Anatomical and Dosimetric Changes during IMRT for Oropharyngeal Cancer Detected by Weekly Cone-Beam CT With Deformable Image Registration

Abstract

Re-planning is often necessary during IMRT for head and neck cancer due to tumor regression and changes of body contour. CBCT was obtained for patients with OPC once a week to confirm anatomical and dosimetric changes of tumor and organs at risk (OAR). Ten patients with OPC treated by a two-step IMRT method were included. For all patients, treatment-planning computed tomography (CT) was done twice before and at the fourth week for initial and boost IMRT, respectively. All 10 patients were given 70 Gy/35 fractions with concurrent chemotherapy (CDDP; 5, Cetuximab; 4, CBDCA 1). On board CBCTs were obtained on days 1, 8, 15, 22, and 29 of IMRT. Deformed volumes of tumor and OARs in the CBCT were compared with those in the planning CT. Dose distribution at the isocenter level was recalculated based on weekly CBCT image and compared with the initial planned dose distribution. Area of body surface, PTV D2, D50 and D98, and Dmax of the spinal cord on the isocenter level were selected for analysis. After volumes on initial treatment-planning CT were deformed into CBCT images on days 1-29, change rates from CBCT images on day1 were determined for CBCT images from day 8 to day 29. Change rates of more than 5% in area or dose were regarded as significant change. Results: Average change rates and number of patients with significant change were shown in Table 1. Area of body surface decreased gradually. After day 22, five of the 10 patients showed decrease of body area of more than 5%. Area of body surface and Dmax of the spinal cord were inversely correlated (r = -0.47). After day 22, Dmax of the spinal cord increased more than 5% for eight patients (80%). Notably, PTV D2, D50 and D98 were not changed during the period from day 1 to day 29. In patients with OPC treated by a two-step IMRT method, weekly CBCT demonstrated decrease of body surface and increase of Dmax of the spinal cord after 22-29 days of treatment. Although changes in PTV doses were minimal, re-planning may be necessary to protect spinal cord toxicity when significant decrease of body area was noted.Abstract 2812; Table 1Average change rate and number of patients with significant changeAverage change rateNumber of patients with significant changeDay1Day8Day15Day22Day29Day1Day8Day15Day22Day29Body surface(cm2)100%97%97%96%96%Body surface(cm2)03356PTV D98 (Gy)100%99%99%99%98%PTV D98 (Gy)00001PTV D50100%100%100%100%100%PTV D5000000PTV D2100%100%100%100%100%PTV D200001Spinal cordDmax (Gy)100%101%102%101%104%Spinal cordDmax (Gy)02589 Open table in a new tab