The Pituitary Gland After Radiation Therapy
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The role of radiation therapy in the management of pituitary adenomas remains controversial among endocrinologists. If radiation therapy is generally accepted in cases of obvious regrowth of a nonfunctioning pituitary adenoma remnant years after surgery if there are no further surgical options, its place is more discussed as a systematic therapeutic adjunct after partial debulking of the same adenomas. Indeed, increased volume of such a pituitary nonfunctioning adenoma remnant does not occur in all patients while pituitary radiotherapy is not without side effects, particularly hypopituitarism and its associated excess mortality. Five-year incidence of hypopituitarism is approximately 20 %, but increases to 80 % by 10–15 years. Visual complications, stroke, and secondary tumors today constitute a very low risk (Figs. 29.1 and 29.2). In general, the risk of recurrence after radiotherapy treatment is very low but not zero (Fig. 29.3). For functioning pituitary adenoma remnants observed after surgery, radiation therapy may be an option as well as medical treatment. The efficacy of radiation therapy is appreciable not only on tumoral control but mainly on hormonal normalization, which is obtained in about half of the cases. Time of remission varies from 12 to 60 months. Different modalities of radiation therapy are available. Radiosurgery delivers a high radiation dose, 12–20 Gy, in a single session. Four forms of radiosurgery are available: Gamma Knife surgery, linear accelerator, CyberKnife, and proton therapy. All four are reserved for tumoral remnants <30 mm in diameter, located at distance from the optic pathways, the ideal indication being intrasellar pituitary adenoma remnant with cavernous sinus extension. Conventional fractionated external beam radiotherapy delivers 45–50 Gy over a 5- to 6-week period and can be offered to patients with large tumors even if the tumor target is less than 5 mm remote from the optic chiasm. The role of MRI is to determine precisely the tumoral extent and tumoral volume to help the radiation oncologist decide the radiation planning; evaluation of cavernous sinus extension and visual pathway relationships are essential. After radiation therapy, MRI evaluates tumoral shrinkage, which can differ with radiation therapy modalities, dose of radiation, location of the tumor, and tumoral cell sensibility. It is accepted that an objective response rate, i.e., >25 % shrinkage, is obtained in more than 80 % of patients harboring a pituitary adenoma remnant at 4 years. There is no significant difference for nonfunctioning versus functioning pituitary adenomas. The response rate at early time points, i.e., between 6 months and 2 years after radiation therapy, is not clearly known. Nevertheless, it is important to determine as early as possible whether radiation therapy in a particular patient will have tumoral efficacy or not. It is our policy to track meticulously the changes in the size of pituitary tumors by using dedicated MRI techniques. Tumoral volume calculated from pituitary remnant dimensions in three orthogonal planes may not be accurate, and may lead to bias and direct volume calculation by the computer. We prefer to consider the transverse and craniocaudal dimensions only, measured on coronal T2WI, serial MRI being obtained with rigorous identical parameters and the coronal plane being strictly perpendicular to the line joining the inferior surface of the genu and the splenium of corpus callosum (Figs. 1.1 and 29.4). By this means, it seems possible to detect early tumoral shrinkage between 6 and 12 months after radiation treatment (Fig. 29.5). The use of coronal T2WI also may permit detection of tumoral signal change before any tumoral size change: in our practice, demonstration of a heterogeneous T2 hyperintensity of the pituitary adenoma remnant frequently precedes objective tumoral shrinkage and asserts that radiation therapy has begun to act (Fig. 29.6).