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1.The history is bit inadequate, as you have not described the lump and its location on the chest, however i will try to explain you the possible causes.
2.The most possible cause could be infective aetiology, hair follicular infection with collection within - which needs antibiotic in early un complicated condition, where late and complicated needs excision.
3.Other possible cause will be
lipomas which needs to monitored unless until symptomatic where excised.
4.Finally bone lesions arise on the chest wall, and
hernias. Which need to examine and do needful treatment.
5.An ultrasund scan will help in the assessment of the nature of the lump, its source and extensions.
6.Suggest to get examined and diagnosed and treated appropriately.
7.Probably the doctor would have visually able to make out the type of lump he is dealing so started treatment. Good luck.
Hope i have answered your query,
any thing to ask do not hesitate to ask.
http://doctor.healthcaremagic.com/doctors/dr-ganesh/62888