Introduction: Sarcoidosis is a multisystem granulomatous disease of unknown etiology, most commonly affecting the lungs and thoracic lymph nodes. Primary cavitary sarcoidosis (PCS) is a rare variant (reported prevalence below 0.5%) in which cavitation represents the presenting manifestation, distinguishing it from the broader entity of pulmonary cavitary sarcoidosis (approximately 2%). PCS poses a diagnostic dilemma as it mimics other cavitary lung diseases, requiring exclusion of pulmonary tuberculosis, atypical mycobacterial infections, fungal disease and vasculitis.

Clinical Case Description: A 24-year-old male presented with erythema nodosum but was otherwise asymptomatic. A chest CT performed during the initial investigation revealed mediastinal and hilar lymphadenopathy along with multiple peribronchovascular lung cavitations across several lobes. This radiological pattern raised suspicion of infectious etiologies or systemic vasculitis, prompting an extensive diagnostic workup.

Microbiological cultures and PCR tests for Mycobacterium tuberculosis, atypical mycobacteria and fungi were negative, as were serological testing for Aspergillus, autoimmune panel and screening for immunosuppression.

In parallel, elevated levels of angiotensin-converting enzyme and non-caseating epithelioid granulomas identified in EBUS-TBNA and bronchial biopsies supported the diagnosis of cavitary sarcoidosis.

During an 18-month follow-up, while the patient remained asymptomatic, a slight worsening of the carbon monoxide diffusion capacity was observed. Repeat imaging showed that while lymphadenopathy regressed, lung cavities remained persistent and slightly larger in size.

Faced with radiological and functional progression, a multidisciplinary team decided to initiate treatment with Methotrexate.

Conclusions: This case underscores the uncommon presentation of PCS and the importance of the systematic exclusion of other etiologies. Radiological progression or persistence of significant cavitary lesions may justify the initiation of immunosuppressive therapy such as Methotrexate, even in the absence of symptoms, to prevent long-term morbidity and complications. Notably, recent evidence (PREDMETH trial, NEJM 2025) supports Methotrexate as a viable first-line alternative to corticosteroids with a more favourable adverse event profile, a relevant consideration in young, asymptomatic patients.