ESTRO 36 Abstract Book

S34 ESTRO 36 _______________________________________________________________________________________________

Table 1. Total number of features per category with a minimum CCC>0.85 before and after wavelet filtering in all three datasets. Shape features are not calculated after wavelet filtering. Conclusion We found a high agreement between radiomics feature stability based on 4DCT and test-retest data in lung cancer, meaning that 4DCT can be used as alternative to test-retest to eliminate unreliable features. For oesophageal cancer a subset of 205 stable CT radiomics features was identified, of which 64 (31%) were not stable in NSCLC. This underlines the need for tumour-specific feature selection prior to model building. OC-0068 Heterogeneous dose escalation in lung: How robust are high FDG-uptake volumes during radiotherapy? A. Haraldsen 1 , C. Lutz 2 , L. Hoffmann 2 , A. Khalil 3 , D. Møller 2 1 Aarhus University Hospital, Department of Nuclear Medicine & PET Center, Aarhus C, Denmark 2 Aarhus University Hospital, Department of Medical Physics, Aarhus C, Denmark 3 Aarhus University Hospital, Department of Oncology, Aarhus C, Denmark Purpose or Objective The Danish NARLAL 2 Trial (NCT02354274) compares 18F- fluorodeoxyglucose (FDG)-guided dose escalation to the current standard radiotherapy (RT) treatment (66Gy/33fx homogeneously). The dose is escalated heterogeneously delivering mean doses up to 95Gy/33fx to the high FDG uptake volumes of the primary tumour aiming to improve local control for patients with locally advanced Non-small Cell Lung Cancer (NSCLC). In this study, we investigated the robustness of these high FDG uptake regions within the Gross-Tumour-Volume (GTV) during the first weeks of RT and we calculated the influence of these changes on the dose coverage. Material and Methods We evaluated three successive FDG-PET/CT scans performed on 20 patients. The baseline scan (PET/CT0) was dated 14 days prior to RT. The first and second scan during RT were performed on the same scanner at day 7 (PET/CT1) and 14 (PET/CT2). These two scans were rigidly registered to CT0 on the primary tumour. A sub-volume, defined by a threshold of 50% of the Standardized Uptake Value peak (SUVpeak), was delineated within the GTV on each scan (V0 50, V1 50 and V2 50 ). The overlap of the volumes was evaluated with the widely used overlap fraction (OF) and the OF-boost (see definitions in Fig. 1). The OF-boost specifies the fraction of respectively V1 50 or V2 50 that is included in the initial escalation volume, V0 50, and is thus more clinically relevant for this trial. The escalated dose plans were then recalculated on CT2 and the dose to V2 50 was compared to the planned dose to V0 50 on CT0 to estimate the effect of varying FDG uptake on the dose coverage.

Results Median values for OF 0-1

and OF 0-2

were 0.79 [0.56;0.91] and

0.85 [0.58;0.95]. OF-boost 0-1 yielded slightly lower median values of 0,77 [0,52-0,91] and 0.82 [0,58-0,92], respectively. For 70% of the patients, we found an OF-boost>70% for all scans (Fig. 2 (a)). These patients showed only small or no dose decrements to V2 50 (maximum 1,3 Gy), indicating that a 70% OF-boost is sufficient to maintain the initial escalation dose. In 30% of the patients, the OF-boost dropped below 70% (Fig. 2 (b)) and the mean dose to the high FDG uptake volumes decreased with up to 4 Gy. For these patients, we found that SUV peak declined to ~2 times background activity, rendering the definition of V1 50 and V2 50 questionable. and OF-boost 0-2

Conclusion For the majority of the patients, the high FDG uptake sub- volume and the escalated dose level were maintained during RT. For 30% of the patients, the OF-boost decreased below 70% and a drop in dose coverage to the high FDG uptake volumes was observed. For these patients the definition of a 50% of SUV peak volume during RT was questionable due to low SUV peak values.