The incidence of RCC has increased in recent years, and obesity, a known risk factor for RCC, is becoming increasingly common. An intriguing paradox exists wherein obesity not only increases the risk of developing RCC but is also associated with a better prognosis in patients with RCC. The mechanism of this “obesity paradox” in RCC is poorly understood. However, the correlation between BMI and the characteristics of RCC remains controversial. Considering the confounding factors caused by different subtypes that usually correspond to varying degrees of malignancy [19, 20], we aimed to investigate the association between BMI and the clinicopathological characteristics of ccRCC, the most common f RCC subtype. We compared pathological characteristics—including pathological tumor diameter, pT stage, Fuhrman or WHO/ISUP grade, renal capsular invasion, perirenal fat or renal sinus fat invasion, and vein cancerous embolus which are closely associated with the prognosis in patients with RCC treated with surgery, among the patients classified into normal-weight, overweight, and obese groups based on the Chinese BMI classification.
Notably, in our cohort, the mean age of the obese group was 55 years, which was lower compared to other groups. The overweight and obese groups also exhibited higher male-to-female ratios. Peired et al.[21] revealed that males are twice as likely to develop kidney cancer as females. In our study, males accounted for over three times the number of females in both the overweight and obese groups and roughly twice the number in the normal-weight group. This suggests that RCC susceptibility to sex may be amplified by a high BMI. Sun et al.[22] suggested that estrogen plays an important role in the etiology of RCC and may modify the association between obesity and RCC risk in women. Moreover, we found that the overweight and obese groups had a higher proportion of smokers than the normal-weight group. This trend extended to comorbidities such as hypertension and diabetes, with overweight and obese patients being more prone to smoking and these health conditions. However, no significant difference was observed in the ASA score, which was used to evaluate the preoperative fitness of patients.
Regarding pathological diagnosis, our study did not reveal significant differences in pathological tumor diameter or pT stages among the normal-weight, overweight, and obese groups, which is consistent with the findings of Tsivian et al.[23] However, Choi et al.[24] revealed that overweight and obese patients had smaller tumors in a clinical cohort of 1543 patients with RCC. Jeon et al.[25] reported that these patients had a lower pT stage than normal-weight patients. In addition, we demonstrated a significant difference in the distribution of tumor nuclear grades, as evaluated by the Fuhrman or WHO/ISUP grades. Specifically, high BMI was associated with low nuclear grade in patients with ccRCC. The obese group had a significantly higher rate of low nuclear grade than the normal-weight group, whereas the overweight group exhibited a higher rate of low nuclear grade, though this difference was not statistically significant. This finding is consistent with Tsivian et al.’s study, that included all kidney cancer subtypes [23]. They revealed that a higher BMI was associated with a lower Fuhrman grade of RCC in clinically localized renal masses, and partly explained the better survival rates in patients with higher BMI. Although Seon et al.[26] did not find any significant association between nuclear grade and BMI in a cohort of 2329 patients who underwent curative surgery for RCC, they reported that 55.6% of patients with Fuhrman grade 1–2 were in the normal-weight group, 58.8% in the overweight group, and 61.6% in the obese group (p = 0.062). Furthermore, we found that the rates of renal capsular invasion, perirenal fat or renal sinus fat invasion, and vein cancerous embolus were significantly lower in the obese group than in the normal-weight group. No significant difference was observed between the overweight and normal-weight groups. According to available research[27–32], the presence of renal capsular invasion, perirenal fat or renal sinus fat invasion, or vein cancerous embolus is strongly associated with RCC prognosis after surgery, especially in terms of overall survival and postoperative local recurrence or distal metastasis.
To the best of our knowledge, this is the first study to report an association between BMI and pathological indices that play key roles in the prognosis of patients with RCC. Many studies have shown that obese patients with RCC have longer overall survival and cancer-specific survival[6, 11, 26, 33], including those with metastatic RCC [34, 35]. A prospective trial conducted by Donin et al.[36] revealed that obese patients had significantly improved overall survival compared with normal-weight patients in a cohort of high-risk patients with RCC from 14 countries. Nevertheless, the mechanism of the “obesity paradox,” where obesity increases the risk of developing RCC while being associated with a better prognosis, remains unclear. Our findings may partly explain the better prognosis in patients with higher BMI and “obesity paradox” in RCC. Obese patients showed less aggressive pathological features, such as low tumor nuclear grade and low rates of renal capsular invasion, perirenal fat or renal sinus fat invasion, and vein cancerous embolus, and therefore presented better overall survival and cancer-specific survival outcomes.
The results of our study should be carefully interpreted and prospectively validated because of some limitations. First, the retrospective nature of the study could have introduced an inherent selection bias. For instance, 3856 records of patients with ccRCC were available; however, 1315 records were excluded because of incomplete data. In addition, the BMI data was preoperative, and we did not consider BMI changes in the early years of surgery or postoperative BMI changes. Second, our cohort spanned over ten years, which could have introduced unintended errors in data collection and inconsistencies in recording indices. For example, the Fuhrman grade was preferred to evaluate tumor nuclear grade in early years, while the WHO/ISUP grade was preferred in recent years. Third, some noteworthy pathological variables, such as sarcomatoid differentiation, tumor necrosis, and pathological nodal stage, were not included in our analysis for various reasons, potentially limiting our conclusions.