Learning Objectives: 1) To clarify the natural history of vestibular schwannoma and outline treatment options 2) To present two cases of abnormal vestibular schwannoma growth following STRS 3) To discuss potential reasons for tumour growth after STRS 4) To confirm the need for lifelong radiological and clinical surveillance following STRS.

Introduction: Vestibular schwannomas, are benign tumours of Schwann cell origin that occur on the eighth cranial nerve. Commonly presenting symptoms include hearing loss, tinnitus and balance disturbance. Tumour progression can lead to brainstem compression, cranial neuropathies and hydrocephalus. Smaller, slow-growing tumours can be safely observed, but larger tumours necessitate treatment in the form of either surgery or STRS. The literature states that tumours up to 3 ;cm in diameter can be successfully controlled in the majority of patients with STRS, and a recent Cochrane review concludes that the treatment method for large vestibular schwannomas should be chosen on an individual basis, taking into consideration the patient's preferences, clinician experience and the availability of radiotherapeutic equipment.

Methods: We present two cases of vestibular schwannoma which were treated with STRS, and decreased in size during the two years following treatment, following which they began to exhibit further growth.

Discussion: Pseudoprogression of vestibular schwannomas for up to two years following STRS is a well-documented phenomenon, following which the oedematous tumour regresses in response to the STRS.

Potential reasons for tumour growth over two years after STRS are malignant transformation of the tumour, and late failure of STRS. Although rare, there is a documented risk of malignant change following exposure to radiation. Late failure of STRS is possible if, despite an early response to STRS, living cells within the tumour develop an adequate blood supply for growth.

Conclusions: Vestibular schwannoma patients warrant lifelong radiological and clinical surveillance following STRS, as there is a small chance of initial regression followed by further growth. These cases therefore require surgery, for tumour removal and histological diagnosis.