Literally within minutes of publishing my previous post following up on a digital case challenge on bronchioloalveolar carcinoma (BAC) of the lung, I discover this month's (October 2010) Archives of Pathology and Laboratory Medicine on my desk which contains a nice article by Lindsay Schmidt and Jeffrey Myers titled, "Bronchioloalveolar Carcinoma and the Significance of Invasion: Predicting Biologic Behavior" (Arch Pathol Lab Med 2010;134:1450-1454).
The issue is how to designate tumors that show stromal invasion but are mostly BAC (think Miracle Max from The Princess Bride: "It just so happens that your friend here is only MOSTLY dead. There's a big difference between mostly dead and all dead."). Ah, but how much is mostly? This paper summarizes studies that demonstrate the presence of invasion only becomes important for prognosis only when the invasive component measures more than 0.5 cm in greatest dimension. Unfortunately, the WHO Classification rather dogmatically defined BAC as a tumor showing "growth of the neoplastic cells along pre-existing alveolar structures without evidence of stromal, vascular or pleural invasion" yet enigmatically did not distinguish cytologic features as part of the diagnosis. The authors discuss the alternative viewpoints to address this situation by either a new diagnostic term (microinvasive or minimally invasive adenocarcinoma) versus just revising the current definition of BAC to include those tumors with less than 0.5 cm of invasion.
A worthy pearl: 2 features most helpful in identifying invasion: (1) a myofibroblastic stromal response (distinguished from "alveolar collapse" and central scar) and (2) isolated tumor cells.
Check out the paper, I especially like the gross photographs. I've seen worse.
Just for fun, check out Miracle Max: