Our study supports the impression that mucinous adenocarcinomas of the prostate may not carry the usual characteristics of nonmucinous tumors on T2-weighted MRI (low signal intensity) and MR spectroscopy (increased choline and decreased citrate peaks), presumably because of the large volume of extracellular mucin they harbor. Our results concur with the description provided by Outwater et al. [9
] in which mucinous adenocarcinomas presented with high signal intensity on T2-weighted images. This characteristic, described in other mucinous tumors seen elsewhere in the body [14
], seems to render prostate mucinous adenocarcinomas occult against the high T2 signal intensity of the normal peripheral zone background. However, to our knowledge, this is the first description of the MR spectroscopy findings seen in this subtype of prostate cancer. We were unable to detect any malignant metabolism in areas where lakes of extracellular mucin were identified at histopathology, making two of our cases completely invisible on MR spectroscopy.
The analysis of our cases raises an additional question. It is not unusual to identify variable amounts of intraluminal mucin in nonmucinous prostate adenocarcinoma specimens, and some tumors present with less than 25% of their volumes composed of lakes of extracellular mucin. Could this fact at least in part explain why some prostate adenocarcinomas are not detected on MRI and MR spectroscopy? Further and larger studies that control for factors known to influence tumor detection—e.g., tumor volume—and investigation of the correlation between the amount of mucin seen in cancer and imaging accuracy are necessary to answer this question.
Our study has limitations. First, this was a retrospective study, and we likely incurred a selection bias because we included only patients who underwent radical prostatectomy. It is possible that these were patients with more localized disease and, consequently, our results would not be generalizable to patients with larger and more extensive mucinous adenocarcinomas. However, these tumors are more likely to contain larger pools of extracellular mucin, and one would expect to identify the same imaging features we describe. Second, as in many case series, our sample size is small and may not represent all mucinous adenocarcinomas; however, the MRI features we describe agree with those previously reported [9
], suggesting that our MR spectroscopy findings are valid. Third, we did not include a control group in our study. A case-control study would allow us to compare the incidence of the MRI and MR spectroscopy findings seen in patients with mucinous adenocarcinoma and other groups of patients, for instance, those with nonmucinous adenocarcinomas or men without cancer. Nonetheless, because of the small number of patients with mucinous tumors, we would not be able to perform a meaningful statistical analysis to draw any definite conclusions from such study. Last, although histopathologic examination with tumor maps is considered the standard of reference for radiologic tumor detection, it can be difficult to correlate the location of histopathologic and imaging findings. Most of our histologic step sections were quartered during processing, the standard procedure at our institution at the time these patients underwent surgery, instead of being prepared as whole-mount sections.
In conclusion, MRI and MR spectroscopy do not appear to be able to reliably detect the lakes of extracellular mucin seen in mucinous adenocarcinomas of the prostate. This raises the question of whether the presence of mucin in some nonmucinous tumors is responsible for the less than optimal detection of cancer with both MRI and MR spectroscopy. Further and larger studies are needed to test this hypothesis.