Retrospective investigation of change in target and organ at risk for head and neck cancer patients during intensity modulated radiotherapy and volumetric arc radiotherapy

During prolonged treatment of Head and Neck cancer radiotherapy, volumetric and dosimetric changes have occurred. These changes may cause under dosing of target volumes and/or overdosing of surrounding normal and critical tissue. The aim of the study is to investigate anatomic and dosimetric change in patient under treatment of head and neck cancer. This study was a retrospective review of 10 nasopharyngeal cancer patients treated IMRT or VMAT techniques. A CT0 for treatment and two subsequent CTs at the 15th (CT15) and 25th (CT25) fractions were acquired. Two simulated adapted plans and hybrid plan were made with CT15 and CT25. Primary tumor and lymph nodes volumes decreased at rate of 22.7% to 32.2% and 31.5% to 39.4% respectively. ipsilateral and contralateral parotid gland volumes decreased at rate of 29.5% to 40.6% and 25,5% to 33.5% respectively. These volume changes were significant. Neck diameter reduction was 5.7% and 7.4% respectively, but the difference was significant. Compared to the initial plans no significant differences of dose coverage CTVs and PTVs and maximum dose of spinal cord and brain steam were observed. The mean dose (Dmean) of ipsilateral and contralateral parotid increased at ratio of 13.5% to 20.2% and 10.9% to16.5% for adaptive plans and 16.5% to 22.1% and 14.8% to 19% for hybrid plans respectively. The mean dose changes for parotids were significant. As a result of the study, it was observed that the volume of tumor and critical organs changed during the treatment of head and neck tumors and the doses of these volumes changed accordingly. Correction of these changes during treatment will increase the success of the treatment.