Current T staging for renal cell carcinoma does not capture the full picture of tumor volume, which data have shown to be more predictive of survival and recurrence in certain types of cancer.
Jacob Jorns, MD
Current T staging for renal cell carcinoma (RCC) is based on maximal tumor diameter. But this assessment does not capture the full picture of tumor volume (TV), which data have shown to be more predictive of survival and recurrence in pT1a clear cell RCC (ccRCC).
Surpassing the standard one-dimensional view in prognostic power, researchers have found, is a calculation for TV that captures height, width, and length of the tumor, taking into account that tumors often develop not in uniform round spheres, but in irregular and varying widths, lengths, and depths.
Jacob Jorns, MD, of the Mayo Clinic in Jacksonville, Florida, presented data from two analyses focused on TV in RCC during the 2013 Annual Conference of the Southeastern Section of the American Urological Association. One analysis substantiated the shortcomings of the T-staging recommendation of largest tumor diameter and introduced a more robust assessment—true tumor volume (TTV). Another assessed the correlation between radiologic and pathologic TV in localized RCC.
For the former study1, the researchers evaluated records from 2180 patients who underwent radical or partial nephrectomy for pT1a ccRCC between the years of 1985 and 2006, and for whom the three tumor dimensions were available in pathology reports, enabling tumor size and volume to be compared. TV was defined as (π/6 x height x length x width). TTV was calculated by plugging the three tumor dimensions reflected in pathology reports into the TV formula. Alternative tumor volume (ATV), utilizing the onedimensional views of standard T staging, was calculated by using the largest tumor dimension to represent all three values—height, length, and width—in the TV formula, as a means of comparison.1
For 1547, or 71%, of the 2180 patients whose tumor size and volume could be compared, each of the three tumor dimensions differed one from another. The mean difference between ATV and TTV was 194 cm3 (standard deviation 485; median 42; range 0-11,608), and statistically significant (P<.001).
The difference between the TTV and ATV calculations increased significantly with increasing stage of a patient’s primary tumor (P<0.001).
Motivated by this finding that TTV had more prognostic value than size alone, the same group went on to evaluate the correlation between radiographic and pathologic TV in localized RCC.2 The aim, the authors wrote, was to determine if presurgical imaging could be employed that would be as accurate as pathology in determining TV, thus helping to inform treatment decisions. Several investigations have looked at preoperative imaging of tumor size, Jorns noted, but this may be the first study to assess TV in this manner.
Using the institution’s nephrectomy registry, the researchers identified 144 patients whose records included three TV measurements. They then correlated preoperative TV imaging with TV as determined by pathologic specimens, to identify and measure differences.
The investigators found a strong correlation between the TV measurements arrived at through imaging and via surgery (r=0.95). The correlation was similar for CT versus MRI (r=0.93, r=0.98), men versus women (r=0.94, r=0.98), clear cell versus other subtype (r=0.94, r=0.97), and obese versus non-obese (r=0.99, r=0.89).
TV did not differ between CT and MRI, but the correlation between the results of imaging and pathology was weaker among pT1a tumors when compared with T2 and T3 tumors. A stronger correlation was noted among patients with a BMI >30 than with thinner patients, suggesting that obesity may promote better imaging.
The finding that neither imaging modality proved better than the other is potentially meaningful because, as Jorns noted, with an increased incidence of small localized renal tumors, there is a need for more and better preoperative tools for decision-making.
David Thiel, MD
Commenting on the data from both studies was corresponding author David Thiel, MD, associate professor of Urology at the Mayo Clinic in Florida. “It turns out that RCC tumors are not round balls but ellipsoids, and utilizing one-dimensional tumor size to indicate true tumor volume is inaccurate,” he said. “Our study shows that tumors very rarely have the same size in any of the three dimensions. And the imaging study demonstrates that our current cross-sectional imaging is very accurate in determining true tumor volume.
“Given our recent data that RCC tumor volume is more predictive of survival than tumor size alone, and with the recent attention given to watchful waiting for small renal masses as well as alternative therapies such as ablation, we need to start talking in terms of tumor volume rather than tumor size alone to improve prognostic accuracy.”