The influence of MRI scan position using a novel head and neck immobilization device on radical radiotherapy for nasopharyngeal carcinoma.

Abstract

e18030 Background: To investigate the influence of MRI scan position on the registration accuracy, gross tumor volume (GTV) delineation, and radiotherapy planning for radical treatment of nasopharyngeal carcinoma when using a novel head and neck immobilization device. Methods: 20 patients with nasopharyngeal carcinoma received a standard diagnostic MRI (MRI D ), as well as a computed tomography (CT) and an MRI in the radiotherapy position (MRI RT ) using a novel head and neck immobilization device. Clinicians delineated the gross tumor volume (GTV) on CT images, including the CT viewing the MRI D separately (GTV C ), the CT registered to MRI D (GTV D ), and the CT registered to MRI RT (GTV RT ). Similarly, the planning target volumes (PTVs) were delineated as PTV C , PTV D , and PTV RT respectively. The registration quality was evaluated by assessing the differences in tumor volume across the three set-ups. Intensity-modulated radiotherapy (IMRT) plans were developed for PTV C , PTV D , and PTV RT . To determine the dose received by the reference PTV RT , the optimization process was conducted based on PTV C and PTV D while calculating the dose to PTV RT . Statistical significance was determined using the two-tailed Mann-Whitney test or two-tailed paired student t-test. Results: Compared to the distance from the structure center in MRI D , the registration accuracy between CT and MRI RT significantly improved, with a geometric mean error of 1.6 mm versus 4.3 mm. The mean GTV C (90.33 cm³) was significantly larger than GTV D (82.52 cm³, p = 0.019) or GTV RT (75.82 cm³, p = 0.012). When optimizing the IMRT plans for PTV C and calculating the mean dose to PTV RT , neither the dose to 99% (63.7%) nor 95% (88.5%) of the PTV met the required clinical dose constraints of 90% and 95%, respectively. Similarly, when optimizing the IMRT plans for PTV D , the mean dose to PTV RT failed to meet the clinical dose constraints of 99% (30.8%) and 95% (75.6%) of the PTV. Only by optimizing for PTV RT were all clinical dose constraints achieved. Conclusions: Significant improvements in CT-MR image registration accuracy, target delineation clarity, and PTV dose coverage are observed when MRI scans are performed in the radiotherapy position using a novel head and neck immobilization device for patients with nasopharyngeal carcinoma. Meanwhile, the novel head and neck immobilization device has been patented by the authors and is available at a cost of only a few hundred RMB, making it affordable and easy to implement. Its widespread adoption could significantly enhance the precision of nasopharyngeal carcinoma radiotherapy across various radiotherapy centers, bringing considerable social benefits.