ESTRO 2023 - Abstract Book

S805

Monday 15 May 2023

ESTRO 2023

patient specific dose escalation factor to the HTV (Hypoxic Target Volume) is defined using the calculated OER normalised to normoxic OER of the patient population. We investigate a split-treatment approach for varying hypoxic tumour sub volumes at different points of time during treatment; at baseline and after 9-11 fractions of radiotherapy for a cohort of seven HNSCC (Head and Neck Squamous Cell Carcinoma) patients (fig 1).This split-treatment approach is compared against three plans; conventional IMRT/IMPT plans and hypoxia-adapted proton plan. Dosimetric differences within the plans were translated into predicting clinical outcomes with the help of tumour control probability (TCP) and normal tissue complication probability (NTCP) models.

Results We observe a median increase of 60% in hypoxic volume after 9-11 fractions of radiotherapy along with spatial changes. However, the amount of hypoxia decreases indicating a median lower dose escalation factor of 9% as compared to 15% for overcoming the baseline hypoxia. We predict an improved median tumour control probability of 10% with the dose escalated plan and 7% with the split-treatment proton dose escalation compared to conventional proton plans. TCP is a function of the change in hypoxic volume with time as depicted in fig 2b. Larger hypoxic volume changes and a larger dose boost leads to an increased TCP. Median NTCP's over all patients were reduced for the three toxicities (xerostomia, dysphagia and acute mucositis) for all proton plans compared to the IMRT plans (deltaNTCP's < 0 in fig 2a). Furthermore, dose-escalated proton plans don’t lead to an increase in OAR dose as compared to conventional proton plans except in the cases where the organ at risk (e.g., superior pharyngeal constrictor muscles) lies within the planning volume which leads to an increase NTCP for dysphagia (fig 2a).

Conclusion An evolving hypoxia based adaptive planning was studied. A workflow to consider varying hypoxia during the treatment has been designed. In conclusion, we observe better tumour control with a similar dose to the OARs with an adaptive planning approach compared to conventional proton plans. MO-0957 Optimal time for early therapeutic response prediction in NPC with multi-parametric MRI W.L. Mui 1,2 , W.M. Lee 2 , W.T. Ng 2 , H.F. Lee 2 , V. Vardhanabhuti 3 , S.Y. Man 1 , T.T. Chua 4 , X.Y. Guan 2 1 Hong Kong Sanatorium and Hospital, Department of Radiotherapy, Hong Kong, Hong Kong (SAR) China; 2 The University of Hong Kong, Department of Clinical Oncology, Hong Kong, Hong Kong (SAR) China; 3 The University of Hong Kong, Department of Diagnostic Radiology, Hong Kong, Hong Kong (SAR) China; 4 Hong Kong Sanatorium and Hospital, Department of Medicine, Hong Kong, Hong Kong (SAR) China

Purpose or Objective