Rare Breast Cancer Subtypes: Histological, Molecular, and Clinical Peculiarities

Breast cancer encompasses a collection of different diseases characterized by different biological and pathological features, clinical presentation, response to treatments, clinical behavior, and outcome. On the basis of cell morphology, growth, and architecture patterns, breast cancer can be classified in up to 21 distinct histological types. Breast cancer special types, including the classic lobular invasive carcinoma, represent 25% of all breast cancers. The histological diversity of breast carcinomas has relevant prognostic implications. Indeed, the rare breast cancer group includes subtypes with very different prognoses, ranging from the tubular carcinoma, associated with an indolent clinical course, to metaplastic cancer, whose outcome is generally unfavorable. New approaches based on gene expression profiling allow the identification of molecularly defined breast cancer classes, with distinct biological features and clinical behavior. In clinical practice, immunohistochemical classification based on the expression of human epidermal growth factor receptor 2 and Ki67 is applied as a surrogate of the intrinsic molecular subtypes. However, the identification of intrinsic molecular subtypes were almost completely limited to the study of ductal invasive breast cancer. Moreover, some good‐prognosis triple‐negative histotypes, on the basis of gene expression profiling, can be classified among the poor‐prognosis group. Therefore, histopathological classification remains a crucial component of breast cancer diagnosis. Special histologies can be very rare, and the majority of information on outcome and treatments derives from small series and case reports. As a consequence, clear recommendations about clinical management are still lacking. In this review, we summarize current knowledge about rare breast cancer histologies. 摘要 乳腺癌包括一系列不同的疾病，这些疾病的特点为具有不同的生物学和病理学特征、临床表现、对治疗的反应、临床行为以及预后。根据细胞形态学、生长和结构模式，乳腺癌最多可以分成 21 种截然不同的组织学类型。乳腺癌特殊类型（包括典型浸润性小叶癌）占所有乳腺癌的 25%。乳腺癌的组织学多样性有相关的预后启示。实际上，罕见的乳腺癌组包括预后截然不同的亚型，从小管癌（具有惰性的临床过程）到化生性癌（预后通常不良）。基于基因表达分析的新方法可以识别根据分子确定的乳腺癌类型，这些类型具有截然不同的生物学特征和临床行为。在临床实践中，基于人表皮生长因子受体 2 和 Ki67 表达的免疫组织化学分类代替固有分子亚型得以应用。然而，固有分子亚型的识别几乎完全局限于浸润性乳腺导管癌的研究。而且，根据基因表达分析，一些预后良好的三阴性组织分型可能归入预后不佳组。因此，组织病理学分类依然是乳腺癌诊断的关键环节。特殊组织学类型非常罕见，有关结果和治疗的大部分信息源于较小的系列和病例报告。因此，有关临床管理的明确建议仍然缺乏。本综述总结了当前有关罕见乳腺癌组织学类型的知识。 (The Oncologist) 2014;19:805–813 The histological diversity of breast carcinomas has relevant prognostic implications. Special histologies can be very rare, and the maj