Gamma and X-based stereotactic EBRT in pituitary disease: what they are and when to use them

John Newell-Price

Senior Lecturer and Consultant Endocrinologist, University of Sheffield

Summer School 5-8 July 2005
St Aidan’s College, Durham University, Durham, UK

Radiotherapy remains an important tool in the management of functioning and non-functioning pituitary tumours. Regardless of the mode of delivery the rationale behind treatment is to deliver ionising radiation to the target, whilst causing as little damage to other structures, in an effort to control tumour volume and hormone hypersecretion.

In conventional external beam radiotherapy (EBRT) this is achieved by fractionation, but the radiation extends some way beyond the target. Keeping the daily fraction dose to <180cGy protects important structures, such as the optic chiasm. The 10-year recurrence rate of non-functioning pituitary tumours is approximately 40%, but this is reduced to approximately 10% by the use of EBRT, with a very low incidence of optic neuropathy and second CNS tumours. This technique is particularly suited to large inoperable t umours, large residual tumours following surgery including those encroaching on the chiasm, or where no tumour can be seen on a post-operative MRI scan in the context of Cushing’s disease or acromegaly.

Focused radiotherapy can deliver a single highly localised dose of radiation to the target, avoiding other structures. The two main modes of delivery are the dedicated Gamma knife unit, and using a conventional linear accelerator (LINAC) with X-knife software. Another means of delivering focused radiotherapy is stereotactic conformal radiotherapy, which is EBRT with beams shaped to the tumour, delivered by fractionation.

In the X-knife technique a conventional LINAC gantry is used to deliver focused radiotherapy by means of arcing the radiotherapy beam by rotating the arm of the gantry over the patient, so delivering a high dose to the centre of the arc. The head is fixed by means of a custom-made bite piece. Data on the use of this technique, as adjuvant therapy to EBRT, in highly active GH-secreting adenomas demonstrates an additional benefit in enhancing biochemical control.

In the Gamma knife technique the skull is placed in an external frame under local anaesthetic in the morning, and MRI planning used to identify the target in 3-dimensional space. The precision of the radiotherapy beam of the Gamma knife unit is 0.1mm. Following dose planning, radiosurgery is performed on the same day with the patient fully conscious, although general anaesthesia is possible for those with claustrophobia and children. The main limitation to this technique is the proximity of the optic chiasm, with doses kept to <8Gy in the single dose fraction. In Sheffield approximately 600 patients per year are treated by the gamma knife technique, with pituitary disease representing 3% of this workload. Dosimetry planning is more favourable for the gamma knife technique compared to X knife.

In our series of 20 patients treated for acromegaly the median 50% isodose to the periphery of the tumour was 25 Gy (19-35), using 2-10 fields, and <8Gy to the optic chiasm in all cases. At final follow-up there was a mean 54% fall in IGF1, with 40% of patients achieving safe GH levels at 5 years. Only one patient developed a new anterior pituitary hormone deficiency, at 3.5 years. There have been no cranial nerve defects, or deterioration in vision. These data compare favourably with other series.

Gamma knife radiotherapy is a highly effective form of radiotherapy for GH-secreting tumours, with an apparently lower incidence of new pituitary hormone deficits compared to conventional 3 field radiotherapy, but longer follow up is needed. Data on the outcome in Cushing’s syndrome is, however, less favourable.

The opinions expressed in this paper are those of the speaker and do not necessarily reflect the views of the Society

Revised: 28-Jul-2005