Evaluation of the positional differences between bone matching and fiducial marker matching in target localization of the expiratory breath-hold radiation therapy for pancreatic cancer.

Abstract

517 Background: In radiation therapy, the accuracy of target localization is very important to increase the agreement between the planned treatment and the actual dose delivered to the patient. Therefore, image-guided techniques, such as bone matching, soft-tissue matching, fiducial marker (FM) matching and so on, are used to improve the precision and effectiveness of radiation therapy. In this study, we evaluate the positional differences of the target localization between bone matching and FM matching in the expiratory breath-hold radiation therapy (BH-RT) for pancreatic cancer. Methods: We analyzed 15 patients who underwent BH-RT for pancreatic cancer using FM matching between July 2016 and August 2017 in our institution. The FMs were implanted in the pancreatic cancer using endoscopic ultrasound. Prior to each treatment session, bone matching using orthogonal kilovolt x-ray images and FM matching using cone beam computed tomography at end-exhalation (BH-CBCT) was performed. A total of 226 matching data from 15 patients was analyzed. The positional differences of these matching were evaluated in the left - right (LR), anterior - posterior (AP) and superior - inferior (SI) directions. Results: The systematic and random errors between bone matching and FM matching were 3.6 and 3.0 mm, 3.3 and 2.0 mm, and 3.7 and 3.5 mm in the LR, AP and SI directions, respectively. Absolute positional differences of > 5 mm were observed in 78 fractions (34.5%), 40 fractions (17.7%), and 69 fractions (30.5%), and those of even > 10 mm were observed in 14 fractions (6.2%), 2fractions (0.6%), and 11 fractions (4.9%) in each direction, respectively. There were no significant correlations between positional differences in SI and both LR and AP. Conclusions: In pancreatic irradiation using BH-RT, it was considered that bone matching alone was insufficient for accurate target localization and other target matching techniques, such as FM matching, were required to achieve effective treatment.