AJR:209, December 2017 1285 cm and cysts containing thick calcifcations or multiple thin septa [4, 8]. Bosniak category III and IV cysts are le- sions with progressive malignancy rates, and surgical resection is often considered on the basis of urologic guidelines [9–11]. Although category III lesions are considered suspicious for malignancy given the presence of thick septa or septal enhancement, they include a broad spectrum of benign differential diag- noses, such as chronic abscess, chronically infected or hemorrhagic cyst, simple cyst af - ter alcohol ablation, localized cystic disease, mixed epithelial and stromal tumor, and mul- tilocular cystic nephroma [12, 13]. Catego- ry IV lesions are characterized by a solid en- hancing component and are highly suspicious for malignancy, particularly the intracystic type of renal cell carcinoma (RCC), which is a less aggressive subtype than conventional Malignancy Rate, Histologic Grade, and Progression of Bosniak Category III and IV Complex Renal Cystic Lesions Pedro N. Mousessian 1 Fernando I. Yamauchi Thais C. Mussi Ronaldo H. Baroni Mousessian PN, Yamauchi FI, Mussi TC, Baroni RH 1 All authors: Departmento de Radiologia, Hospital Israelita Albert Einstein, Avenida Albert Einstein, 627/701, Morumbi, 05652-900, São Paulo, SP, Brazil. Address correspondence to F. I. Yamauchi (fernando.yamauchi@einstein.br). Genitourinary Imaging • Original Research AJR 2017; 209:1285–1290 0361–803X/17/2096–1285 © American Roentgen Ray Society R enal cystic lesions are extremely common, with a prevalence of at least 50% in populations older than 50 years [1], and they are of- ten incidentally discovered on imaging ex- aminations. The Bosniak classifcation for renal cystic lesions aims to predict the risk of malignancy by means of a standardized re- port based on CT fndings [1–4], criteria that were later extended to MRI [3, 4], suggesting appropriate management regardless of lesion cause [5–7]. Bosniak categories I and II represent simple and minimally complicated cysts (small hem- orrhagic cysts or cysts containing thin septa), respectively, and do not require monitoring or specifc treatment [1–4]. Bosniak category IIF corresponds to indeterminate but probably be- nign fndings that require follow-up, including an intrarenal hemorrhagic cyst larger than 3.0 Keywords: Bosniak classifcation, CT, kidney cysts, MRI, renal cysts DOI:10.2214/AJR.17.18142 Received February 23, 2017; accepted after revision April 24, 2017. OBJECTIVE. The primary purpose of this study is to determine the malignancy rate, his- tologic grade, and initial stage of surgically treated complex renal cysts classifed as Bosniak category III or IV. For nonsurgical lesions, a secondary objective was to evaluate lesion pro- gression on follow-up examinations. MATERIALS AND METHODS. We searched our database for cystic lesions classi- fed as Bosniak III or IV category on CT or MRI from January 2008 to April 2016. Surgically resected lesions, per category, were correlated with information on pathologic reports to ob- tain malignancy rates. For malignant lesions, histologic grade and initial stage were evalu- ated. Imaging follow-up of at least 2 years was used to evaluate progression of clinically fol- lowed lesions. RESULTS. We included 86 lesions in 85 patients in the fnal analysis. Of the 60 surgically resected lesions (70%), 46 (77%) were malignant and 14 (23%) were benign. Malignancy rates were 72% for Bosniak category III lesions and 86% for Bosniak category IV lesions. Most malignant cysts were early-stage (pT1) cysts with low histologic grades (89% of Bosniak III lesions and 91% of Bosniak IV lesions). Follow-up studies of the surgically resected lesions did not show local recurrence, metastasis, or lymph node enlargement. Among patients with lesions managed by watchful waiting ( n = 26), all lesions remained unchanged in terms of size and complexity after at least 2-years of follow-up. CONCLUSION. Although high malignancy rates were observed for both Bosniak cat- egory III and IV lesions, our results suggest that such malignant cysts are usually early-stage tumors with a low histologic grade. Lesions that underwent follow-up remained unchanged on control examinations. These fndings may indicate low aggressiveness of these lesions, sup- porting the idea that more conservative approaches may be used. Mousessian et al. Bosniak Category III and IV Complex Renal Cystic Lesions Genitourinary Imaging Original Research Downloaded from www.ajronline.org by 52.73.204.196 on 05/17/22 from IP address 52.73.204.196. Copyright ARRS. For personal use only; all rights reserved