Benign CNS tumors
Pituitary adenoma
A benign tumor under the brain where radiation controls growth and, for hormone-secreting tumors, quiets overproduction.
Definition
Pituitary adenomas are benign tumors of the anterior pituitary gland. They cause symptoms two ways: mass effect on the optic chiasm (bitemporal visual field loss, headaches) and hormone overproduction — prolactin (galactorrhea, amenorrhea), growth hormone (acromegaly), ACTH (Cushing disease), or rarely TSH. Nonfunctioning adenomas are the most common. Because the gland sits in the sella turcica directly beneath the optic chiasm, treatment must preserve vision and normal hormone function.
How radiation treats it
Radiation stops adenoma cell proliferation, preventing regrowth after subtotal transsphenoidal resection and controlling unresectable or recurrent tumor. In functioning adenomas, radiation gradually reduces hormone hypersecretion over one to three years as the secretory cells are depleted. SRS delivers ablative-level focal dose in a single session while the sharply conformal plan spares the chiasm, which tolerates only about 8–10 Gy.
Side effects
- Acute effects are minimal: mild headache or fatigue.
- Hypopituitarism is the main late effect, developing in a substantial minority over years and requiring endocrine monitoring and replacement.
- Rare visual injury when the chiasm dose exceeds tolerance; SRS requires at least 3–5 mm of separation between tumor and chiasm.
Simulation process
A thin-slice contrast MRI fused to a stereotactic CT defines the tumor, cavernous sinus extension, and the optic chiasm and nerves, which are contoured with strict constraints. For SRS, a frame or high-precision mask is applied; for fractionated treatment, a tight-fitting thermoplastic mask with daily image guidance is used. Hormone status (prolactin, IGF-1, cortisol) is documented before treatment as a baseline for response.
Treatment process
Stereotactic radiosurgery to 15–25 Gy in a single fraction (lower doses for nonfunctioning, higher for secretory adenomas), fractionated stereotactic RT to 21–25 Gy in 5 fractions, or conventionally fractionated 45–50.4 Gy in 25–28 fractions for larger or chiasm-hugging tumors. Tumor shrinkage is slow; hormone normalization in secretory adenomas can take one to three years. Patients undergo lifelong endocrine and MRI follow-up.
Key takeaways for students
- The optic chiasm is the dose-limiting structure — separation from the tumor decides SRS eligibility.
- Functioning adenomas need higher marginal doses and years of hormone follow-up.
- Endocrine replacement for hypopituitarism is the most common long-term consequence.