Dosimetric impact of coplanar and non-coplanar arc arrangements in VMAT-based lung SABR

Although coplanar and non-coplanar volumetric modulated arc therapy (VMAT) have been widely investigated, the effect of organizing two full rotations as sequential sub-arcs within one Monaco beam entry or as one arc in each of two beam entries remains insufficiently characterized for lung stereotactic ablative radiotherapy (SABR). This study compared four Monaco workflows differing in arc organization and coplanarity.

This retrospective dosimetric planning study included 20 medically inoperable patients with early-stage non-small cell lung cancer (18 peripheral and two central tumors). Four VMAT-based SABR plans were generated per patient: Gp1, one coplanar beam entry with one full arc; Gp2, one coplanar beam entry with two sequential full sub-arcs; Gp3, two coplanar beam entries with one full arc each; and Gp4, two non-coplanar beam entries with one full arc each at couch angles of 15° and 345°. All 80 plans were generated by one medical physicist in Monaco version 5.11.02 using identical within-patient contours and optimization objectives and a prescription of 60 Gy in eight fractions. Plan-quality indices, monitor units (MU), Monaco-estimated beam-delivery time, and organ-at-risk (OAR) doses were compared using Bonferroni-adjusted repeated-measures comparisons. Estimated beam-delivery time excluded setup, imaging, couch motion, repositioning, and verification.

Target coverage and conformity index were comparable across groups. Compared with all coplanar configurations, Gp4 reduced total lung V5 (22.6% vs. 25.9-26.5%; p ≤ 0.001), total lung V10 (12.2% vs. 13.7-14.2%; p ≤ 0.041), and contralateral lung V5 (9.9% vs. 18.2-19.2%; p < 0.001). Gp4 had a lower gradient index than Gp2 (5.63 vs. 6.08; p = 0.020), but not than Gp1 or Gp3. Esophageal and spinal cord near-maximum doses were lower with Gp4 than with all coplanar groups, whereas mean heart dose was higher (2.1 Gy vs. approximately 1.6 Gy; p = 0.010-0.027). Although Gp2 and Gp3 used the same number of rotations, Gp3 had lower gradient index, MU, and TPS-estimated beam-delivery time than Gp2 (p ≤ 0.015).

Modest non-coplanarity produced a planning trade-off between improved low-dose lung and selected serial-OAR sparing and increased mean heart dose. Beam-entry organization was also associated with differences in dosimetric and TPS-estimated delivery metrics when the total number of rotations was unchanged.

Fixed ±15° non-coplanar VMAT may be considered when pulmonary or serial-OAR sparing is prioritized, provided that cardiac dose, collision clearance, imaging workflow, and total treatment-room time are evaluated individually. For Monaco lung SABR using two coplanar rotations, assigning one arc to each of two beam entries yielded lower gradient index, MU, and TPS-estimated beam-delivery time than placing two sub-arcs within one beam entry. These findings are system-specific planning observations rather than evidence of clinical outcome superiority.

Stereotactic ablative radiotherapy is a precise form of radiation treatment used for some people with lung cancer. This study compared four ways of organizing radiation treatment plans for 20 people with early-stage lung cancer to see how well they treated the tumour while protecting healthy organs. This study found that a non-coplanar approach, which delivers radiation from slightly different angles, reduced radiation to the lungs and some nearby organs but increased the average dose to the heart, while different arc arrangements also affected treatment efficiency. This matters because understanding these trade-offs can help clinicians choose treatment plans that best balance safety, effectiveness, and efficiency for individual patients.