In this small series, CVM of the orbital apex treated with FSRT exhibited a 100% response rate to radiotherapy and ongoing local control. In addition, it was associated with a favorable clinical response in terms of vision, proptosis, and pain.
Surgical resection of tumors extending into the orbital apex is challenging and carries a high risk of operative injury to the optic nerve and its vascular supply, or to the nerve supply of the extra- and intraocular muscles, and sensory nerves. The surgical risks are highlighted in a series of 214 CVM of the orbit managed with anterior and lateral orbitotomy, where 22.9% had a motility deficit, 13.6% had ptosis, 12.1% had a deterioration in their vision, and 4.2% had total loss of vision after the operation.25 In another surgical series of 14 patients, there was deterioration of vision in 14% of patients at 6 months post procedure.13
Stereotactic radiotherapy is an important alternative to consider in this scenario. The largest series of radiation therapy for CVM of the orbit is 23 patients receiving 12 to 20 Gy in a single fraction with stereotactic radiosurgery.21 All had a decrease in tumor size, while 11 of 14 patients with visual symptoms showed subsequent improvement and the majority that were affected by proptosis also improved. Two other series containing 1 and 3 patients with CVM, respectively, receiving single fraction stereotactic radiosurgery confirm similar tumor regression.18,19 However, the risk of toxicity increases markedly beyond doses of 12 Gy in a single fraction.26–28 Optic neuropathy is a late toxicity that may occur years following treatment and may not have been captured in the larger series, which had a median follow up of only 12 months.21,29
FSRT uses the same principles and equipment as single fraction stereotactic radiosurgery with the same benefits in terms of accuracy and conformity, but reduces the risk of optic neuropathy through the radiobiological advantage of fractionated delivery, which uses smaller daily doses over an extended treatment duration.30 This allows the normal tissue to repair between fractions leading to a significantly lower risk of toxicity.22 All patients treated had radiation doses to the optic nerve below 55 Gy, which is generally accepted to be associated with a risk of optic neuropathy of less than 3%.28 The radiation dose to the lens was low with a mean 1.44 Gy (0.53–2.22 Gy) but was within the threshold dose for cataractogenesis (> 1–2 Gy).31
Our findings using FSRT were similar to the Canadian case series of 5 patients involving the orbital apex that described an average tumor shrinkage of 60%, and rapid resolution of the visual deficit at 3 months.23 The study described very similar delivery techniques with doses ranging from 40 Gy in 20 fractions to 55.1 Gy in 29 fractions. The mean follow up was 23.4 months (range, 5–50 months) in this series with no evidence of progression or complications from treatment in any of the cases. A case report of orbital CVM treated with FSRT reported a reduction in lesion size and improvement in visual function.20 Together with these studies, our series provides further evidence of FSRT as a valid treatment option for this condition.
The authors found that FSRT is a safe and effective treatment for CVM of the orbital apex with an average 64% volume reduction at 12 months and improvement in visual function, proptosis, and pain, maintained over a median follow up of 33 months. There were no complications from the treatment in our cohort. FSRT is a safe and effective treatment modality to consider in CVM of the orbital apex.
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