田村 史子¹, 角田 博子², 八木下 和代², 鹿俣 直樹³

Fumiko TAMURA¹, Hiroko TSUNODA², Kazuyo YAGISHITA², Naoki KANOMATA³

¹聖路加国際病院臨床検査科, ²聖路加国際病院放射線科, ³聖路加国際病院病理診断科

¹Clinical Laboratory Department, St. Luke's International Hospital, ²Department of Radiology, St. Luke's International Hospital, ³Department of Pathology, St. Luke's International Hospital

キーワード : breast cancer, small breast cancer, breast ultrasonography, recall criteria, breast cancer screening

目的：近年超音波（US）検査で小さな乳癌が検出されることが増加してきた．乳房US検診要精検基準ではUS上5 mm以下の腫瘤は基本的に精検不要，形状不整な場合は要精検とすることになっているが，小さいがために形態の評価が難しい．今回US上5 mm以下の充実性腫瘤として描出された乳癌がどのように検出され，どのようなタイプの乳癌であったかを検討し，今後のUS判定に役立てることを目的とした．対象と方法：2016年5月～2021年4月に当院で手術された連続症例のなかで，US画像を確認できた5 mm以下の乳癌症例について，病理組織型，サブタイプ，患者背景，発見契機，US所見などを後方視的に検討した．結果と考察：16名16病変が該当し，組織型は浸潤癌8例，非浸潤性乳管癌（DCIS）8例であった．すべて無自覚で，検診やサーベイランス，術前検査における偶発病変であった．浸潤所見をとらえられた症例はすべて浸潤癌であった．遺伝性乳癌卵巣癌症候群症例を除くと，いずれも悪性度の低いホルモンレセプター陽性の乳癌であり，次の検診で検出したとしても生命予後に影響のない乳癌である可能性が示された．結論：5 mm以下の腫瘤でもUS上の浸潤所見は正確に浸潤癌を判定できた．浸潤所見のない5 mm以下の腫瘤は，日本乳腺甲状腺超音波医学会の診断樹では基本的に精検不要とされるが，今回の結果でも平均的リスクの場合，すべて低悪性度のDCISであった．今後，過剰診断，過剰治療についても考える必要があるといえる．

Purpose: In recent years, small breast cancers have been increasingly detected using ultrasonography (US). According to the recall criteria established by the Japan Association of Breast and Thyroid Sonology (JABTS), a mass measuring ≦5 mm and lacking ultrasonographic findings of invasion does not require further examination, and only masses with irregular margins should undergo further examinations. However, the small size of these lesions makes it difficult to evaluate their morphology. Therefore, to contribute to future US judgement, this study aimed to examine how breast cancers identified as solid masses measuring ≦5 mm on US were detected and what type of breast cancer they constituted. Subjects and Methods: We retrospectively reviewed the histopathologic types, subtypes, patient backgrounds, detection opportunities, and US findings of breast cancer lesions measuring ≦5 mm with confirmation on US among consecutive cases operated on at our hospital from May 2016 to April 2021. Results and Discussion: Sixteen patients with 16 lesions were included: eight with invasive carcinoma and eight with ductal carcinoma in situ (DCIS). All patients were unaware of their status, and lesions were detected on screening or surveillance, or as incidental lesions during preoperative examinations for other breast cancers. Excluding patients with hereditary breast ovarian cancer syndrome, all patients had hormone receptor-positive low-grade breast cancer, indicating that these cancers might not have affected the patients' prognosis whether they were detected at that time or at the next year's screening. Conclusion: Invasive US findings can accurately determine invasive cancer, even for masses measuring ≦5 mm. In the JABTS diagnostic tree, masses measuring ≦5 mm without invasive findings are basically considered to not require a further examination. All of the average-risk cases were low-grade DCIS even in the present results. It is necessary to consider overdiagnosis and overtreatment in the future.