If doctors can't figure out where an adenocarcinoma starts, how can they treat it successfully?

Answer 1

They need to do a biopsy of the cancer. In general a pathologist or usually two or three will look under a microscope to try and identify what type of cell is cancerous and where in the body that cell originated from (if they see breast cells they will know the cancer came from that location) Sometimes it is very difficult to identify advanced cancer cells because they are so mishapen that they no longer resemble the original normal cell. This is known as an Unknown Primary:

http://www.thedoctorsdoctor.com/diseases/metastastic_malignancy_unknown_primary.htm

NCI: Unknown Primary
http://www.cancer.gov/cancertopics/pdq/treatment/unknownprimary/HealthProfessional

It is best to be persistent and ask for a definitive diagnosis if at all possible. Even if it means sending the biopsy to more than one pathologist. If that is impossible than an oncologist uses all his experience and skill to come up with a treatment plan based on what they know about where in the body the cancer was found.

Answer 2

Its all on the cellular level with mutations in the cells which cause them to become malignant and start replicating abnormally.
I spoke with a cell biologist about 5 yrs ago and he says being able to look at cells under a microscope makes him understand how cell division makes big nasty cancer tumors. Its a goof in the cell division that causes cancer.