Letter: Importance of Cobalt-60 Dose Rate and Biologically Effective Dose on Local Control for Intracranial Meningiomas Treated With Stereotactic Radiosurgery

To the Editor:

 Meningiomas are among the most common primary brain tumors, representing about 30% of these tumors. They are classified by the World Health Organization as Meningioma grade I, II, and III. Despite being mostly benign and with a low rate of recurrence, there are those that have an increased risk of recurrence and greater aggressiveness, resulting in a worse prognosis, and recently molecular genomics and epigenetics got included as WHO grade.¹ The identification of prognostic variables in meningioma survival patients will provide a more accurate decision for radiotherapy as an adjuvant treatment in the postoperative period.² One technique used to improve patient survival is stereotactic radiosurgery (SRS). SRS is a neurosurgical procedure that provides highly targeted radiation, allowing high doses of treatment.³

We have read and appreciated the recent article published in Neurosurgery by Huo et al⁴ on the association of cobalt-60 dose rate and biologically effective dose (BED) on the outcome of patients with meningioma treated with SRS. The authors retrospectively reviewed a case series of 336 patients submitted to cobalt-based SRS and found that patients treated with low dose rates and those with grade I or unresected meningiomas treated with lower BEDs were at a higher risk of local failure. In addition, no benefits were found by dose escalation beyond the 12 Gy dose; on the contrary, patients treated with higher doses (greater than 12 Gy) had a higher cumulative incidence of radiological and symptomatic edema. These findings establish new parameters in studies of SRS for meningioma and might help the pursuit of better outcomes.

However, we identify some data absence that should be considered for evaluating outcomes. In this study, the authors⁴ recognize dose treatment as location based and acknowledge supratentorial/convexity region, but do not show any comparison between these fields nor details on additional meningiomas and their locations. This absence of information regarding this location may bias the interpretation of the results as it has been reported by others. Almeida et al,⁵ in a cross-sectional study with 593 patients, found that supratentorial areas have twice the chance of recurrence than convexity brain areas. For a more precise decision-making approach, we suggest including the locality of the meningioma in the statistical model to obtain a more precise dose rate and BED's outcome.

In addition, there is no information regarding meningioma's Simpson grade or any kind of resection status which are good prognostic parameters.⁶ A cohort with 1041 patients⁷ grouped Simpson's grades I and II as gross total resection (GTR) and grade III, IV, and V as subtotal resection (STR) and noticed that the resection extension also influences the progression-free survival (PFS). Patients who underwent a GTR had a superior PFS than the ones who underwent the STR. In addition, studies^6,8 related that adjuvant radiotherapy increased the PFS in STR, but had no effect on the GTR. Thus, only superficially acknowledging previous surgery or radiotherapy could lead to bias, so each category should be detailed.

Moreover, Huo et al⁴ did not mention the number of dose shots used and the number of cases used. The lack of information provides questions whether the authors considered the treatment as SRS or fractionated radiotherapy (FRT). Morgan et al⁹ for example, referred to 5 or fewer fractions as SRS, whereas more than 5 fractions were considered FRT. They concluded that FRT decreased the risk of treatment-related edema. The current information would be an essential discussion topic in the study by Huo.⁴

The Radiation Therapy Oncology Group 0539 study¹⁰ found that high-risk patients who had newly diagnosed or recurrent WHO grade III meningioma, as well as WHO grade II meningioma of any extent of resection and newly diagnosed after STR, exposing that one should not only consider the tumor grade to consider the dose and treatment used. In this study,¹⁰ they urge that postoperative intensity-modulated radiotherapy is a promising treatment option for high-risk patients. For this reason, we suggest the authors⁴ consider the dose choice and investigate the interference of this on the results.

Finally, this article suggests that dose rate and BED for cobalt-60 SRS should be incorporated into analysis for predicting and achieving better outcomes in meningiomas treatment. Nonetheless, Gamma knife SRS limitations, such as reduced treat-able range,¹¹ compared with the benefits and risks of other radiosurgery techniques available today, such as treatment with linear accelerators, mainly because of the multileaf collimator, the volumetric modulated arc therapy, the image-guided radiotherapy, and the intensity-modulated radiotherapy is observed that the control of meningiomas is better in patients undergoing treatment.^10,12-15 Considering these data, replacing Gamma knife with more advanced and secure techniques also confers a better prognosis for the patient.

Funding

This study did not receive any funding or financial support. Dr Zaninotto declares National Institutes of Health funding.

Disclosures

The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.