Aside from clinical manifestations, the two subtypes differ histologically, with the tyrosine kinase receptor ALK being expressed almost exclusively in the systemic form of the disease, and being involved in pathogenesis through alterations in the ALK gene.
But systemic ALK-negative disease is less well understood, and reports have suggested that five-year survival is less than 50 percent compared with 70 percent in ALK-positive systemic disease.
Yet all the women in this study for whom histologic findings were available were ALK-negative, and no deaths have been reported.
"This is a somewhat surprising finding, given the usual poor prognosis of extranodal ALK-negative cases," observed Mattke and colleagues.
Therefore, because implant-associated ALK-negative disease behaves clinically more like cutaneous than systemic disease, some researchers have suggested that it should be considered a distinct disorder.
In addition, treatment "may therefore only require surgical removal of the affected implant and capsule and clinical follow-up, as opposed to aggressive adjuvant chemotherapy and/or radiation therapy," the investigators suggested.
Limitations of the study included possible inconsistencies in interpretation of pathology findings and underreporting of the condition.