The advent of PET/CT imaging has improved baseline staging and surveillance for disease recurrence in cancer patients. The ability to co-register the anatomical data of CT with the functional data of PET in a hybrid PET/CT image provides diagnostic information unmatched by either modality alone. PET/CT imaging also has some specific pitfalls related to abnormal foci of FDG uptake, including correction artifact; physiological uptake in muscle, fat, or bowel; and nonmetastatic uptake in inflammation or benign tumors [12
]. We demonstrate in our large series of cases that isolated pelvic hot spots have an overall incidence of 1.1% (8/700), with the majority (5/8 or 62.3%) being due to physiological or benign uptake in female reproductive organs. It may be possible to minimize physiologic FDG uptake in the uterus and ovaries by understanding how this uptake relates to the stages of the menstrual cycle or to reproductive hormonal shifts. Further understanding of such phenomena can prove useful in detecting pelvic tumors accurately in females where detection would be otherwise confounded by the presence of benign pelvic hot spots. For example, women could be scheduled for PET/CTs during the secretory phase of their cycle to minimize uterine uptake or during the proliferative phase of their cycle to minimize corpus luteal cyst uptake. Other isolated pelvic hot spots on PET/CT such as uptake in inflamed bowel or within leaked urine are more difficult to address systematically and likely require a case-by-case approach with knowledge of detailed clinical history (i.e., Crohn’s disease patient with active disease, leukocytosis, etc.) and/or notes from the radiology technologist (i.e., patient with leaked urine during the examination).
Irrespective of the considerations above, one clear result of our study is that solitary pelvic hot spots appear unlikely to represent metastatic disease. We had only one case (1/8, 12.5%) of malignant disease in our series, but not related to metastases. This is an important result that may provide reassurance to an anxious patient and allow for a more conservative approach to the work-up of such a finding. It is also worth noting that an isolated pelvic hot spot might give rise to greater concern in community practice, where PET/CT studies are often performed without intravenous iodinated contrast. The ability to recognize benign uptake in a fibroid or corpus luteum cyst may be very limited in this scenario, and the reassurance provided by our study would then become even more important. While we cannot conclude that a solitary pelvic hot spot will never be due to metastatic disease, such an occurrence would seem rare, given that we did not see this once in 700 cases. It is important to note, though, that in patients with colorectal cancer, and conceivably with other primary malignancies arising in the pelvis, the likelihood of an isolated hypermetabolic focus in the pelvis being local recurrence or residual tumor may be relatively higher when compared to other malignancies.
Our study has some limitations. First, this was a retrospective study with all inherent limitations of such a design. Second, the research was done at a single tertiary institution with a population that may not reflect the general population; however, our findings were mostly related to physiologic changes and uterine leiomyomas rather than rare pathologic conditions or specialized treatments. Third, we did not stratify patients based on their initial diagnosis, but pooled all into one group. It is conceivable that within certain group of patients, for instance, a patient with a primary anorectal cancer, an isolated hot spot would more likely represent malignancy rather than a benign hot spot. The fact that we identified only one case of isolated malignant focus may be explained by our research design. On the other hand, our large sample size probably mitigates any detrimental effects of this bias. Fourth, we had only one reader reviewing all images and that precludes any assessment of interobserver variability. Although this may be a significant limitation on research assessing the accuracy of a diagnostic method, it is less likely to have a major impact on a descriptive study. In addition, it is conceivable that that the single reader missed an isolated hot spot within the pelvis that would have been otherwise detected by a second radiologist. This is felt to be unlikely as there were three independent opportunities to identify such findings: the PET images, the CT images, and the related report. Fifth, our definitions of hot spot did not use any strict SUV numbers. SUV measurements are, however, subject to variability due to physiological aspects (such as blood glucose level, body habitus, and organ of interest) and measurement variability, including camera calibration, time of scanning postinjection, size of regions of interest, and choice of backgrounds. Irrespectively, the use of a minimal SUV threshold to establish the presence of an isolated hot spot would not have changed the conclusions of this study, as we would not have increased the number of malignant lesions identified. In summary, understanding the incidence and radiologic appearance of isolated hot spots in PET/CT imaging is pivotal in accurate diagnosis of malignant vs. benign disease. Our results show that isolated pelvic hot spots at PET/CT imaging in an oncological population are not common and usually benign; physiological endometrial or ovarian uptake is the single commonest cause.