Benchmarking techniques for stereotactic body radiotherapy for early-stage glottic laryngeal cancer: LINAC-based non-coplanar VMAT vs. Cyberknife planning

Abstract

Introduction: Stereotactic body radiation therapy (SBRT) was found effective in treating laryngeal cancer with only five treatment fractions by a recent clinical trial (NCT01984502, ClinicalTrials.gov). Nevertheless, this trial used the Cyberknife system, which is not widely accessible enough to benefit all patients affected by laryngeal cancer. Our study investigates the feasibility of larynx SBRT treatment planning on a conventional gantry-based LINAC and compares its plan quality with that from the Cyberknife.

Materials & methods: Ten larynx SBRT cases were originally treated by Cyberknife using fixed cones in our institution, with plans created and optimized using the Monte-Carlo algorithm in the MultiPlan treatment planning system. These cases were retrospectively planned in the Eclipse planning system for a LINAC with the same prescription dose. We used volumetric modulated arc therapy (VMAT) for larynx SBRT planning in Eclipse and incorporated non-coplanar arcs to approach the Cyberknife's large solid angle delivery space. We used both anisotropic analytical algorithm (AAA) and Acuros XB (AXB) algorithm for dose calculation and compared their accuracy by measurements on an in-house larynx phantom. We compared the LINAC VMAT plans (VMAT-AAA and VMAT-AXB) with the original Cyberknife plans using dosimetric endpoints such as the conformity index, gradient indices (R50, R20), OAR maximum/mean doses, and the monitor units.

Results: Phantom measurement showed that both the AAA and the AXB algorithms provided adequate dose calculation accuracy (94.7% gamma pass rate on 2%/2 mm criteria for AAA vs. 97.3% for AXB), though AXB provided better accuracy in the air cavity. The LINAC-based VMAT plans achieved similar dosimetric endpoints as the Cyberknife planning, and all plans met the larynx SBRT dosimetric constraints. Cyberknife plans achieved an average conformity index of 1.13, compared to 1.20 of VMAT-AXB and 1.19 of VMAT-AAA. The VMAT plans spared the thyroid gland better with average Dmean of 2.4 Gy (VMAT-AXB) and 2.7 Gy (VMAT-AAA), as compared to 4.3 Gy for Cyberknife plans. The VMAT-AAA plans had a slightly lower contralateral arytenoid Dmax (average: 15.2 Gy) than Cyberknife plans (average: 17.9 Gy) with statistical significance, while the contralateral arytenoid Dmax was similar between VMAT-AXB and Cyberknife plans with no statistically significant difference. Cyberknife plans offered slightly better R50 (average: 5.0) than VMAT-AXB (5.9) and VMAT-AAA (5.7) plans. The VMAT plans substantially reduced the plan MUs to less than 1/3 of the Cyberknife plans, and the differences were statistically significant. The other metrics were similar between VMAT and Cyberknife plans with no statistically significant differences.

Conclusions: Gantry-based LINACs can achieve similar plan quality to Cyberknife systems. Treatment outcome with both methods remains to be investigated.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1

Image rendering of the 5-arc template for LINAC larynx SBRT planning

Fig. 2

(a) CT slice of an example larynx SBRT patient, where the PTV contour covers a considerable amount of air cavity. (b) Our in-house larynx wax phantom used for dose measurement and dose accuracy validation/comparison. (c) Mapping of a larynx SBRT plan onto the larynx phantom and the corresponding dose distribution calculated by AXB. (d) Dose distribution measured by the EBT3 film. The film was sandwiched between the phantom slabs at the slice location of (c). (e) 2D dose difference map for the Eclipse AAA algorithm. The brighter pixels indicate larger dose differences, as shown by the color bar. (f) 2D dose difference map for the Eclipse AXB algorithm. The PTV contour was overlaid onto both (e) and (f). The difference maps (without underlying CTs) were attached at the lower-left corners of (e) and (f)

Fig. 3

Isodose map comparisons in three views between an original Cyberknife plan and the non-coplanar VMAT LINAC plans (VMAT-AXB and VMAT-AAA) for one studied case

Fig. 4

DVH comparison between an original Cyberknife plan and the non-coplanar VMAT LINAC plans (VMAT-AXB and VMAT-AAA) for one studied case

Fig. 5

Distribution boxplots of all metrics evaluated in the plan comparison study for the Cyberknife, VMAT-AXB, and VMAT-AAA plans. Each boxplot contains 10 data points. In each boxplot, the upper edge, central line, and lower edge of the box represent the 75th percentile (Q3), median, and 25th percentile (Q1) of the data, respectively. The lower whisker extends to the datum no smaller than Q1 − 1.5 × (Q3 − Q1), and the upper whisker extends to the datum no larger than Q3 + 1.5 × (Q3 − Q1). The “+” in the plots are outliers outside the whiskers