Dosimetric evaluation and clinical feasibility of Halcyon-E O-ring linear accelerator for single fraction coplanar...

This study aimed to evaluate the Halcyon-E O-ring gantry linear accelerator, which lacks non-coplanar beam delivery capability, for single-fraction stereotactic radiosurgery (SRS). The validation process consisted of three steps: dosimetric analysis, clinical evaluation, and patient-specific quality assurance, along with a comparative analysis with plans generated on a C-arm linear accelerator.

A total of 96 patients who underwent single-fraction SRS using a Novalis Tx C-arm linear accelerator (linac) were replanned on an O-ring gantry linac. All patients were treated using the volumetric modulated arc therapy (VMAT) technique. The original plans on the C-arm linac included both coplanar and non-coplanar beams, whereas the O-ring plans used only coplanar arcs. The arc lengths on the O-ring gantry were adjusted iteratively to achieve dose coverage and organ-at-risk (OAR) doses comparable to those of the C-arm linac. Target dose coverage was assessed using metrics such as the percentage dose received by 98% and 2% of the planning target volume (PTV D98% and D2%), Paddick Conformity Index (PCI), and the dose to 0.1 cc of serial OARs. Paired sample t-tests were used to evaluate statistical significance (two-tailed p < 0.05). All O-ring gantry plans were reviewed by the clinical team and classified as ``accepted'' or ``rejected'' based on dosimetric parameters (target coverage, conformity, OAR doses, low-dose spillage) and clinical factors (disease burden, malignant or benign, expected life expectancy). All plans underwent patient-specific quality assurance using gamma analysis and dose to monitor-unit verification with a 0.01-cm³ ionization chamber in a solid-water phantom.

Data are presented for C-arm plans first, followed by O-ring plans. PTV D98% was 99.1 ± 5.6% for the C-arm and 98.2 ± 5.5% for the O-ring, while D2% was 109.7 ± 5.7% and 110.1 ± 5.5%, respectively. The variations in both parameters were statistically insignificant (p = 0.097 and p = 0.367). The average maximum brainstem dose was 0.7% (p = 0.498) lower in the O-ring plans. The mean PCI was the same for both plans, at 0.8 ± 0.1. The C-arm linac required fewer monitor units (MU), with an average reduction of -205.1 ± 693.9 MU (p = 0.081). Statistically significant differences were observed in the D0.1 cc doses for the left eye (p = 0.028), optic chiasm (p = 0.009), right optic nerve (p = 0.032), and V12Gy received by normal brain tissue (p < 0.0001). C arm plans delivered lower doses for all organs; however, only the left eye, optic chiasm, right optic nerve and V12Gy differences were statistically significant. After thorough evaluation by the clinical team, 92.7% (89 out of 96) of the O-ring plans were deemed acceptable. Seven plans were considered unacceptable based on clinical or dosimetric criteria. Three plans were rejected because of higher doses to serial organs despite comparable PTV coverage between the C-arm and O-ring plans. The remaining four plans exceeded the V12Gy (Brain-GTV) ≥ 10 cc constraint. All quality assurance results were within tolerance limits, and the difference between the two groups was not statistically significant using a paired sample t-test (p = 0.089 and p = 0.118 for volumetric and point dose measurements, respectively).

The Halcyon O-ring linear accelerator, using only coplanar arcs, can be effectively used for single-fraction stereotactic treatment for the majority of patients. Following the completion of the validation process, single-fraction SRS treatments were initiated using the Halcyon linear accelerator.

Stereotactic radiosurgery is a type of precise radiation treatment used to target small areas, often in the brain, in a single session. This study tested whether a newer type of treatment machine that uses a simpler beam setup could produce plans similar to a standard machine by re-planning treatments for 96 people and comparing results. This study found that most plans from the new machine provided similar tumour coverage and were acceptable to the clinical team, although doses to some sensitive areas were slightly higher in some cases. This matters because it suggests this machine could be used safely for many people, helping expand access to treatment.