Adaptive Radiotherapy (ART) Using In Vivo Portal Dosimetry Constancy on Halcyon LINAC for Head and Neck Cancer VMAT

Introduction: This study evaluated the potential of the Varian Halcyon LINAC and its associated Portal Dosimetry (PD) software, using in vivo megavoltage portal images, as a tool for in vivo portal dosimetry constancy (IVDc) in head and neck cancer (HNC) patients receiving volumetric modulated arc therapy (VMAT).Material and Methods: A retrospective study included 100 HNC patients (mean age 58.4 ± 9.2 years; 72 males, 28 females) treated with three arc VMAT plans on the Halcyon LINAC. A prescription dose of 7000 cGy was delivered over 35 fractions to high risk volumes and 6300 cGy to intermediate risk volumes. Portal images from all fractions (3500) and arcs (10,500) were compared to first fraction baselines via gamma analysis with a 3%/2 mm criterion and 95% passing threshold. Phantom tests evaluated sensitivity to thickness changes.Results: Gamma passing rates exhibited a steady decline over the treatment course (R²=0.9724), with an average 93.9 ± 3.6% passing rate, dropping below 90% after 25 fractions. Failures correlated with thickness variations exceeding 1 cm, which were detected with high accuracy during phantom tests. Pretreatment patient-specific QA yielded 100% pass rates.Conclusion: This study demonstrates the potential of the Halcyon LINAC's integrated PD system and Varian PD software for IVDc in HNC patients treated with VMAT. The results confirm the feasibility of using in vivo MV portal images acquired on the Halcyon LINAC to assess dose consistency throughout treatment.