腹部CT検査で腎に1cmのhigh density（高吸収）な嚢胞様の腫瘤が。明らかに良性病変に見えますが、本当に良性といえるのでしょうか？
久しぶりにMD consult（電子教科書データベース）にアクセス。Bosniak分類というのを見つけました。ボスニア分類と読むのでしょうか？
Hyperdense lesion with no enhancement（造影効果のない高吸収病変）はBosniak分類ではII、悪性腫瘍のリスクは0-5%で、定期的な検査が必要、ということでした。
Bosniak分類について日本語の発表はあるようですが、分類そのものの日本語の情報源は検索でうまくみつけられませんでした。Bosniak分類の原著論文（1997年）はオンラインで入手できませんでしたが、2005年のBosniakらのreviewが見つかりました。
Urology. 2005 Sep;66(3):484-8.
An update of the Bosniak renal cyst classification system.
Israel GM, Bosniak MA.
http://www.ncbi.nlm.nih.gov/pubmed/16140062
Table I . Bosniak renal cyst classification system
I
A benign simple cyst with a hairline thin wall that does not contain septa, calcifications, or solid components. It measures water density and does not enhance.
II
A benign cyst that may contain a few hairline thin septa in which “perceived” enhancement may be present. Fine calcification or a short segment of slightly thickened calcification may be present in the wall or septa. Uniformly high attenuation lesions <3 cm="" so-called="" high-density="" cysts="" that="" are="" well="" marginated="" and="" do="" not="" enhance="" included="" in="" this="" group="" category="" require="" further="" evaluation="" br="">
IIF (F for follow-up)
Cysts that may contain multiple hairline thin septa or minimal smooth thickening of their wall or septa. Perceived enhancement of their septa or wall may be present. Their wall or septa may contain calcification that may be thick and nodular, but no measurable contrast enhancement is present. These lesions are generally well marginated. Totally intrarenal nonenhancing high-attenuation renal lesions >3 cm are also included in this category. These lesions require follow-up studies to prove benignity.
III
“Indeterminate” cystic masses that have thickened irregular or smooth walls or septa in which measurable enhancement is present. These are surgical lesions, although some will prove to be benign (eg, hemorrhagic cysts, chronic infected cysts, and multiloculated cystic nephroma), some will be malignant, such as cystic renal cell carcinoma and multiloculated cystic renal cell carcinoma.
IV
These are clearly malignant cystic masses that can have all the criteria of category III, but also contain enhancing soft-tissue components adjacent to, but independent of, the wall or septum. These lesions include cystic carcinomas and require surgical removal.
IIは経過観察不要となっています。これでひとまず安心です。
臨床では役に立ちそうです。これから腎嚢胞を見たら、「Bosniak分類では・・・」と考えてみたいです。