Introduction Amyloid deposits within the seminal vesicles are a com- mon finding at autopsy. The reported incidence ranged from 9 to 16 % [1, 2] in an unselected population of men over the age of 50 years. The incidence increased with age and was found in 21 % of men over 75 years [2]. Despite the high incidence, only two reports in the radiologic literature have described the MR appearance of seminal vesicle amyloidosis [3, 4]. In both cases amy- loid deposits were misinterpreted as tumor extension into the seminal vesicles due to prostate or bladder can- cer. With the increasing use of MR as a staging modality, more men with vesicula amyloidosis will undergo such an examination. Recognizing this entity may prevent overstaging the tumor. We describe a patient in whom seminal vesicle amyloidosis was misinterpreted as ex- tension of prostate cancer into the seminal vesicles on MR images obtained with an endorectal coil. Case report A 61-year-old previously healthy man was referred to the urologic department because of a palpable nodus in the right lobe of the prostate. Transrectal US examina- tion demonstrated a hypoechoic region in the base of the prostate extending into both seminal vesicles. Biop- sies revealed bilateral moderately differentiated adeno- carcinoma of the prostate. Magnetic resonance images were obtained with a 1.5-T Siemens SP system (Sie- mens, Erlangen, Germany), using a Medrad (Pittsburgh, Pa.) endorectal coil by using a 26-cm field of view, 5-mm section thickness, 1-mm intersection gap with a 512 × 216 matrix. Axial T1 (TR 420/TE 22) and axial, sagittal, and coronal fast-spin-echo (FSE) T2-weighted images (TR 2940/TE 160, 13 echo trains) were per- formed. The T2-weighted images demonstrated an area of low signal intensity bilaterally in the base of the prostate (Fig.1a–c). This was interpreted as prostate cancer ex- tending into the base of both seminal vesicles. Because radical surgery is not the treatment of choice in patients with seminal vesicle invasion, additional US-guided needle biopsies of the prostatoseminal vesicular junc- tion were obtained. One of the three cores demon- strated moderately differentiated adenocarcinoma. Seminal vesicle epithelium was not present. One core showed small eosinophilic deposits at the end of the bi- opsy. In retrospect, this deposit was amyloid material. Because seminal vesicle invasion could not be con- firmed with these biopsies, radical prostatectomy was thought to be the best treatment. Following radical prostatectomy, the specimen was fixed with formalin and embedded in paraffin. Four-mi- cron-thick tissue sections were stained with hematoxylin and eosin. The specimen showed moderately differenti- ated bilateral adenocarcinoma of the base of the pros- tate. The tumor volume was 2.0 cc. Because routine stains showed amorphic eosinophilic material in the wall of the seminal vesicles suspected for amyloidosis, additional staining with Congo red was performed. The Eur. Radiol. 7, 552–554 (1997)  Springer-Verlag 1997 European Radiology Case report A myloidosis of the seminal vesicles simulating tumor invasion of prostatic carcinoma on endorectal MR images G. J. Jager 1 , E.T.G.Ruijter 2,3 , J. J. M. C. H. de la Rosette 2 , Ch. A . van de Kaa 3 1 Department of Radiology, University Hospital Nijmegen, P.O. Box 9101, NL-6500 HB, Nijmegen, The Netherlands 2 Department of Urology, University Hospital Nijmegen, P.O. Box 9101, NL-6500 HB, Nijmegen, The Netherlands 3 Department of Pathology, University Hospital Nijmegen, P.O. Box 9101, NL-6500 HB, Nijmegen, The Netherlands Received 30 May 1996; Accepted 15 July 1996 Abstract. Amyloid deposits within the seminal vesi- cles are a common finding at autopsy. The incidence increases with age. Amyloid deposits can mimic tu- mor extension into the seminal vesicles due to pros- tate or bladder cancer on T2-weighted MR images. We describe a case of seminal vesicle amyloidosis demonstrating the MR appearance and the character- istic pathologic findings. Recognizing seminal vesicle amyloidosis may prevent overstaging prostate cancer on MR images. Key words: Amyloidosis – Seminal vesicles – MRI – Prostate cancer Correspondence to: G. J. Jager