Comparison of Conventional and Optical Positioning System for Non-Coplanar Beam Radiotherapy of Nasopharyngeal Carcinoma

摘要

Purpose:A new positioning method called Optical Positioning System (OPS) method has been developed based on the Polaris spectra system. Both the OPS method and the Conventional Positioning System (CPS) method were used in the non-coplanar beam radiotherapy of nasopharyngeal carcinoma. In this research, the results of the two methods were compared in terms of their performance. Method: Eleven patients diagnosed with nasopharyngeal carcinoma were enrolled for non-coplanar beam radiotherapy to prescribed dose of 67 Gy-73 Gy. The patients were positioned by OPS other than CPS, and each patient was checked by physicist using EPID. The deviation of the positioning was taken into the original plan as input respectively. The dose distribution was recomputed. The performance of the OPS method and the CPS method were compared, using the following criteria: Gross Tumor Volume (GTV) D98%, Clinical Tumor Volume (CTV) 095%, max dose and average dose to tolerated dose ratio of the organs-at-risk (OARs). Results: Both the two positioning methods showed comparable immutability of GTV 098% and CTV D95%. The OPS positioning was more immutable in contrast. In terms of protecting the OARs, the OPS positioning keeps both the max dose and average dose of the lens much lower than the CPS. For the CPS the dose received by OARs increased more than 30% and the max dose even exceeded the tolerated dose of lens in 2 cases when the distance between beams and the OARs was less than 4 mm. Conclusion: The results shows a distinct advantage in using OPS method especially when the beams are very close to the OARs for non-coplanar beams radiotherapy of nasopharyngeal carcinoma. While the CPS method usually limits the treatment effect or sacrifices the OARs, the OPS positioning can obtain both good treatment effect and OARs sparing.