Evaluation of plan quality and treatment efficiency for single-isocenter/two-lesion lung stereotactic body radiation therapy.

Purpose/objectives: To evaluate the plan quality and treatment delivery efficiency of single-isocenter/two-lesions volumetric modulated arc therapy (VMAT) lung stereotactic body radiation therapy (SBRT).

Materials/methods: Eight consecutive patients with two peripherally located early stage nonsmall-cell-lung cancer (NSCLC) lung lesions underwent single-isocenter highly conformal noncoplanar VMAT SBRT treatment in our institution. A single-isocenter was placed between the two lesions. Doses were 54 or 50 Gy in 3 and 5 fractions respectively. Patients were treated every other day. Plans were calculated in Eclipse with AcurosXB algorithm and normalized to at least 95% of the planning target volume (PTV) receiving 100% of the prescribed dose. For comparison, two-isocenter plans (isocenter placed centrally in each target) were retrospectively created. Conformity indices (CIs), heterogeneity index (HI), gradient index (GI), gradient distance (GD), and D were calculated. The normal lung V5, V10, V20, mean lung dose (MLD) and other organs at risk (OARs) doses were evaluated. Total number of monitor units (MUs), beam-on time, and patient-specific quality assurance (QA) results were recorded.

Results: The mean isocenter to tumor distance was 6.7 ± 2.3 cm. The mean combined PTV was 44.0 ± 23.4 cc. There was no clinically significant difference in CI, HI, GD, GI, D , and V20 including most of the OARs between single-isocenter and two-isocenter lung SBRT plans, evaluated per RTOG guidelines. However, for single-isocenter plans as the distance between the lesions increased, the V5, V10, and MLD increased, marginally. The total number of MUs and beam-on time was reduced by a factor of 1.5 for a single-isocenter plan compared to a two-isocenter plan. The single-isocenter/two-lesions VMAT lung SBRT QA plans demonstrated an accurate dose delivery of 98.1 ± 3.2% for clinical gamma passing rate of 3%/3 mm.

Conclusion: The SBRT treatment of two peripherally located lung lesions with a centrally placed single-isocenter was dosimetrically equivalent to two-isocenter plans. Faster treatment delivery for single-isocenter treatment can improve patient compliance and reduce the amount of intrafraction motion errors for well-suited patients.