ESTRO 2023 - Abstract Book

S654

Monday 15 May 2023

ESTRO 2023

The difficulty of defining and delineating the clinical target volume for high-grade gliomas (HGGs) is a well-recognized problem within the ESTRO community which lead to the formation of a working group working group aiming at replacing the cumbersome manual process of defining the CTV with a largely computerized process that allows user adjustments. Within this framework, the present work introduces a model for determining the tumor invasion outside the observable gross tumour volume (GTV) and its capacity of predicting the outcome of the treatment and hence guiding the definition and delineation of CTV for HGGs. Materials and Methods A model for the tumor invasion of the normal tissue using a Fisher-Kolmogorov equation which describes tumor cell concentration in space and time by accounting for cell proliferation and cell diffusion was applied on a cohort of 93 HGGs patients. The infiltration of the tumor cells into the normal tissue was quantitatively described by defining volumes encompassed by isocontours with different cell densities assuming that the density of the cells defining the GTV is 8000 cells/mm3, the same as the detectability threshold on MRI scans. The correlation between the volumes defined by different isocontours and the treatment outcome expressed as overall survival (OS) time was sought in terms of ROC curves and Kaplan-Meier survival analysis and compared with the predictions made based on the GTV volume. Results Among the volumes defined by isocontours, those corresponding to 1000 cells/mm ³ (Fig. 1.a) and 2000 cells/mm ³ , respectively, resulted in the highest correlation with the OS as rendered by the ROC analysis (Area -Under-the-Curve, AUC, of 0.767, p-value < 0.001) comparable with the predictions based on the GTV (AUC = 0.726, p-value < 0.001). The Kaplan- Meier survival analysis, however, using the 1000 cells/mm ³ isocontour (Fig 1.b) and the ROC optimal cut-off volume (i.e. Youden threshold) for patient group selection rendered a p-value < 0.0001 and a hazard ration (HR) of 2.7 while for the GTV (Fig 1.c) the predictions were much poorer from the statistical point of view (HR = 1.6 and p = 0.029).

Figure 1: a) Example of a GTV and isocontour corresponding to a predicted tumor cell density of 1000 cells/mm3. The simulated infiltration appears anisotropic as it conforms to natural barriers and has a preference of spreading via white matter. b) Kaplan-Meier survival analysis when using a binary survival classification based on volume of GTV, and c) volume of V1000. There is greater separation between survival curves when using the V1000 as parameter, than when using the GTV, indicating that V1000 is a better predictor of overall survival. Conclusion The volume defined by the isocontour of 1000 cells/mm3 obtained by modelling the tumour invasion is a stronger predictor of overall survival than the GTV indicating the importance of using mathematical models for cell invasion to assist the definition of the clinical target distribution for HGGs patients. OC-0784 Dosimetric comparison between IMRT and Hyperarc in re-RT of recurrent nasopharyngeal carcinoma K.K. Tong 1 1 Pamela Youde Nethersole Eastern Hospital, Department of Clinical Oncology, Hong Kong , Hong Kong (SAR) China Purpose or Objective Re-irradiation of locally recurrent nasopharyngeal carcinoma (rNPC) is usually performed with Intensity Modulated Radiation Therapy (IMRT). However, it is often associated with sub-optimal target coverage due to tight constraints of the previously heavily irradiated organ-at-risks (OARs). Although manual use of non-coplanar beams might improve the situation, the process is often un-intuitive and in-efficient. In recent years, there are growing interests in expanding the use of an automated non-coplanar volumetric modulated arc therapy technique ‘Hyperarc’ (HA), which was originally introduced for stereotactic radiosurgery, onto extra-cranial cases. This study aims to investigate the potential dosimetric advantages of HA in terms of target volume coverage, plan quality metrics and sparing of OARs in re-RT of rNPC. Materials and Methods Twenty patients with locally rNPC who have previously received a 70Gy-irradiation for primary disease were retrospectively included in this study. For each patient, treatment plans for the recurrent disease were computed using IMRT and HA. The first-priority cumulative BED limits for OARs were defined as 130% of the primary course BED limit. In case the first-priority constraints could not be fulfilled, the secondary-priority cumulative BED limits, which were defined as 150% of the primary course BED limit for all OARs, except 160% for temporal lobes, were followed. The planning goal was to give maximal permissible dose to PTV within the OAR dose limits. Dose delivered to target volumes, plan quality metrics (i.e. PTV V100%, conformity index CI, homogeneity index HI, high-dose spillage and gradient index GI), and dose delivered to the planning-