ESTRO 2021 Abstract Book

S1277

ESTRO 2021

treatment benefit relative to photon treatment in terms of differences in NTCP (∆NTCPs). For thoracic indications, treatment quality may be compromised by various sources of range uncertainty such as anatomical changes, respiratory motion, setup and machine errors, and more. Therefore, in this study we propose a fraction-wise method to assess the impact of these uncertainties on the robustness of ∆NTCPs determined for modality selection at any time during the treatment course. Materials and Methods Ten thoracic patients (lung , esophagus and lymphoma cases) treated at our center were retrospectively analysed. For every treatment fraction, spot delivery was linked to ten distinct breathing cycle phases of the repeated weekly CT through the use of treatment delivery logs and ANZAI signals. Fraction doses were obtained after warping weekly verification 4DCT phase doses to the reference end-of-exhale phase and subsequently accumulated on the planning 4DCT end-of-exhale phase, which is where the OARs were delineated. The dose reconstruction and accumulation (4DREAL) was performed in an automated workflow. For every fraction, relevant OAR doses D mean (heart), D mean (lungs-GTV), and D mean (esophagus) were obtained from the 4DREAL and converted to an NTCP value (NTCP fraction ) using the clinical protocols for the respective indications as shown in Table 1. Mean OAR dose variability was assessed. The expected NTCP of the proton plan (NTCP planning ) and the photon plan (NTCP VMAT ) were obtained from the clinical records. NTCP fraction , NTCP planning and the accumulated treatment course NTCP treatment were then compared to NTCP VMAT to determine

the respective NTCP quantities. V95(CTV) and D98(CTV) were assessed as well.

Results Mean dose variations in OARs were within 5% for every reconstructed fraction. The fraction-wise 4D reconstructed ∆NTCP fraction varied within 2 %, while the reconstructed ∆NTCP treatment was found to be 0.48 ± 0.08 % lower on average than ∆NTCP planning . V95(CTV) and D98(CTV) (as a percentage of prescribed dose) of accumulated treatment course doses were above 99.35% and 98.38% for all ten patients, respectively.

Figure 1 shows the different endpoint NTCP fraction

over a full lung cancer treatment course.