Direct Plan Comparison of RapidArc and CyberKnife for Spine Stereotactic Body Radiation Therapy

We compared the treatment planning performance of RapidArc (RA) vs. CyberKnife (CK) for spinal stereotactic body radiation therapy (SBRT). Ten patients with spinal lesions who had been treated with CK were re-planned with RA, which consisted of two complete arcs. Computed tomography (CT) and volumetric dose data of CK, generated using the Multiplan (Accuray) treatment planning system (TPS) and the Ray-Trace algorithm, were imported to Varian Eclipse TPS in Dicom format, and the data were compared with the RA plan using analytical anisotropic algorithm (AAA) dose calculation. The optimized dose priorities for both CK and RA plans were similar for all patients. The highest priority was to provide enough dose coverage to the planned target volume (PTV) while limiting the maximum dose to the spinal cord. Plan quality was evaluated with respect to PTV coverage, conformity index (CI), high-dose spillage, intermediate-dose spillage (R50% and D2cm), and maximum dose to the spinal cord, which are criteria recommended by RTOG 0631 spine and 0915 lung SBRT protocols. The mean CI +/- SD values of PTV were 1.11 +/- 0.03 and 1.17 +/- 0.10 for RA and CK (p = 0.02), respectively. On average, the maximum dose delivered to the spinal cord in CK plans was approximately 11.6% higher than in RA plans, and this difference was statistically significant (p < 0.001). High-dose spillages were 0.86% and 2.26% for RA and CK (p = 0.203), respectively. Intermediate-dose spillage characterized by D2cm was lower for RA than for CK; however, R50% was not statistically different. Even though both systems can create highly conformal volumetric dose distributions, the current study shows that RA demonstrates lower high- and intermediate-dose spillage than CK. Therefore, RA plans for spinal SBRT may be superior to CK.

💡 Research Summary

This study directly compares the treatment planning performance of RapidArc (RA) and CyberKnife (CK) for spinal stereotactic body radiation therapy (SBRT). Ten patients who had previously received CK treatment for spinal lesions were retrospectively replanned using RA, which employed two full 360‑degree arcs. Original CT scans and volumetric dose data generated by the Multiplan system with the Ray‑Trace algorithm were exported in DICOM format and imported into the Varian Eclipse treatment planning system, where dose calculations were performed with the Analytical Anisotropic Algorithm (AAA). Both planning approaches used identical dose‑priority objectives: adequate coverage of the planned target volume (PTV) and strict limitation of the maximum dose to the spinal cord, reflecting the criteria recommended by RTOG 0631 (spine) and RTOG 0915 (lung) SBRT protocols.

Plan quality was assessed using several quantitative metrics. The conformity index (CI) measured how closely the prescription isodose matched the PTV, with values closer to 1 indicating superior conformity. High‑dose spill was defined as the percentage of total volume receiving more than 105 % of the prescription dose. Intermediate‑dose spill was characterized by two parameters: R50 % (the ratio of the 50 % prescription isodose volume to the PTV volume) and D2cm (the maximum dose measured 2 cm away from the PTV in any direction). Finally, the maximum spinal cord dose (Dmax) was recorded to evaluate normal‑tissue sparing.

The results demonstrated that RA achieved a statistically superior CI (mean ± SD = 1.11 ± 0.03) compared with CK (1.17 ± 0.10; p = 0.02), indicating tighter dose conformity around the target. The spinal cord Dmax in CK plans was on average 11.6 % higher than in RA plans (p < 0.001), highlighting a clear advantage for RA in protecting critical neural structures. High‑dose spill was lower for RA (0.86 %) than for CK (2.26 %), although this difference did not reach statistical significance (p = 0.203). For intermediate‑dose spill, the D2cm metric favored RA, showing reduced dose at a 2 cm margin, while R50 % values were comparable between the two modalities.

The authors discuss that the AAA‑based volumetric intensity‑modulated radiation therapy (VMAT) employed by RA can generate more precise, non‑linear dose distributions than the ray‑trace based CK calculations, especially in the context of steep dose gradients required for spinal SBRT. Limitations of the study include the small cohort size and the fact that CK’s non‑coplanar, non‑isocentric beam delivery and its unique tracking capabilities were not fully replicated in the RA plans. Consequently, while the dosimetric superiority of RA is evident in this dataset, broader validation with larger, multi‑institutional cohorts and correlation with clinical outcomes such as pain relief, neurological toxicity, and local control is necessary.

In conclusion, both RA and CK are capable of producing highly conformal spinal SBRT plans, but RA demonstrates lower high‑ and intermediate‑dose spill and better spinal cord sparing. These findings suggest that, for the specific clinical scenario of spinal metastases treated with SBRT, RapidArc may offer dosimetric advantages that could translate into improved therapeutic ratios.