Pituitary irradiation is used to decrease the rates of tumor recurrence and to reduce the excess hormone secretion of functioning pituitary adenomas. There is some debate regarding the case selection of patients best treated by this modality. Conventional irradiation of the pituitary fossa is three-field external beam irradiation. This stereotactic technique delivers charged particle beams that can be targeted to deposit ionizing radiation at a specific depth. Radiotherapy of this type is delivered in fractions (usually less than 200 cGy/d) designed to reduce damage to the optic chiasm, that is believed to be largely mediated through effects on the blood vessels supplying this structure.
Alternative methods of radiation delivery include the use of photon techniques. Examples include the design of the Swedish y unit, termed the Gamma Knife, and linear accelerator methodology, termed the X-knife (see Chapter 14).
The main adverse consequence of pituitary irradiation is hypopituitarism, the incidence of which is greater the more compromised the function before radiation delivery. The somatotroph is most sensitive to radiation damage, followed by gonadotrophs and corticotrophs. The TSH axis is relatively insensitive, and DI as a result of loss of posterior pituitary function is exceedingly rare. Rates of hypopituitarism vary from series to series, but in patients who are irradiated for acromegaly, 25% required new endocrine replacement after 5 yr, with this figure rising with further passage of time (39). When assessed 10 yr after fractionated radiotherapy, 47% of patients were hypogonadal, 30% hypoadrenal, and 16% hypothyroid in another series (40). The rate of GH deficiency is likely to be significantly greater than 50%.
Recurrence of functioning tumors and regrowth of primary tumors after pituitary irradiation have also been the subject of debate. Ten-year recurrences for acromegaly (8%), CD (12%), NFPA (17%), and prolactinoma (24%) have been reported (41). In addition, regrowth of tumors has been reported varying from 21% to 32% during roughly a 10-yr period, significantly lower rates than observed in the nonirradiated population (42). As noted, our own recurrence rates appear to be considerably less than these figures.
After irradiation of pituitary tissue, regular surveillance is mandatory to detect development of hypopituitarism, particularly GH deficiency. Basal pituitary profiles, including measurement of IGF-1 and assessment for the clinical features of GH deficiency, should be performed regularly. Dynamic testing should be performed in cases of clinical doubt. In addition, regular imaging is required to assess tumor response to radiotherapy and to facilitate early detection of recurrence. Initial MRI/computed tomography (CT) imaging may be performed 6 mo after surgery (less than this and perioperative changes are likely to obscure the assessment), and regularly thereafter with increasing time interval depending on clinical response. Accurate assessment of visual fields is a useful adjunct. The response of functioning tumors to irradiation should also be regularly assessed, as outlined in the previous subheadings. The responses in CD and acromegaly need especially vigilant monitoring, because typically these patients are on medical treatment (e.g., adrenolytic drugs, somatostatin analogs), which must be titrated to response.
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