99mTechnetium pentavalent dimercaptosuccinic acid scintigraphy in the follow-up of clinically nonfunctioning pituitary adenomas after radiotherapy

BACKGROUND: It is still difficult to differentiate pituitary adenoma remnants from postradiotherapy fibrosis by computed tomography (CT) or magnetic resonance imaging (MRI), especially in patients with clinically nonfunctioning pituitary adenomas (NFA), lacking circulating markers to follow disease progression or cure. OBJECTIVE: We investigated the usefulness of scintigraphy with technetium-99m pentavalent dimercaptosuccinic acid [^99mTc(V)DMSA], shown previously to detect most pituitary GH- and PRL-secreting adenomas and NFA, with tumour-to-background ratios (T/B) as high as 25-fold. PATIENTS: Eighteen patients with NFA (study group), 10 patients with GH- and three patients with PRL-secreting adenomas (control group), all of whom had undergone previous surgery. DESIGN: The study was an open longitudinal design. Pituitary CT/MRI and ^99mTc(V)DMSA scintigraphy was performed before and 1, 3 and 5 years after conventional radiotherapy. Tumour size was measured as maximal diameter of the residual lesion, while uptake of ^99mTc(V)DMSA was measured as a T/B ratio. RESULTS: At study entry, pituitary ^99mTc(V)DMSA uptake was found in 13 NFA (72.2%), seven GH-secreting (70%) and all PRL-secreting adenomas; remnant tumour was documented by CT/MRI in all 31 patients. Maximal remnant diameter was significantly higher in patients with positive (13.3±0.9 mm) than in those with negative scintigraphy (7.0±0.3 mm, P <0.001). During the 5-year follow-up postradiotherapy, a significant decrease in ^99mTc(V)DMSA uptake (9.7±0.8 vs. 3.2±0.5, P <0.0001) occurred in all but three patients. Two NFA patients died of tumour invasion 19 and 36 months after radiotherapy and one acromegalic patient had no change in his hormone levels. In the eight negative patients (five NFA and three GH), scintigraphy remained negative throughout follow-up. A remarkable shrinkage of the remnant tumour was observed in both the patients with negative (from 7.0±0.3 to 1.9±0.6 mm, P <0.001) and in those with positive scintigraphy (from 13.3±0.9 to 7.3±0.6 mm, P <0.001). At the end of the study, CT/MRI showed evident remnant tumour in 13 of 16 NFA (81.2%), nine GH-secreting (90%) and all three prolactinomas (100%), while the scintigraphy was negative (T/B <1) or faintly positive (T/B 1-2) in eight of 16 NFA (50%), five GH-secreting (50%) and one prolactinoma (33.3%). CONCLUSIONS: Functional imaging of pituitary remnant adenomas (> 10 mm in size) by ^99mTc(V)DMSA depicts viable pituitary adenoma remnants. This approach may be of clinical value in patients with clinically nonfunctioning adenomas to monitor the effects of radiotherapy.