A malignancy limited to the organ of origin; no spread beyond organ of origin; infiltration past basement membrane of epithelium into stroma of organ.
Tumor can be widely invasive or even show metastases within the organ itself and still be considered "confined to organ of origin") or localized. Usually a straightforward stage determination for organs which have definite boundaries (prostate, testis, stomach, etc.) or sites where there is a clear line between the organ of origin and the surrounding region. (Exception: skin)
For internal organs, it is generally impossible to determine whether tumor is localized without exploratory surgery. However, the increasing sophistication of many imaging techniques is predicted to eventually make exploratory surgery obsolete.
Read both pathology reports and operative reports for comments on gross evidence of spread, microscopic extension and metastases. Also read diagnostic imaging reports for mention of distant disease. If path report, operative report and other investigations show no evidence of spread, tumor may be assumed to be localized.
When staging cases are diagnosed clinically only, it is better to record as "stage not recorded" than "localized" when there is no other evidence of spread. Recognize the names of different structures within the organ (such as lamina propria, myometrium, muscularis) so that reference to invasion of such structures will not be interpreted as regional spread.