November 2010, Issue 47

Novel Radiation Technique for the Treatment of Neovascular Age-Related Macular Degeneration

Marcos Ávila, MD
Federal University of Goiás
Brazil

Radiotherapy for neovascular age-related macular degeneration (AMD) has been studied since the early 1990s. One of the studies considered to be a more “modern” review of external beam radiation for neovascular AMD was published in 1996.¹ Although there was a good amount of optimism for this concept early on, it has been difficult to assess the efficacy of radiation for neovascular AMD because the studies that have been performed offer no consensus. For example, there are as many published studies that show a positive effect^2-8 as those showing an equivocal or no effect.^9-14 The various studies used differing doses and fractionations of radiation and this may explain why the results are so variable.

The main challenge in many of these studies that focus on the use of radiation for the destruction of angiogenic cells has been avoiding damage to other areas of the retina. External beam radiation is not precisely localized, which reduces the efficacy and the ability to target a specific area with a high dose of radiation.

The method of radiation delivery, type of radiation administered, and the dose delivered are very important factors in determining therapeutic outcomes. The biggest difference between the various types of radiation is their penetrating power. Alpha particles penetrate poorly and can be stopped by a piece of paper. Gamma rays require large lead-lined, lead-and-concrete-mixed walled rooms to protect those outside the room when the radiation is taking place. The penetration of beta rays falls between alpha and gamma rays and can be stopped by fairly simple measures. Strontium 90 beta radiation is most intense at the source and, within a few millimeters, there is very little radiation present. Beta radiation penetration can be halted by surrounding tissue in the retina, which is essentially water. Therefore, it is appropriate for targeted delivery to the retina.

The Epimacular Brachytherapy System (Neovista, Fremont, CA) uses 24 Gy strontium 90 brachytherapy. The device brings the actual radiation to the lesion itself with a rapid falloff. The volume of tissue affected by the radiation is small, along the order of a few millimeters.

The majority of clinical trials that used radiation for AMD in the past did so with external beam delivery. It is very difficult to deliver a precise dose of radiation using external beam techniques to volumes smaller than 4 centimeters in diameter.

Epimacular brachytherapy has shown promising results in pilot studies.^15-16 The safety and efficacy of this intervention is being evaluated further in two large phase III randomized trials.^15-16

The CABERNET trial (n=450), which is the pivotal phase III, multicenter (45 sites) trial, is designed to evaluate the safety and visual acuity outcomes of treatment-naïve patients in two arms: patients treated with the Epimacular Brachytherapy System and two intravitreal injections of ranibizumab 1 month apart, and patients treated with intravitreal ranibizumab monthly for the first three injections, then followed by quarterly injections. Both groups receive ranibizumab rescue treatment based on predefined retreatment criteria

The MERLOT trial has a target recruitment of 363 subjects and is based in the United Kingdom. The trial receives support from the National Institute for Health Research (NIHR) via the Comprehensive Clinical Research Network (CCRN). The CCRN aims to provide support for studies that are scientifically robust, and address areas of importance to the National Health Service. Patients in the MERLOT study are randomized to either epimacular brachytherapy or anti-VEGF monotherapy (control). Both groups receive ranibizumab rescue treatment based on predefined retreatment criteria. The co-primary outcome measures are mean number of anti-VEGF injections over 12 months, and mean ETDRS visual acuity.

Summary
Radiation preferentially damages the cells that mediate vision loss in neovascular AMD. It was, therefore, somewhat unexpected that early studies of external beam radiation failed to show significant benefit. Interest in radiation treatment of neovascular AMD has recently been rekindled, using devices that provide more focal delivery and less collateral damage than external beam radiation therapy. Large randomized clinical trials of epimacular brachytherapy are underway.

REFERENCES

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