Description
Head and neck cancers are among the most complex sites to deliver radiation therapy due to patient anatomy, compact nature of critical structures, and dynamic changes in structures over the course of treatment. This is coupled with the fact that detrimental side effects can occur due to over dosing of critical organs or under dosing the tumor. In this study, patient treatments were retrospectively analyzed utilizing a prototype of the Varian Eclipse™ treatment planning system integrated with their new DART™ technology. Ten patients with head and neck cancer who received daily cone-beam imaging CTs, as part of their radiation therapy, were chosen from the University of California San Diego Moores Cancer Center database. These 10 patients had between 9 and 33 treatment fractions of intensity-modulated radiation therapy (IMRT), with 8 of the 10 patients treated with static beam angles and the other 2 patients treated using RapidArc™ therapy. DART™ is an image registration and dose accumulation system that stands for Dynamic Adaptive Radiation Therapy. With the aid of DART™, accumulated dose measurements to both the tumor and healthy tissues over the entire course of treatment were calculated through retrospective analysis. This process was accomplished through rigid and deformable registrations of their daily cone-beam CT scans and cumulative dose measurements. Volumetric changes were compared to their original planning CT image. Structure volume changes, structure displacements, and dose deviations were analyzed to assess the need for more adaptive treatment planning. Results showed notable changes to the parotid glands in all patients, in both structure shrinkage and dose escalation, which were often correlated with a medial displacement of the glands towards the high-dose region. It was estimated that the mean rate of parotid volume reduction was 0.18 cm3/treatment day with a range of 0.08 -- 0.31 cm3/treatment day. GTV, CTV, and PTV regions also showed significant volume reductions which were shown to be more pronounced during the first half of treatment. In 6 out of 9 patients with contoured parotids, the absorbed dose increase to these glands was greater than 3%. For 3 of these patients, the dose accumulated was greater than 10% of the original planned value. The GTV, CTV, and PTV showed smaller variations, and in only 1 out of the 10 patients studied did the dose accumulation increase greater than 3%. Patients with the greatest number of treatments showed the greatest structure volume changes, yet no patterns of structure dose escalation could be linked to the number of treatment fractions. While the program used was just a prototype, hopefully this project could be integrated clinically in the future to develop more adaptive treatment plans allowing for better treatment accuracy and patient care.
