correlation between ADC and SUVmax (Kendall’s tau b = - 0.4872, p-value = 0.0216). The volumes of DWITV and PETTV were similar with no significant difference in the GTV and LN group. DCE and JCE similarity indexes were significantly different in the LN group for DWITV and PETTV after deformable registration. (DSC: DWITV p = 0.045, PETTV p = 0.005; JSC: DWITV p = 0.038, PETTV p = 0.017). Conclusion The use of PET/MR in RT planning, illustrated that DWITV and PETTV had similar volume sizes. DWITV and PETTV had smaller volumes than GTVs defined by treating oncologists. These smaller defined target volumes by DWI and PET images could lead to tighter treatment volumes. Deformable registration of PET/MR was essential in the neck region to provide greater overlapping with LN targets and to achieve accurate target delineation. Target volume delineation could be improved by using multi-parametric PET/MR images in RT planning for head and neck cancers treatment. PH-0169 Dosimetric comparison of IMRT and IMPT in the treatment of recurrent nasopharyngeal carcinoma H.M. Hung 1 , O.C.M. Chan 2 , C.H. Mak 1 , W.T. Ng 1 , M.C.H. Lee 2 , W.M. Hung 1 1 Pamela Youde Nethersole Eastern Hospital, Clinical Oncology, Chai Wan, Hong Kong SAR China ; 2 Pamela Youde Nethersole Eastern Hospital, Medical Physics, Chai Wan, Hong Kong SAR China Purpose or Objective Re-irradiation of locally recurrent nasopharyngeal carcinoma (rNPC) is always challenging. Target volume coverage is often suboptimal as organs at risk (OARs) usually receive significant dose in the primary course of radiotherapy. While most patients with rNPC are treated with Intensity Modulated Radiation Therapy (IMRT), there is growing interest in the potential dosimetric and clinical advantages of Intensity Modulated Proton Therapy (IMPT) to provide a wider therapeutic window. This study aims to evaluate the dosimetric performance of IMPT compared to IMRT in terms of target volume coverage and sparing of neurological OARs. Material and Methods Twenty patients with locally rNPC who received second course of IMRT were retrospectively re-planned using IMPT (Varian Eclipse External Beam Planning System: Version 15.6). All of them had been previously treated with IMRT to 70Gy in the primary treatment. Compared with 9-fields sliding window IMRT re-irradiation, 3-4 fields robust- optimized IMPT plan was computed with field-specific target and uncertainties (range +/-3 % and ISO shift 3 mm, total 12 perturbation scenarios). The planning goal was to maximize dose coverage of volume of 60Gy to Clinical Target Volume (CTV) without exceeding the neurological OARs tolerances. The accumulative dose limits of OARs were defined as 130 % of dose constraints of OARs of primary course. The dose received by Gross Tumor Volume (GTV) (V100%), CTV (V100% and V98%), planning neurological OARs volume (PRV)(i.e. brainstem, spinal cord, optic nerves, optic chiasm, and temporal lobes), internal carotid arteries (ICA) and nasopharyngeal (NP) mucosa were compared. Results IMPT plans significantly improved the prescribe dose coverage received by GTV V100% (+10.8%), CTV V100% (+10.4%) and CTV V98% (+2.3%) with p-values of <0.05. IMPT plans also achieved much better neurological OARs sparing, which showed significant reduction (p < 0.05) in average maximum dose (D max) to spinal cord (-2.51Gy); brainstem (-3.01Gy); optic chiasm (-3.02Gy); left optic

nerve (-1.72Gy); right optic nerve (-2.04Gy); left temporal lode (-3.45Gy) and right temporal lode (-2.41Gy). These superiorities were attributed by the physical property of Bragg peak with proton beam. However, there was a paradoxical increase in the mean hotspot dose with IMPT which was 16. 5% higher than that of IMRT, and this led to significant increase in doses to ICA and NP mucosa. Conclusion Target under-dosage is a potential cause of treatment failure in rNPC patients undergoing re-irradiation. IMPT is a promising novel treatment technique that demonstrates superiority in providing more conformal dose coverage compared to IMRT. However, clinicians need to be aware that dose to the ICA and NP mucosa are often exceedingly high, which may result in increased risk of carotid blow- out and massive epistaxis. Careful clinical and dosimetric evaluation is required when deciding on the treatment strategy. PD-0170 Volumetric regression in brain metastases after stereotactic RT: Time course and significance F. Putz 1 , D. Oft 1 , R. Perrin 1 , V. Mengling 1 , T. Weissmann 1 , J. Roesch 1 , S. Mansoorian 1 , L. Distel 1 , C. Bert 1 , R. Fietkau 1 1 Friedrich-Alexander-Universität Erlangen, Strahlenklinik, Erlangen, Germany Purpose or Objective Currently there is insufficient understanding of the natural course of volumetric regression in brain metastases treated with stereotactic radiotherapy (SRT). More evidence on the volumetric definition of response following stereotactic radiotherapy is needed to establish volumetric criteria for standardized assessment in clinical trials. Material and Methods Volumetric analysis via whole-tumor segmentation in contrast-enhanced 1 mm³-isotropic T1-Mprage sequences before SRT and during follow-up. A total of 3145 MRI studies of 419 brain metastases from 189 patients were segmented. Progression was defined using a volumetric extension of the RANO-BM criteria. A subset of 205 metastases without evidence of progression and follow-up ≥3 months was used to study the natural course of volumetric regression after SRT. Predictors for volumetric regression were investigated. The prognostic significance of volumetric response at 3 months (defined as ≥20% and ≥65% volume reduction, respectively) for subsequent local control was evaluated for all available metastases. Results Median relative metastasis volume post-SRT was 66.9% at 6 weeks, 38.6% at 3 months, 17.7% at 6 months, 2.7% at 12 months and 0.0% at 24 months. Radioresistant histology and FSRT vs. SRS were associated with reduced tumor regression for all time points. In multivariate linear regression, radiosensitive histology (p=0.006) was the only significant predictor for metastasis regression at 3 months. Volumetric regression ≥20% at 3 months post-SRT was the only significant prognostic factor for subsequent local control in multivariate analysis (HR 0.63, p=0.023), whereas regression ≥65% was no significant predictor. Poster discussion: CL: CNS