Prostate Cancer Tumor Size Often Underestimated by MRI, Study Finds

Magnetic resonance imaging (MRI) is often used in the diagnosis and management of prostate cancer, but the method frequently underestimates tumor size, a study from the UCLA Jonsson Comprehensive Cancer Center found.

The study, published in The Journal of Urology, aimed to assess radiologic tumor size vs actual pathological tumor size in patients with prostate cancer and to identify predictors of pathological tumor size.

Researchers assessed a cohort of 441 men with biopsy-proven prostate cancer and 3 or higher index lesions based on the Prostate Imaging Reporting and Data System version 2 (PI-RADSv2) who later underwent radical prostatectomy. They defined radiologic tumor size as the maximum tumor diameter determined by the MRI and compared those measurements to those found in the whole-mount histopathology after resection.

The analysis included 461 lesions from the 441 patients identified. The mean radiologic tumor size was 1.57 cm, whereas the mean pathological tumor size was 2.37 cm. Although tumor size was consistently underestimated in the radiologic setting, tumors that were smaller and had lower PI-RADSv2 scores tended to be further underestimated than others.

In cases where tumors were categorized as grade 5 on the Gleason Grade Group (GG), pathological tumor size was larger compared with GG grade 1 tumors, with a mean change of 0.37 cm (P = .014). Lesions rated 5 on the PI-RADSv2 index also had pathological tumor sizes larger than PI-RADSv2 4 lesions, with a mean change of 0.26 (P = .006). Tumors with higher prostate-specific antigen density were also larger.

Overall, there was not much correlation between radiologic tumor measurements and pathological tumor size in the cohort. The researchers saw correlation coefficients ranging from 0.1 to 0.65. Inaccurate tumor measurements can make it difficult for physicians to see where the outer edges of the tumor end and healthy tissue begins, potentially leading to insufficient treatment when the measurements are underestimated.

“Multiparametric magnetic resonance imaging frequently underestimates pathological tumor size and the degree of underestimation increases with smaller radiologic tumor size and lower PI-RADSv2 scores,” the authors concluded.

For the treatment of smaller tumors and those rated low on the PI-RADSv2 scale, they suggest that a larger ablation margin might be necessary as physicians estimate focal therapy treatment margins.

Reference

Pooli A, Johnson DC, Shirk J, et al. Predicting pathological tumor size in prostate cancer based on multiparametric prostate magnetic resonance imaging and preoperative findings. J Urol. 2021;205(2):444-451. doi:10.1097/JU.0000000000001389