How can you tell if a lung adenocarcinoma is a second primary lung cancer or a metachronous lung metastasis from a previously diagnosed and treated rectal cancer? do treatment and prognosis differ in these two scenarios?

Biopsy of the current lung tumour will help to decide whether it is a primary lung tumour or secondary to rectal cancer. Based on the type of the cancer the treatment may vary.

A biopsy will be needed to clarify origin as treatment/approach will be different based on primary. Keep oncologist involved and communication with your medical team efficient.