Dear patient Cancer ot metastasis is rare at your age and your need not to worry about it. You might be having osteochondroma of left sided rib which is benign bone tumour. This need to be investigated with 1. Xray of chest postero anterior view and 2. Ct scan of chest to confirm diagnosis and know the exact site of origin of tumour. Please visit radiologycenter nearby you and get it done. You need to consult expert orthopaedic surgeon with report. If diagnosis is confirmed treatment is surgical excision of osteochondroma. All the best.