Osteosarcoma is the most frequent primary malignant bone tumor in adolescents and young adults. The most common sites are the metaphysis of long bones, particularly around the knee. Common sites of metastasis include the lungs, but pleural effusion as an initial presentation is rare [1] . The presence of pleural effusion may complicate both diagnosis and management. We present a case of a 17-year-old boy with osteosarcoma and concurrent pleural effusion who achieved complete recovery after multimodal therapy.

Osteosarcoma requires a multimodal treatment approach combining neoadjuvant chemotherapy, surgical excision with wide margins, and adjuvant chemotherapy. This strategy improves local control and reduces the risk of systemic spread. The standard MAP regimen (high-dose methotrexate, doxorubicin, and cisplatin) remains the backbone of therapy, with limb-salvage procedures preferred over amputation whenever feasible [6] .

This patient responded well to chemotherapy, with histological analysis showing >90% tumour necrosis, indicating a favourable prognosis. The resolution and non-recurrence of the pleural effusion after tumour control for 3 years further support its reactive nature.

Historically, the prognosis for osteosarcoma was poor, but survival has improved substantially with modern therapy [7] . The 5-year overall survival for localized disease is now around 60% - 70%. Close surveillance with periodic chest imaging is critical, especially during the first 2 - 3 years after treatment, when the risk of relapse is highest.

In our case, the patient has remained disease-free at 18 months and 3 years follow up with full functional recovery of the limb and normal pulmonary function, highlighting the effectiveness of timely multidisciplinary management.

Only a handful of published case reports describe osteosarcoma presenting with pleural effusion without pulmonary metastases, underlining the rarity of this clinical association [8] . These reports emphasize that effusion often resolves with primary tumour treatment, as observed here. Awareness of this phenomenon is important to avoid over staging the disease or unnecessarily labelling the patient as having metastatic disease.

This case underscores a rare but clinically important presentation of osteosarcoma with reactive pleural effusion in an adolescent, occurring without evidence of metastatic disease. Early diagnostic thoracentesis played a pivotal role in distinguishing non-malignant from malignant effusions, thereby preventing disease over-staging and guiding appropriate therapy. Multidisciplinary management—including chemotherapy and limb-salvage surgery—resulted in excellent oncologic and functional outcomes. Recognizing such atypical manifestations is essential to avoid misinterpretation and ensure timely, curative treatment in similar cases.