What is the significance of fibrocollagenous tissue found on pleural fluid cytology and what further diagnostic steps should be taken?

Fibrocollagenous Tissue in Pleural Fluid Cytology

Clinical Significance

The presence of fibrocollagenous tissue fragments in pleural fluid cytology most likely represents a solitary fibrous tumor of the pleura (SFTP), a rare mesenchymal neoplasm that requires tissue biopsy for definitive diagnosis and complete surgical excision for treatment. 1, 2

Fibrocollagenous tissue is not a typical finding in routine pleural fluid cytology and should immediately raise suspicion for SFTP rather than the more common causes of pleural effusion such as adenocarcinoma or mesothelioma. 1, 2, 3

Diagnostic Approach

Immediate Next Steps

• Obtain image-guided core needle biopsy or thoracoscopic biopsy to establish the definitive diagnosis, as cytology alone cannot reliably distinguish benign from malignant SFTP or exclude other spindle cell neoplasms. 3, 4

• Request CD34 immunostaining on the cytology cell block if available, as CD34 positivity supports the diagnosis of SFTP (positive in approximately 85% of cases). 1, 2

• Obtain contrast-enhanced CT chest to characterize the mass, assess for pleural-based origin, and determine whether the tumor is pedunculated (arising from visceral pleura) or sessile (arising from parietal pleura or chest wall), which impacts surgical planning. 5, 6

Cytologic Features of SFTP

The characteristic cytologic findings that should prompt consideration of SFTP include:

• Irregular ropy fragments of dense collagen intimately admixed with spindle cells, creating a distinctive appearance not seen in typical malignant effusions. 2, 3

• Scattered fusiform or oval cells with bland nuclear features, evenly distributed chromatin, and minimal pleomorphism in benign variants. 1, 2

• Loose aggregates of spindle cells enmeshed in collagenous matrix, rather than the papillary groups or cytoplasmic vacuolation typical of adenocarcinoma. 2, 3

Distinguishing Benign from Malignant SFTP

Cytology has limited ability to predict clinical behavior, but certain features suggest malignancy:

• Highly cellular clusters with mitotic activity favor malignant SFTP, though significant cytologic atypia may be absent even in malignant variants. 3

• Greater cellularity with dispersed cells rather than cohesive clusters suggests malignant behavior. 1

• Definitive determination of malignancy requires histologic assessment for high cellularity with nuclear crowding, high mitotic activity (>4 mitoses per 10 high-power fields), and pleomorphism. 3, 4

Differential Diagnosis

The finding of fibrocollagenous tissue with spindle cells requires exclusion of:

• Neurogenic tumors (schwannoma, neurofibroma) - typically S100 positive, CD34 negative
• Mesothelioma - calretinin and CK5/6 positive, CD34 negative 7
• Sarcomatoid carcinoma - cytokeratin positive, CD34 negative
• Synovial sarcoma - TLE1 and bcl-2 positive
• Hemangiopericytoma - CD34 may be positive but shows characteristic staghorn vessels 1

Treatment Algorithm

For Confirmed SFTP

• Complete surgical excision with negative margins is the definitive treatment for both benign and malignant SFTP, with cure rates of 88-92% overall. 6, 4

• Video-assisted thoracic surgery (VATS) with intraoperative margin assessment is the preferred approach for pedunculated tumors arising from visceral pleura, typically requiring wedge resection. 4

• Thoracotomy with wider resection is required for sessile tumors, which may necessitate formal lung resection or en bloc chest wall resection due to higher recurrence risk. 6, 4

Prognosis and Follow-up

• Benign SFTP has an 8% local recurrence rate that is usually amenable to curative re-excision with long-term survival. 6

• Malignant SFTP, particularly sessile variants, has a 63% recurrence rate even with complete resection, and most patients with recurrence die within 2 years. 6

• Long-term surveillance is mandatory as recurrences can occur years after initial resection, especially in malignant or sessile tumors. 6, 4

Critical Pitfalls to Avoid

• Do not assume this represents typical malignant effusion from adenocarcinoma or mesothelioma, as the treatment paradigm is entirely different (surgical excision rather than systemic therapy or pleurodesis). 7, 6

• Do not rely on cytology alone for diagnosis - tissue biopsy is mandatory as cytology cannot reliably predict behavior or exclude other spindle cell neoplasms. 3

• Do not perform simple thoracentesis or pleurodesis without tissue diagnosis, as this represents a potentially curable tumor requiring complete surgical excision. 4

• Do not assume benign histology guarantees cure - even benign SFTP requires complete excision with negative margins and long-term follow-up due to recurrence risk. 6, 4