Likelihood Assessment: Prolactinoma vs. Malignancy

This is almost certainly a prolactinoma, not a malignant lesion, and the dramatic response to cabergoline strongly confirms this diagnosis. 1, 2

Why This is Highly Likely a Prolactinoma

The clinical presentation is classic for prolactinoma:

Primary amenorrhea with galactorrhea in an adolescent female is pathognomonic for prolactinoma 1, 3
The 2 cm sellar lesion with suprasellar extension and optic chiasm compression is consistent with a macroprolactinoma 1
The peripheral enhancement pattern on MRI is typical for prolactinoma 1

The prolactin levels, while variable, fit the expected pattern:

Initial prolactin of 255 ng/mL (first measurement) is appropriate for a 2 cm macroprolactinoma 1, 3
The second measurement of 136.47 ng/mL before treatment still indicates macroprolactinoma (prolactin >100 ng/mL typically indicates macroadenoma) 3
After only 4 doses of cabergoline 0.25 mg, prolactin dropped to 63 ng/mL—this represents a 75% reduction and is highly specific for prolactinoma 1, 2, 4

The fluctuation in prolactin levels before treatment (255→136.47 ng/mL) does not argue against prolactinoma:

Prolactin levels can vary significantly based on timing of blood draw, stress, sleep, and other physiological factors 1
The Endocrine Society guidelines specifically note that "inconsistent symptoms and laboratory results can occasionally arise" in prolactinomas 1
Both values remain elevated and consistent with macroprolactinoma 3

You should have checked for the "high-dose hook effect":

For large pituitary lesions with unexpectedly low or normal prolactin levels, serial dilutions should be performed to detect this artifact 2, 5
However, your patient's prolactin levels are appropriately elevated for the tumor size, making hook effect unlikely 2

The dramatic prolactin reduction after only 4 doses is virtually pathognomonic for prolactinoma:

Cabergoline normalizes prolactin in 60-83% of prolactinomas, with most patients showing significant reduction within 2-3 months 1, 2, 4
Your patient achieved a 75% reduction in just 2 weeks (4 doses)—this rapid response is characteristic of prolactinoma and would not occur with other sellar masses 4, 6
Malignant lesions do not respond to dopamine agonists in this manner 1

Multiple factors argue strongly against malignancy:

Craniopharyngiomas (the main differential mentioned in the MRI report) do not cause hyperprolactinemia of this magnitude and do not respond to cabergoline 1
Malignant sellar lesions (metastases, lymphoma) would not show this dramatic response to dopamine agonist therapy 1
The patient's age (adolescent), sex (female), and clinical presentation (amenorrhea, galactorrhea) are classic for prolactinoma, not malignancy 3
The peripheral enhancement pattern on MRI is typical for prolactinoma, not malignant lesions 1

The probability of malignancy in this case is <1%. 1, 3

Continue cabergoline with dose escalation as needed:

Current dose of 0.25 mg (presumably twice weekly based on standard dosing) is producing excellent response 1, 2, 7
Target prolactin normalization (<25 ng/mL) and tumor shrinkage 1, 2
For macroprolactinomas, standard doses up to 2 mg/week are typically required, though some patients respond to lower doses 1, 7, 4
If prolactin doesn't normalize with standard doses, increase gradually to 3.5 mg/week, or up to 7 mg/week in exceptional cases 1, 7

Monitoring protocol:

Recheck prolactin levels in 1-3 months, then every 3-6 months until stable 2, 5
Repeat MRI at 3-6 months to assess tumor shrinkage 1, 2
Monitor for visual field improvement given the optic chiasm compression 1
Baseline echocardiogram now, then every 5 years if dose remains ≤2 mg/week, or annually if dose exceeds 2 mg/week 2, 7, 8

Watch for cerebrospinal fluid leak:

Medication-induced tumor shrinkage in lesions with sphenoid bone invasion can cause rhinorrhea (CSF leak) 1, 7
This typically occurs within the first 3-17 months of treatment 1
If rhinorrhea develops, test nasal secretions for β2-transferrin or β-trace protein 1

Do not pursue biopsy or surgery at this point:

The excellent response to cabergoline confirms the diagnosis 1, 2
Surgery is reserved for patients who fail medical therapy or develop worsening vision despite treatment 2, 7
Surgical outcomes are worse in pediatric patients compared to adults 2

Do not be concerned about the prolactin variability:

Prolactin levels naturally fluctuate 1
The therapeutic response to cabergoline is far more diagnostically significant than the absolute baseline values 4, 6

Do not stop cabergoline prematurely:

Treatment typically continues for at least 2 years after prolactin normalization and tumor disappearance on MRI 2
Younger patients with high initial prolactin levels (markers of larger tumors) are less likely to achieve complete remission and more likely to relapse after discontinuation 1