Early Clinical Experience with Varian Halcyon V2 Linear Accelerator: Dual -Isocenter IMRT Planning and Delivery with Portal Dosimetry for Gynecological Cancer Treatments

Abstract

A linear accelerator was recently released with new upgraded features. One of new features is to allow treatment length > 28cm using multiple- isocenters (maximum of 36cm in superior and inferior directions). As one of the first users of Halcyon V2, we report our clinical experience with Halcyon V2 for a dual - isocenter IMRT planning and delivery for gynecological cancer patients and examine the feasibility of in-vivo portal dosimetry. Eight extended field IMRT plans for gynecological cancer patients were generated with a treatment planning system using two isocenters to treat pelvis and pelvic/or para-aortic nodes region. Auto feathering technique was employed to split fields between isocenters during IMRT optimization. The prescription dose was 45Gy in 25 fractions (fxs) with simultaneous integrated boost (SIB) dose of 55Gy or 57.5 Gy in 25 fxs to involved nodes. All treatment plans, pre-treatment patient specific QA and treatment delivery records including daily in-vivo portal dosimetry were retrospectively reviewed. For in-vivo daily portal dosimetry analysis, each fraction was compared to the reference baseline (1st fraction) using gamma analysis criteria of 4%/4mm with 90% of total pixels in the portal image planar dose. All 8 extended field IMRT plans met the planning criteria and delivered as planned (a total of 200 fractions). Conformity Index (CI) for the primary target was achieved with the range of 0.99-1.14. For organs at risks, most were well within the dose volume criteria. Treatment delivery took from 5.0 to 6.5 minutes. All 8 cases passed pre-treatment QA using gamma analysis criteria of 3%/2mm with 95 % of total pixels. Interfractional in-vivo dose variation exceeded gamma analysis threshold for 8 fractions out of total 200 (4%). Those 8 fractions were found to have relatively large difference in small bowel filling or motion and SSD change at the isocenter compared to the baseline. Halcyon V2 is effective to create complex extended field IMRT plans using dual isocenters with efficient delivery. Also Halcyon in-vivo dosimetry is feasible for daily treatment monitoring for possible (or significant) changes of organ motion, internal or external anatomy and weight which could further lead to adaptive radiation therapy.