Dosimetric evaluation of 4 different treatment modalities for curative-intent stereotactic body radiation therapy for isolated thoracic spinal metastases

To investigate the dosimetric characteristics of 4 SBRT-capable dose delivery systems, CyberKnife (CK), Helical TomoTherapy (HT), Volumetric Modulated Arc Therapy (VMAT) by Varian RapidArc (RA), and segmental step-and-shoot intensity-modulated radiation therapy (IMRT) by Elekta, on isolated thoracic spinal lesions. CK, HT, RA, and IMRT planning were performed simultaneously for 10 randomly selected patients with 6 body types and 6 body + pedicle types with isolated thoracic lesions. The prescription was set with curative intent and dose of either 33Gy in 3 fractions (3F) or 40Gy in 5F to cover at least 90% of the planning target volume (PTV), correspondingly. Different dosimetric indices, beam-on time, and monitor units (MUs) were evaluated to compare the advantages/disadvantages of each delivery modality. In ensuring the dose-volume constraints for cord and esophagus of the premise, CK, HT, and RA all achieved a sharp conformity index (CI) and a small penumbra volume compared to IMRT. RA achieved a CI comparable to those from CK, HT, and IMRT. CK had a heterogeneous dose distribution in the target as its radiosurgical nature with less dose uniformity inside the target. CK had the longest beam-on time and the largest MUs, followed by HT and RA. IMRT presented the shortest beam-on time and the least MUs delivery. For the body-type lesions, CK, HT, and RA satisfied the target coverage criterion in 6 cases, but the criterion was satisfied in only 3 (50%) cases with the IMRT technique. For the body + pedicle-type lesions, HT satisfied the criterion of the target coverage of ≥90% in 4 of the 6 cases, and reached a target coverage of 89.0% in another case. However, the criterion of the target coverage of ≥90% was reached in 2 cases by CK and RA, and only in 1 case by IMRT. For curative-intent SBRT of isolated thoracic spinal lesions, RA is the first choice for the body-type lesions owing to its delivery efficiency (time); the second choice is CK or HT; HT is the preferential choice for the body + pedicle-type lesions. This study suggests further clinical investigations with longer follow-up for these studied cases.