Selection criteria for circular collimator- vs. Multileaf collimator-based plans in robotic stereotactic radiotherapy for brain metastases and benign intracranial disease: Impact of target size, shape complexity, and proximity to at-risk organs

Abstract

Purpose: This study aimed to determine the selection criteria for circular collimator (CC)- and multileaf collimator (MLC)-based stereotactic radiosurgery (SRS)/stereotactic radiotherapy (SRT) plans for brain metastases (BM) and benign intracranial disease (BID) in terms of geometric parameters using CyberKnife (CK).

Methods: Forty-eight and eighty-five patients with BM and BID, respectively, were included. Two plans using CC and MLC were created for each case. Six dosimetric parameters and mathematical scores (MS) were extracted from each plan to assess plan quality. Two geometric parameters in BM-equivalent radius (rGTV) and sphericity index (SI) of the gross tumor volume-and three in BID-rGTV, SI, and the distance between the GTV and organ at risk (dOAR)-were calculated. Their effect on the superiority of CC- or MLC-based plans in terms of dosimetric parameters and MS was evaluated using multiple regression analysis.

Results: The rGTV was associated with improved dosimetric parameters of MLC-based plans, especially the GTV conformity in BM and BID cases (β: 0.70 and 0.51) and the OAR sparing in BM cases (β: 0.82), where β represents the regression coefficient. Based on the MS, where the weights for the GTV coverage and OAR sparing were equal, the thresholds at which the MLC-based plans become comparable or superior to the CC-based plans in BM and BID were rGTV > 7.6 and >17.5 mm, respectively. Meanwhile, SI and dOAR were weakly correlated (β ≤ 0.30).

Conclusions: In SRS/SRT plans for BM and BID cases using CyberKnife, geometric parameters, especially rGTV, must be considered when selecting CC or MLC.

Keywords: Brain SRS/SRT; Circular shape collimator; Clinical/mathematical score; CyberKnife; Geometric parameter; Multileaf collimator; Selection criteria.