RADIATION-INDUCED MALIGNANT TUMORS AFTER STEREOTACTIC RADIOSURGERY

To the Editor: We read with interest the case report “Intracranial Sarcoma in a Patient With Neurofibromatosis Type 2 Treated With Gamma Knife Radiosurgery for Vestibular Schwannoma” by Thomsen et al. (1). The authors consider the possibility that the reported tumor may have been malignant from the start, although they do not think this likely. The only other published case report of a malignant cerebellopontine angle tumor following stereotactic radiosurgery (SR) comes from the Pittsburgh group, who are strong proponents of gamma knife treatment of vestibular schwannomas (VS) (2). These authors propose that this tumor was likely malignant at the outset, being partially controlled by gamma knife for several years. Without the initial pathology results, it is very difficult to refute or confirm these differing interpretations. Indeed, because the diagnosis of VS is based on imaging criteria and not on pathology before SR, it is usually impossible to meet the criteria for radiation-induced tumors, as established by Cahan et al. (3). These criteria state that the secondary tumor arises in the irradiated field; that there is a latency period of at least several years; that there is histologic and/or imaging evidence of the initial tumor; that there is histologic confirmation of the secondary neoplasm; and finally that the secondary tumor must differ histologically from the original irradiated tumor. The risks of radiation include both the induction of new tumors and also the malignant degeneration of previously benign tumors. At present there have been no reports that satisfy Cahan's criteria for either of these two types of malignancy for SR. In fact, there are no reports of secondary tumors separate from the VS but in the scatter field of the SR. We have recently seen and submitted a case report (4) of a 57-year-old woman who had presented 7 years previously with a right cerebellopontine angle 3 cm × 2.5 cm mass on computed tomography (CT) diagnosed as a VS. The patient chose SR treatment at the Karolinska Institute. Gamma knife planning consisted of an average dose of 17.1 Gy, with a maximum central and general peripheral dose of 28 and 11 Gy, respectively. The maximal and minimum peripheral dose was 14 and 8 Gy, respectively. Her clinical condition progressively deteriorated over the next 6 to 8 months, with increasing ataxia and loss of the right corneal reflex. Repeat CT and magnetic resonance (MR) scanning demonstrated minimal increase in the cystic component of the tumor, with mild increase in the brainstem distortion and ventricular size. Microsurgery was undertaken through a combined translabyrinthine and middle cranial fossa approach, with gross total resection of the VS. Pathologic results confirmed a benign VS. Clinical and MR imaging follow-up until early 1999 showed no evidence of tumor recurrence, with only an asymptomatic pseudomeningocele at the operative site. In late 1999, she presented with a rapid and progressive history of headaches, confusion, and left hemiparesis. MR scanning demonstrated a right temporal lobe cystic enhancing lesion. Operative removal and pathologic examination demonstrated typical features of a glioblastoma multiforme, confirmed by immunohistologic staining characteristics. Postoperatively there was clinical improvement; however, she deteriorated after a few weeks, with progressive headaches, lethargy, nausea, and vomiting. MR imaging demonstrating tumor recurrence, and she died shortly afterwards despite a second craniotomy and debulking. Fields from the original planning show some radiation in the inferior temporal lobe, and the exact field and pathology details are included in the original report. This case satisfies all of Cahan's criteria for radiation-induced malignancy. Ultimately, however, it is impossible on the basis of a single report to separate coincidental occurrence from causation. This would require documentation of multiple cases to calculate the increased risk over the sporadic frequency of malignant tumors in the same region. It is interesting to note, however, that of the six reported cases of posterior fossa malignant schwannomas identified by Comey et al. (2), three followed SR. Unfortunately, the reporting of rare events such as tumors in a previous radiation field is beset by problems. Their scarcity makes them necessarily case reports, with fewer and fewer journals accepting case reports. The posterior fossa is a territory shared by neurosurgeons and otolaryngologists. This means that meeting presentations and personal communications (which form 2 of the 6 cases identified in Comey et al.'s report) will be split between two essentially separate communities with no easy overall tally or alerting of members to report these cases. For this reason we felt it important to alert the neurotology community to this case report, which is being published in more detail in a neurosurgery journal. Manohar Bance M.B., M.Sc., F.R.C.S.(C.) Abhijit Guha M.D., M.Sc., F.R.C.S.(C.)

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