ESTRO 2021 Abstract Book

S961

ESTRO 2021

treatment options.

PO-1157 Prediction of delayed lymphopenia after chemoradiotherapy in non-small cell lung cancer B. Kang 1 , J.H. Lee 1 , L. Xue 2 , J. Son 3 , H. Wu 1 , H.J. Kim 1 , H. Kang 1 1 Seoul National University College of Medicine, Radiation Oncology, Seoul, Korea Republic of; 2 Seoul National University College of Medicine, Radiation Oncoloogy, Seoul, Korea Republic of; 3 Seoul National University College of Medicine, Radiation Onocology, Seoul, Korea Republic of Purpose or Objective Consolidation immunotherapy becomes a new standard treatment after definitive chemoradiotherapy (CRT) in locally advanced non-small cell lung cancer (NSCLC). However, the efficacy of immunotherapy can be compromised by treatment-related immune suppression status. We aimed to evaluate clinical and dose- volumetric predictors of delayed lymphopenia at the time of consolidation immunotherapy after CRT in locally advanced NSCLC. Materials and Methods We retrospectively reviewed 318 patients with locally advanced non-small cell lung cancer who received definitive CRT from January 2012 to December 2020. Patients diagnosed between 2012 and 2018 were assigned as training set (n= 244), and rest were selected as validation set (n=74). Absolute lymphocyte count(ALC) was recorded at baseline, during CRT, and 4 to 12 weeks after CRT. Dose-volume histogram parameters for planned target volume, whole body, heart, lung, great vessels, spleen, esophagus and thoracic vertebral bodies were evaluated. Dose-volume histogram parameters reported as mean, maximum and minimum dose with volume receiving 5 Gy (V5), 10 Gy (V10), 20 Gy (V20), 30 Gy (V30), 40 Gy (V40), 50 Gy (V50), and 60 Gy (V60). Overall survival (OS) was analysed using Kaplan-Meier survival analysis, log-rank test, and Cox regression model. Area under the receiver operating characteristic curve (ROC) analysis and multivariate logistic regression were used to determine the predictors of severe lymphopenia. Results Grade 3 lymphopenia (ALC nadir < 500/mm 3 ) were 81.1%, 48.1%, and 11.4% cases during CRT, 0 to 4 weeks, and 4 to 12 weeks after CRT, respectively. Kaplan-Meier survival curves and multivariate cox regression analysis revealed that patients with delayed lymphopenia (grade 3 lymphopenia during 4 to 12 weeks after CRT) was associated with inferior overall survival (P<0.0001). On multivariable logistic regression models, age (OR = 1.116; p = 0.018), baseline ALC (OR=0.998; p=0.010), albumin nadir during CRT (OR = 0.111, p = 0.001), and lung V5 (OR =1.089, p < 0.001) were significant predictors for delayed lymphopenia. The prediction nomogram was developed according to the above four independent variables. The AUC value of the nomogram model was 0.928. In the validation set (n = 74) the performance was 0.859. Conclusion Delayed lymphopenia is an independent predictor of poor overall survival. Age, baseline ALC, albumin nadir during CRT, and lung V5 revealed as the dominant contributor to delayed lymphopenia at the time of consolidation immunotherapy after definitive chemoradiotherapy. PO-1158 Validation of a spatial dose pattern predicting pulmonary toxicity in patients treated with VMAT for a lung cancer V. Bourbonne 1,2 , V. Jaouen 2,3 , M. Hatt 2 , O. Pradier 4,5 , D. Visvikis 5 , F. Lucia 4,5 , U. Schick 4,5 1 University Hospital of Brest, Radiation Oncology Department, Brest, France; 2 University of Brest, LaTIM, UMR 1101, INSERM, Brest, France; 3 Institut Mines - Télécom Atlantique, Departement Image et Traitement de l'Information, Brest, France; 4 University Hospital of Brest, Radiation Oncology Department , Brest, France; 5 University of Brest, LaTIM, UMR 1101, INSERM , Brest, France Purpose or Objective (Chemo)-radiotherapy is the standard treatment for patients with locally advanced lung cancer (LALC) not accessible to surgery. Despite strict application of dose constraints, acute toxicities such as acute pulmonary toxicity (APT) remain frequent, and may impact treatment’s compliance and patients’ quality of life. Previously, on a population treated with intensity-modulated photon therapy or passive scattering proton therapy, spatial dose patterns associated with APT were identified in the lower lungs, especially in the lower right lung. In the present study, we aim to define these spatial dose patterns on a retrospective cohort treated by volumetric-arctherapy (VMAT) and to validate our findings prospectively. Materials and Methods For the training cohort, we retrospectively included all patients treated in our institution by VMAT for a LALC between 2015 and 2018. APT was scored according to the CTCAE v4.0 scale. All dose maps were registered to a thorax phantom using a segmentation-based elastic registration. Voxel-based analysis of local dose differences was performed with a non-parametric permutation test accounting for n = 10.000 permutations, producing a 3-dimensional significance maps on which clusters of voxels that exhibited significant dose differences (p < 0.05) between the two toxicity groups (APT ≥ grade 2 vs APT < grade 2) were identified. A dose cut-off was defined by maximizing the Youden-Index derived from the Receiver Operative Curve. For the validation cohort, we applied the predefined spatial dose pattern and the dose cut-off to an observational prospective cohort. The model was evaluated using the Area under the curve (AUC) and the balanced accuracy (Bacc: mean of the sensitivity and specificity). Results 167 and 42 patients were included in the training and validation cohorts, with respective APT rates of 22.2% and 21.4%. In the training cohort, a cluster of voxels was identified in the lower right lung with a cut off of 30.3Gy for the Dmean (mean-dose). This spatial-based model resulted in an AUC of 0.69 and a Bacc of 0.71. Using the same voxel cluster and the Dmean cut-off on the validation cohort, the model resulted in an AUC of 0.74 and a Bacc of 0.69. Conclusion