As far as I know, x-ray or ultrasound is the most reliable way to find out if there's another one somewhere.
I've taken a lot of information from this thread and emails with pigpal and I plan to ask the vet about them tomorrow.
The only downfall with Murray is that it is almost impossible to talk to get a hold of him- he doesn't have a lot of free time.
Source/History: Adult male guinea pig with rapidly growing mass associated with right teat. No evidence of reproductive system disease. Intact male. Received a 2 cm skin/covered specimen with well-demarcated mass.
Representative sections of the mass are examined. The mass is present subjacent to the teat. It is fairly well demarcated but nonencapsultated and is surrounded by a peripheral margin of normal tissue ringing from .03cm and more.
The center portion of the mass consists of small tubules and cluster formed by mildly atypical cuboidal epithelial cells. Narrow bands of fibrous stroma are interspersed and the mitotic rate is approximately 1/hpf. Some of the tubules contain luminal secretory material.
There are also multifocal areas of necrosis and inflammation present within the mass. In some areas, there is also a prominent mycepithelia stroma between epithelia structures.
Microscopic findings: mammary tubular and variably cystic Adenocarcinoma, well demarcated and well differentiated, guinea pig.
Prognosis - Long term - potentially guarded.
Comments: Mammary neoplasm's can occur in both male and female guinea pigs. The majority of mammary tumors in guinea pigs are consistent with benign fibroadenomas. Approximately 30% are adenocarcinoma, as in this case.
They generally behave as local invasive tumors with rare metastasis reported. In this case, the mass was fairly well demarcated and appeared within surgical margins. This would suggest complete removal; however findings should be used in conjunction with surgical findings.
Lymphatic and/or vascular invasion were not identified; however, I cannot guarantee that they have not yet taken place.
Close monitoring of the mammary region for re-growth is suggested as well as monitoring of regional lymph nodes for any sign on enlargement that would suggest metastasis.