Bone scan revealed small lesions on lungs/ribcage. Could they be anything other than cancer?

Correlation. Bone scan is very sensitive and not specific. Correlative imaging ususally ct of chest would be needed. Lesions of ribs can also be fractures, benign tumors, and congenital anomalies. Lung uptake can also occcur in metabolic conditions, sometimes infection, scars, etc.
More info needed. It is possible that it could be something else but please could you give us a little bit of background history?
Yes. Bone scan is a very sensitive way to detect local changes in bone metabolism but is less specific to define the type of lesions in various circumstances. It is likely that lesions detected in the ribs are related to previous fractures or bone traumatic lesions, even remote, since it can be seen for several years following the trauma.