How would you treat grade 3 cauda equina meningioma status post biopsy only with a history of prior prostate/whole pelvis radiation?

This question is anything but routine. WHO grade 3 spinal meningiomas are exceedingly rare. I’ve been treating meningioma patients my entire career, now beyond 3 decades, and have never seen one. If this patient’s prostate cancer radiation therapy (RT) were many years ago and if his meningioma were located in-field or nearby, the possibility of a secondary malignancy from RT must be considered.

A recent retrospective analysis by Kobayashi and colleagues described 116 spine meningioma patients, 113 (97.4%) of whom were WHO grade 1, 3 (2.6%) WHO 2, and none WHO 3. Any recommendations, of sheer necessity, will be based on first principles, anecdotes, and/or limited literature. Additional confounding features for this patient are related to prior RT. The details are not provided but implications vary considerably depending upon the particulars of his “whole pelvis” RT (e.g. external RT alone, external + brachytherapy, and dosing), its relationship to the location of his meningioma, and the time interval since prior RT.

If further surgery can be safely accomplished, it should be considered. Thorough resection is often difficult to attain for cauda equina tumors without neurologic repercussions but, should there be any doubt, pathology confirmation and molecular assessment might be welcome. With or without further surgery, RT will likely ultimately be required here, whether now or later for symptomatic progression, which is to be expected with a WHO grade 3 meningioma, particularly in a male patient with less than Simpson grade I or II resection [see Kobayashi et al., PMID 34079036]. We are working within NRG to identify clinically applicable molecular markers and effective systemic therapy for patients such as him with high-grade or recurrent meningioma and targeted therapy might be considered here, although to date, the only treatments with validated efficacy remain surgery and RT.

Should his RT have been recent and/or overlapping doses high, additional RT may be hazardous but perhaps not as hazardous as progressive anaplastic meningioma. In general, one would recommend RT for a grade 3 meningioma, even if it might be RT-induced, more assuredly after biopsy only (Simpson grade V), and even in the setting of moderate dose prior RT. However, the optimal method of RT would depend intricately upon the timing, extent, and dose from prior RT.  

Not having those details limits recommendations but IMRT or SBRT may be applicable. In consideration of prior RT, with IMRT for his meningioma, doses <54 Gy in 30 fractions would likely be of limited benefit as would doses <25-30  Gy in 5 with SBRT. If possible, for a WHO grade 3 meningioma, IMRT doses of 60-66 Gy in 30-33 fractions or SBRT doses of 35 Gy in 5 fractions would be preferrable, but this may be too ambitious with prior RT. This scenario is complex for anyone. There is no high-level data and very little direction on the low-level data side of the spectrum.

I agree with the answers provided by Dr. @Dr. First Last and Dr. @Dr. First Last. A WHO grade 3 spinal meningioma with a history of prior radiation to the area (prostate/whole pelvis radiation) is exceedingly rare and requires an individualized treatment plan to find a balance between reasonable tumor control and cauda equina late toxicity. While still not available in the U.S., I wanted to point to BNCT as an option for these patients, that has been used at reactor-based facilities in Japan and will soon be offered at accelerator-based BNCT facilities in hospitals (the University of Helsinki Hospital is preparing for their first patient). In a study conducted between 2005 and 2019, 44 patients with recurrent and refractory intracranial high-grade meningiomas were treated with reactor based BNCT by Dr. Miyatake and colleagues in Japan [see Takai et al. PMID 33984146] The results showed good local tumor control and prolonged survival for these cases. While not directly applicable to the current case, it is of interest to mention that with the approved drug BPA, the Japanese colleagues were able to plan these patients limiting the dose to the pretreated brain to 15 Gy-E (X-ray biologically equivalent dose) and still deliver doses in the range of 37-67 Gy-E to the tumor. The 2021 publication provides Dr. Miyatake's email address, in case someone wants to contact him.