Atypical Hyperdense Thornwaldt Cyst with Lateral Extension
Updated: May 6
Clinical Presentation:
A 70-year-old male patient. A lesion approximately 30x16 mm in size is located in the left parasagittal region of the posterior nasopharyngeal wall.
Key Findings:
Posterior wall localization
Left parasagittal position
Finger-like extension towards the Rosenmüller fossa
On MRI;
At this point, the primary diagnosis considered was: 👉 A Thornwaldt cyst with high protein content?
🚩 However, a recent CT scan of the patient was available. On the CT examination, the density of the lesion measured approximately 60-70 HU.
This HU value is high for a simple cyst.
Typical Thornwaldt cyst:
10–30 HU (serous)
Rarely 40–50 HU if proteinaceous
60–70 HU = Very close to true soft tissue density.
🚩 Furthermore, the lesion was extending towards the Rosenmüller fossa.
🔴 Significant extension into the Rosenmüller fossa.
Thornwaldt cysts are generally midline structures.
Rosenmüller fossa extension is a classic hallmark for NPC (nasopharyngeal carcinoma).
At this point, the question arose: "Is this case just a simple cyst, or is it something more?"
The answers to the following questions are very decisive:
1-Post-Contrast Evaluation:
❌ If there is enhancement → Not a cyst.
✅ If only a thin peripheral wall enhances → In favor of a cyst.
2-Diffusion-Weighted Imaging (DWI)
Cyst: Generally no restriction.
Solid tumor: May show restriction.
3-Nasopharyngeal mucosa:
Is there mucosal thickening or asymmetry over it?
Any associated change at the Eustachian tube orifice?
Must-Consider Differentials:
🔷 1. Nasopharyngeal carcinoma (early / submucosal)
Can have smooth contours (especially in early stages).
Characteristically "loves" the Rosenmüller fossa.
T1 iso.
T2 hyper, lower than CSF.
70–80 HU on CT → Highly alarming.
Was it evaluated after contrast?
👉 If solid-enhancement is present, Thornwaldt is excluded.
🔷 2. Lymphoid hyperplasia / submucosal lymphoid tissue
Age 70 → Favors malignancy, but...
Smooth contours.
Soft tissue density(30-40 HU) .
Generally symmetric, though it can be unilateral.
Due to its cellular nature, it may show mild-to-moderate signal alteration (physiological restriction) on diffusion MRI.
🔷 3. Mucus retention cyst
Generally midline and small in size; lateral extension is rare.
More common in young people and children/adolescents.
T2 signal high (like CSF).
CT HU low (approximately 10–40 HU).
We did not have a current post-contrast examination, but when we looked back at the archive, we found that the patient had a Brain MRI and Brain Diffusion MRI performed 4 years ago.
The lesion was identical in both size and morphology 4 years ago.
🔑 The two most critical data points (locking the diagnosis):
1️⃣ The Time Factor
Stable at the same size / same morphology for 4 years.
Zero progression.
👉 Nasopharyngeal carcinoma is practically excluded.
2️⃣ Diffusion
No restriction identified.
👉 The probability of a solid malignant lesion drops significantly.
When these two pieces of information come together, all previous red flags lose their significance to a great extent.
What remains now?
✅ Thornwaldt cyst – proteinaceous / high-content type
Posterior nasopharyngeal localization.
Smooth borders.
T1 iso.
T2 lower hyperintensity than CSF (due to proteinaceous content).
Bright on FLAIR sequence.
DWI negative.
Stable over the years.
High HU on CT → Indicative of thick secretions / mucoid content.
👉 60-70 HU is no longer a disadvantage; on the contrary, it is an indicator of "chronic, thickened content."
🔍 Regarding the extension into the Rosenmüller fossa:
This point can still be confusing, but:
Thornwaldt cysts can lateralize as they expand.
"Finger-like" extension → Cyst herniation / filling into the mucosal pocket.
No mucosal infiltration, no bone destruction, no effusion.
👉 In favor of benign behavior.
📖 Report Sentence:
"A lesion was identified in the posterior wall of the nasopharynx, localized left parasagittally and extending towards the Rosenmüller fossa, with smooth borders, isointense on T1, hyperintense with lower signal than CSF on T2, markedly hyperintense on FLAIR sequences, showing high density on CT but no diffusion restriction. When compared with images from 4 years ago, the lesion remained stable in size and morphology and was considered compatible with a Thornwaldt cyst with high protein content."
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