ESTRO 2021 Abstract Book

S816

ESTRO 2021

Conclusion Paranasal sinus tumours are heterogeneous group of histological tumours. Due to variable histology, no single treatment algorithm has been validated. Surgery in the mainstay of therapy for these patients, but combination of radiation and chemotherapy improves end-points outcomes in advanced cases. PO-0982 early efficacy prediction of NPC based on 3D-ADC during radiotherapy: a phase II prospective study M. Feng 1,2 , L. Yan 3 , X. Du 1 , H. Wang 1 , J. Ren 1 , M. Wang 1 , Q. Yin 1 , X. Lai 1 , L. Li 1 , M. Lan 1 , S. Lu 1 , Y. Huang 4 , F. Li 4 , X. Xu 5 , W. Wang 1 , J. Lang 1 1 Sichuan Cancer Hospital and Institute, Sichuan Cancer Center, School of Medicine, University of Electronic Science and Technology of China, Department of Radiation Oncology, Sichuan Cancer Hospital & Institute, Sichuan Cancer Center, Radiation Oncology Key Laboratory of Sichuan Province, Chengdu, China; 2 The Third People's Hospital of Sichuan Province, Department of Oncology, The Third People's Hospital of Sichuan Province, Chengdu, China; 3 Chengdu Medical College, Department of Radiation Oncology, Sichuan Cancer Hospital & Institute, Sichuan Cancer Center, Radiation Oncology Key Laboratory of Sichuan Province, Chengdu, China; 4 Sichuan Cancer Hospital and Institute, Sichuan Cancer Center, School of Medicine, University of Electronic Science and Technology of China, Department of Radiation Oncology, Sichuan Cancer Hospital & Institute, Sichuan Cancer Center, Radiation Oncology Key Laboratory of Sichuan Province, Chengdu, China; 5 University of Electronic Science and Technology of China, University of Electronic Science and Technology of China, Chengdu, China Purpose or Objective Concurrent chemoradiotherapy (CCRT) is the main treatment model for locally advanced nasopharyngeal carcinoma (LANPC). Previous study found the apparent diffusion coefficient (ADC) value of primary tumor partly correlated to prognosis for NPC patients. Recent studies reported that the changes of ADC value at pre- and post-treatment was a more sensitive predictor for prognosis. The prognosis for LANPC patients who received the same treatment strategy varies greatly. The early predictor was urgently needed to help for adjusting the treatment strategy. Thus, we aimed to observe the dynamic changes of ADC of gross target volume (GTV) and metastatic lymph nodes (LN) for NPC patients during radiotherapy (RT), and explored the potential correlations between ADC changes and treatment response. Materials and Methods A total of 50 squamous NPC (II-IVa, the 8 th TNM staging system) patients treated with CCRT were enrolled in this study. All patients received a planned total dose of 66 to 70Gy in 33 fractions at 5 fractions per week using Image-guided intensity modulated radiotherapy (IGRT). The regimen was cisplatin-based chemotherapy for 3 cycles. The patients received diffusion weighted imaging ( DWI) scan at pre-RT, the 5 th and 15 th fractions and immediately post-RT on a 3.0T MRI system (b=0, 500, 800s/mm 2 ) system. The ADC values of GTV and LN were acquired using MIM software. RECIST1.1criteria was used to evaluate the tumor response. Multivariate analyses were calculated using the logistic regression model. Receiver-operating characteristic (ROC) curve was used to determine the cut-off value of ADC. Results After the completion of CCRT, the objective response rate (ORR) for all the patients was 100% (CR 58%, PR 42%). The mean ADC value of GTV and LN gradually increased from pre-RT to post RT. These changes generally occurred in two stages. For the first stage (from pre-RT to the 5 th fraction), the mean ADC value of GTV and LN increased significantly by 20.78% and 22.96% respectively, while the volumes of GTV and LN had minimal changes (GTV:18.42cm 3 vs 16.72cm 3 , LN: 4.67cm 3 vs 4.12cm 3 ). As for the second stage (from the 5 th fraction to post-RT), the mean ADC value of GTV and LN continued to increase by 45.58% and 41.2% respectively, while the volumes decreased by 86.07% (GTV) and 61.6% (LN). Multivariate analysis showed the mean ADC changes of GTV from pre-RT to the 5 th fraction (ΔADC5) was the only independent prognostic factor for treatment response of NPC. ROC showed that the cut-off value for ΔADC5 was 158.82mm 2 /s (sensitivity:87%; specificity:65%). Conclusion Mean ADC values of GTV and LN increased dramatically during RT for NPC patients treated with radiotherapy. The mean ADC began to increase at the 5 th fraction, while the change of volume was minimal. Early ADC changes at the 5 th fraction might be a new and sensitive biomarker to predict the prognosis for NPC patients independently of volume change.