Discussion
APF is a clinical phenomenon without a direct effect on patients, yet for surgeons it complicates dissection and release of the kidney7,8. Dissection of perirenal fat is an important component of partial nephrectomy and donor nephrectomy. APF can cause excessive hemorrhaging during surgery, trauma to the renal capsule, iatrogenic vascular injury, and prolongation of operative time6,8,9. Hemorrhaging from fatty tissue and vascular injury can complicate endourological manipulations. During laparoscopic and robot-assisted partial nephrectomy APF impedes kidney dissection. Furthermore, prolonged surgery time and renal capsular trauma can interfere with the renal transplantation process. Thusly, APF is a challenging phenomenon, both for open and endoscopic procedures. In addition to the radiological features, the present study focused on the pathological aspects of APF.
In the most recent studies APF was qualified as present or absent; only Narita et al. classified cases as non-APF, APF, and severe APF, as in the present study10-12. We acknowledge that APF differs in severity from patient to patient. Dissection in some patients is sometimes so difficult that decapsulation occurs multiple times and occasionally complete removal of perirenal fat cannot be achieved; however, in some cases with APF dissection is managed with only minimal trauma and hemorrhaging. For the aforementioned reasons we find that it is appropriate to characterize APF as easy, moderate, and difficult.
The correlation between perirenal fat dissection complexity and dissection time in the present study confirms the pertinence of the dissection classification used. Zheng et al. classified APF according to dissection duration, as <10 min, 10-20 min, and >20min13. Their dissection time groupings were comparable to those in the present study, in which mean dissection time in the easy, moderate, difficult dissection complexity patients was 5.6±2.9min, 12.7±2.1min, and 35.1±13.0min, respectively.
Retroperitoneal fat thickness is reported to affect duration of surgery, as it did in the present study. In addition to male gender, Ito et al. showed that retroperitoneal fat thickness is positively correlated with surgical duration3. In a prospective study of 100 robotic partial nephrectomies it was reported that surgery time in patients with APF was longer than in those without APF12. A study that included 92 donor nephrectomies observed that APF and severe APF significantly prolonged surgery10. In the present study dissection time was significantly affected by APF; dissection time was longest in the patients with difficult dissection complexity. Total duration of surgery was not a focus of the present study because it can be deceptive. Total surgery time in donors during the transplantation process is affected by many factors; as such, we think that dissection time is a more reliable measurement for assessment of APF.
For the prediction of APF the image-based Mayo Adhesive Probability(MAP) score has been introduced4. This 5-point scoring system uses posterior perinephric fat thickness and perinephric fat stranding. The utility of the MAP score has been verified by several researchers7,14. Furthermore, many studies show that there is a clear association between APF and perirenal fat thickness6,10,11. Perirenal stranding observed in radiological images is not always an indicator of APF and stranding is a subjective parameter. To assess the clinical relevance of the perirenal field’s radiological features, the perirenal field’s HU was measured in the present study, and there wasn’t a significant correlation between the perirenal field’s HU and APF, yet perirenal fat depth differed significantly according to the complexity of perirenal fat dissection. Based on the present findings, we think use of perirenal fat depth measured via pre-operative CT is sufficient for presurgical prediction of APF.
Obesity is thought to play a role in APF, yet there wasn’t a significant correlation between APF and BMI in the present study. Correspondingly, 2 other studies did not note an association between BMI and APF; however, others have observed a marked association6,8,11,14,15. Some studies considered waist circumference a predictor of APF6. The present study analyzed subcutaneous fat thickness as an indicator of internal fat burden, which is assumed to be a more objective parameter than waist circumference, but there wasn’t a significant association between subcutaneous fat thickness and APF.
In addition to a high BMI, many other factors are associated with APF. Khene et al. assessed APF in 202 patients that underwent robot-assisted partial nephrectomy and reported that male gender, old age, a high BMI, smoking, and hypertension are risk factors for APF8. Similarly, Kawamura et al. reported the same risk factors for APF14. Kocher et al. noted a significant association between APF, and increased age and male gender11. In the present study there wasn’t a correlation between APF and any of the aforementioned factors. The donor population in the present study might account for these differences, as the renal donors were for the most part fit and healthy non-smokers.
Radiologically measured posterior fat thickness is a predictor of APF10,11. Additionally, a prospective study that included 125 patients reported that lateral fat thickness was greater in patients with APF6. Moreover, increased posterior fat thickness, visceral fat area, and perinephric fat area are reported to be risk factors for APF10,15. In the present study perirenal fat thickness and perirenal fat depth measured via CT differed significantly according to perirenal fat dissection complexity. In the present study measurement of actual perirenal fat thickness was performed in order to detect the thickest part of the fatty tissue and perirenal fat depth was measured in order to use a more standardized measure. HU measurements of the perirenal field and subcutaneous fat via CT in the present study showed no significant difference according to dissection complexity. Histopathological findings supported this finding, no difference was observed in small capillary density according to dissection complexity. Bylund et al. reported a similar finding; however, Dariane et al. indicated the density of fat was an independent predictor of APF6,15.
Histopathlogical analysis in the present study show increased fibrous tissue in the perirenal fat of APF patients, in contrast to the findings reported by Dariane et al6. This seems to be the major underlying factor that can lead to surgical difficulty. Thicker Gerota’s fascia, and more collagen around blood vessels and in between fat cells focally were observed in the present study’s severe APF patients, but what caused this variance is not clear. Metabolic disorders, such as obesity and diabetes, and collagen vascular diseases or other systemic problems might have led to an increase in collagen deposition in the tissues; however, the present study’s APF and non-APF patients did not differ in these respects. Additionally, there wasn’t a significant difference in mean fat cell diameter, indicating that obese or overweight patients have not accumulated on one arm of the study set. Furthermore, we could not attribute to an inflammatory process for the explanation of fibrosis as both groups of patients have only a very few mononuclear cells over and there. In fact, we think that the amount of fibrous tissue might have been innate and related to the individual’s features. The present finding that the APF patients had a higher number of blood vessels with wall thickness >100µm might be considered supportive of this hypothesis.
The primary limitation of the present prospective study is the small number of patients. As living donor nephrectomy is performed less frequently, the study also included partial nephrectomies. Pathological examination might have been more objective if it had been conducted in donors only.