ESTRO 2022 - Abstract Book

S425

Abstract book

ESTRO 2022

Mini-Oral: 12: Head and neck

MO-0475 Simple strategy to select head and neck cancer patients for adaptive radiotherapy in the first week

Y. Gan 1 , J.A. Langendijk 1 , A. van der Schaaf 1 , L. van den Bosch 1 , E. Oldehinkel 1 , Z. Lin 2 , S. Both 1 , C.L. Brouwer 1

1 University of Groningen, University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands; 2 Shantou University, Cancer Hospital of Shantou University Medical College, Department of Radiotherapy, Shantou, China Purpose or Objective Adaptive radiotherapy (ART) enables correction of dose deviation in organs at risk (OARs) caused by anatomic changes during head and neck RT. The intensive workload for ART requires early identification of head and neck cancer (HNC) patients that benefit most during radiotherapy, not only to save workload/resource, but also to improve therapeutic ratio. The purpose of this study was to present a simple strategy to select candidate of ART in the first week of radiotherapy. Materials and Methods This retrospective analysis was based on 123 HNC patients who underwent photon radiotherapy and had at least 5 weekly in-treatment re-scan CTs (rCTs) available. In total, 15 OARs were segmented on each rCT using a semi auto-segmentation method (Gan et al., 2021). Then, the original treatment plan (VMAT or IMRT) was re-calculated on each rCT with full scale of prescription dose to obtain the actual weekly mean dose (D mean ) of OARs. The actual given D mean was defined as the average of all actual weekly D mean . We then applied the comprehensive NTCP-profile for HNC patients (van den Bosch et al., 2021)to translate the planning D mean and actual given D mean into nominal and actual NTCP values, respectively. The Δ NTCP were obtained by subtracting nominal NTCP with actual NTCP values. The Δ D mean between the first weekly actual D mean and planning D mean was calculated and designated as Δ D mean _ week1 . For each Δ NTCP, the maximum Δ D mean _ week1 of OARs which were involved in the NTCP model was designated as max Δ D mean _ week1 . The max Δ D mean _ week1 was then used to develop different logistic regression models to identify suitable candidates of ART who presented large Δ NTCP with different criterion Δ NTCP from 0 to 10 of percentage point. For different Δ NTCP criterion, the cutoff max Δ D mean _ week1 was calculated (R program package of pROC) to obtain the maximum summation of sensitivity and specificity, patients presenting Δ D mean _ week1 of any OAR ≥ the cutoff max Δ D mean _ week1 were designated as classified candidate of ART, patients presenting any Δ NTCP ≥ criterion of Δ NTCP were classified as true candidate of ART. Major Δ NTCP was defined as ≥ 5% and considered clinically relevant. Results In total, 7832 complete cases of NTCP values for different kinds of toxicity in the 6 th , 12 th , 18 th and 24 th month after the end of radiotherapy together with the corresponding max Δ D mean _ week1 were obtained. The Spearman’s rank correlation coefficient between max Δ D mean _ week1 and Δ NTCP was 0.42 ( p-value <0.001)(Fig.1). For major Δ NTCP, the cutoff max Δ D mean _ week1 was 5.14Gy, the proportion of true candidate of ART was 4.5%, the proportion of classified candidate of ART was 19.1% which included all the true candidate of ART. (Fig.2). Fig.1

Fig.2