Progressive resolution optimizer (PRO) predominates over photon optimizer (PO) in sparing of spinal cord for spine...

we assessed the performance of the optimization algorithms by comparing volumetric modulated arc therapy generated by a progressive resolution optimized (VMAT_PRO) and photon optimizer (VMAT_PO) in terms of plan quality, MU reduction, sparing of the spinal cord (or cauda equina), and plan complexity.

Fifty-seven patients who received spine stereotactic ablative radiotherapy (SABR) with tumors located in the cervical, thoracic, and lumbar spine were retrospectively selected. For each patient, VMAT_PRO and VMAT_PO with two full arcs were generated with using the PRO and PO algorithms. For dosimetric evaluation, the dose-volumetric (DV) parameters of the planning target volume (PTV), organs at risk (OARs), the corresponding planning organs at risk (PRV), and 1.5-cm ring structure surrounding the PTV (Ring_1.5 cm) were calculated for all VMAT plans. The total number of monitor units (MUs) and the modulation complexity score for the VMAT (MCS_v) were compared. To investigate the correlations of OAR sparing to plan complexity, Pearson's and Spearman's correlation tests were conducted between the two algorithms (PO - PRO, denoted as Δ) in the DV parameters for normal tissues, total MUs, and MCS_v.

For the PTVs, Target conformity and dose homogeneity in the PTVs of VMAT_PRO were better than those of VMAT_PO with statistical significance. For the spinal cords (or cauda equine) and the corresponding PRVs, all of the DV parameters for VMAT_PRO were markedly lower than those for VMAT_PO, with statistical significance (all p < 0.0001). Among them, the difference in the maximum dose to the spinal cord between VMAT_PRO and VMAT_PO was remarkable (9.04 Gy vs. 11.08 Gy with p < 0.0001). For Ring_1.5 cm, no significant difference in V_115% for VMAT_PRO and VMAT_PO was observed.

The use of VMAT_PRO resulted in improved coverage and uniformity of dose to the PTV, as well as OARs sparing, compared with that of VMAT_PO for cervical, thoracic, and lumbar spine SABR. Better dosimetric plan quality generated by the PRO algorithm was observed to result in higher total MUs and plan complexity. Therefore, careful evaluation of its deliverability should be performed with caution during the routine use of the PRO algorithm.