Letter

To the Editor: We read with interest the paper comparing volumetric modulated arc therapy (VMAT) and gamma knife (GK) stereotactic radiosurgery (SRS) of multiple brain metastases by Thomas et al1 and the related Letter to the Editor by Ma et al.2 Thomas et al1 concluded that “4-arc VMAT produced clinically equivalent conformity, dose falloff, 12-Gy isodose volume, and low isodose spill,” whereas Ma et al2 affirmed that “[t]heir study applied different prescription isodose volumes for GK and VMAT plans, showing significantly larger prescription isodose volumes for GK than VMAT plans: in short, VMAT delivers faster treatment but with significantly increased normal tissue dose.” Low-dose spread with VMAT is a significant issue of debate, but it could be less crucial in patients with brain metastases. To date, no data have been reported about acute or late side effects of low-dose spread in healthy tissue in patients with multiple brain metastases. Usually these patients have a poor overall survival (OS); according to the new prognostic index3 (graded prognostic assessment [GPA]), the most favorable class (GPA 3.5-4) had a median OS of 11 months, but the other classes had a median OS ranging from 3 to 7 months; according to the oldest recursive partitioning analysis (RPA)3 index, the most favorable class (RPA I) had a median OS of 7.1 months. Considering this limited life expectancy, it is reasonable that no late neurodamage developed in these patients. Thus, the role of low-dose treatment has been overestimated with respect to the potential practical advantages of a VMAT technique compared with GK, according to results from the following reports: a. Noninvasive frame. For Linac-based SRS, a simple thermoplastic mask has been used without the conventional invasive head ring used in GK. The average reported mean localization error was 0.8 mm, which was achieved by controlling the motion of the treatment couch with 6 degrees of freedom.4 b. Image guide radiotherapy. One study showed that automatic 3-dimensional (3D) image matching on bony anatomy using on-board cone beam computed tomography and/or radiography could quantify and correct rotational and translational errors with high accuracy and low patient exposure.5 c. Shorter delivery time. As reported by Thomas et al,1 for flattened filter-free delivery by VMAT at doses that average less than 24 Gy, beam-on time was approximately 2.5 minutes, whereas GK beam-on times ranged from 17.5 to 121.2 minutes (mean, 55.3 minutes). d. Simultaneous whole-brain radiotherapy. As reported by several guidelines, including those from the National Comprehensive Cancer Network, in some patients, SRS is recommended for brain lesions along with irradiation of the entire brain tissue to increase local control. e. Nondedicated machine. Often oligometastatic patients have required other treatment modalities for their cancer (ie, treatment of other metastatic sites). GK equipment cannot provide treatment for extracranial metastases, which means that patients must be moved to other machines or even other centers. Nevertheless, no difference in terms of local control and distant relapse has been reported in the literature between GK and Linac-based SRS, including VMAT.6 In conclusion, both dosimetric and clinical factors must be thoroughly evaluated before selecting the most appropriate radiation treatment approach for brain metastases SRS. Without considering other technical and clinical issues, a pure dosimetric comparison is not enough to confirm the superiority of one technique over another. Disclosures Prof Alongi has received honoraria for past educational seminars for Varian and Brain Lab. The other authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.