Purpose: To evaluate the dosimetric characteristics of ZAP-X and CyberKnife (CK) G4 stereotactic radiosurgery (SRS) for single brain metastasis.
Methods: Ten patients with single brain metastasis treated with CK were re-planned in the ZAP-X planning system using the same prescribed dose. The prescribed dose of planning target volume (PTV) was 20-24Gy for 1-3 fractions. The PTV volumes ranged from 0.68-11.52 cm(3).The same target and organs at risk (OARs) constraints were used, and the prescription dose of PTV was normalized to 70% for both ZAP-X and CK. The dosimetric parameters and total MUs, treatment time (TT) were evaluated for PTV and normal structures. A paired t-test was performed to show the significance of difference.
Results: For coverage, CK is significantly higher than ZAP-X (98.89±0.89% vs 97.69±1.54%，p<0.01)，but ZAP-X shows a higher conformity index (0.80 ±0.05 vs 0.77±0.08)，and a lower Paddick gradient index (2.97±0.26 vs 3.04±0.24) than CK. Because all the prescribed doses of PTV are normalized to 70%, the plans have the same HI. The total MUs of ZAP-X are significantly lower than CK (12159.9 ±5423.0 vs 25940.1 ±2752.3, p<0.01) and shows shorter TT than ZAP-X (25.67 ±6.67 vs 37.89 ±4.94 min) (p<0.01). For V2-V21 and Dmean of brain tissues，there are no significant differences between two modalities, but ZAP-X have a little lower than CK. But CK may have a certain advantage for the target close to OARs, such as lens, due to no function of avoiding certain organs in ZAP-X.
Conclusion: For SRS treating single brain metastasis, ZAP-X can provide a high quality plan equivalent to or even better than CK,especially reducing the treatment time. ZAP-X may become a new SRS platform for the treatment of brain metastasis.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by a grant from Chinese National Science Fund(Grant No.81801799)