Intensity Modulated Radiotherapy on a Novel Ring Gantry-Based Linac with Integrated Dual PET-CT Imaging Systems: Dosimetric Plan Comparison and Initial Clinical Delivery Experience

Abstract

<h3>Purpose/Objective(s)</h3> This study aims to report our initial clinical experience on using a new external-beam radiotherapy delivery system on a ring gantry with dual PET-CT imaging capabilities for IMRT treatments at different anatomical sites. The study also evaluates the system's plan quality compared to an existing ring gantry-based Linac. This new external-beam radiotherapy delivery system has the potential to be used for biology-guided radiotherapy. <h3>Materials/Methods</h3> We performed a single-institution retrospective review of patients treated on the machine since commissioning. The initial five patients treated to a thoracic site and five patients treated to a pelvic site were included in this study. The dose per fraction ranged from 1.8 to 3.0 Gy. The treatment plans for the machine used a 2-cm jaw size along the longitudinal direction and used the IMRT delivery mode. A 64-leaf binary MLC was used to modulate a flattening filter free 6-MV photon beam. A helical tomotherapy plan (HT plan) using a 2.5-cm jaw size was generated for each RFX plan to serve as the backup plan in clinical treatments. Dosimetric parameters used in institutional treatment protocols were compared between RFX plans and HT plans. Daily delivery records were reviewed, and actual beam-on time was compared between the two machine types. <h3>Results</h3> On average, 12 dosimetric parameters (range: 7 – 14) for critical organs and the primary PTV were used in each treatment plan as clinically relevant planning optimization parameters and were analyzed in this study. Among the critical organ dose parameters, 19 (22.4%) dosimetric parameters were lower by more than 10% in the study's plans compared to the HT plans, while 11 (12.9%) dosimetric parameters were higher by more than 10% in the study's plans compared to the HT plans. The other dosimetric parameters had less than 10% or no clinically relevant difference between the two plans. There was no statistically significant difference in integrated dose between the new machine and HT plans. All the plans were normalized so that the prescription dose covered 95% of the primary planning target volume (PTV). The average D_mean of the primary PTV were 103.5±1.1% and 102.6±0.8% of the prescription dose in the new external-beam radiotherapy delivery system and HT plans, respectively, and the average D(1%) of the primary PTV (dose covering the hottest 1% of the PTV) were 107.1±2.0% and 105.2%±1.6% in the new external-beam radiotherapy delivery system and HT plans, respectively (two-tailed p-value = 0.06 in the paired t-test). The beam-on time was not significantly different between the two machines. The average daily beam-on time was 438.8±151.2 seconds and 427.6±183.1 seconds with the new external-beam radiotherapy delivery system and HT systems, respectively (two-tailed p-value = 0.76 in the paired t-test). <h3>Conclusion</h3> Our initial clinical experience with the external-beam radiotherapy delivery system showed that image guided IMRT could be delivered to different anatomical sites with adequate clinical workflow efficiency. The new machine achieved clinically comparable treatment plan quality compared to helical tomotherapy. The beam-on time was comparable to that on helical tomotherapy.