ESTRO 2024 - Abstract Book

S4120

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

Material/Methods:

The first three patients enrolled in an on-going trial (ethical approval H-19083440) in which patients are examined with 18F-labeled fluoroazomycinarabinoside (FAZA) PET/CT at the beginning (scan 1), midway (scan 2), and near-end (scan 3) of their RT course were included in this retrospective planning study. An initial DE plan was created for each patient on their planning scan, where the HV from scan 1 was prescribed to an escalated dose of 78Gy in 34 fractions. The other dose levels were: 68, 60, and 50Gy to planning target volumes (PTVs) 1, 2, and 3, respectively, which corresponded to the clinical (non-DE) plan. HVs were defined as FAZA values within the GTV of ≥ 1.4 times the FAZA signal of a well-oxygenated reference muscle tissue [3]. HVs were rigidly propagated from scan 1 to the planning scan, scan 2, and scan 3 for treatment planning, and redefined at each timepoint using the FAZA scan 2 and 3 to verify continued coverage. Other structures were first deformed in MIM (MIM Software Inc.) from the planning scan to to scan 2 and 3, then reviewed and edited by an oncologist. The initial DE plan was recalculated on scan 2 and 3, and the adaptive plan was reoptimized based on scan 2 and was also recalculated on scan 3. Dose metrics for targets and OARs were extracted from the DE plans for all recalculation/reoptimization scenarios and for the clinically approved plan for comparison.

Results:

All three patients had HVs at scan 1 (HV1). Only patient 1 had a remaining HV at scan 2 (HV2), which was within HV1. No patients had any HV at scan 3 (Table 1).

Table 1: Hypoxic volumes (HV) throughout the RT course.

HV1 (cc)

HV2 (cc)

HV3 (cc)

Patient

1

20.7

5.6

0.0

2

1.1

0.0

0.0

3

2.5

0.0

0.0

Dosimetric results were mixed for the three patients (Figures 1 and 2). The benefits of adaptation were generally small, and the DE plans were mostly robust to anatomical changes, but not in all cases. The largest differences were observed for target coverage and Body D0.1cc for patient 1. In this patient, adapting midway (plan 2) reduced hot spots while maintaining target coverage relative to the initial plan for scan 2; however, plan 2 had reduced target coverage when recalculated on scan 3. Some of these differences for patient 1 could be explained by a significant change in shoulder position on scan 2, relative to scans 1 and 3, combined with a HV at shoulder level.