This study investigated APTWI and its correlation with GS in PI-RADS v2.1 category 3–5 lesions and concluded that APTWI is valuable in diagnosing PCa and evaluating different GG types of PCa. APTmean combined with ADC and PSAD had optimal diagnostic performance in the risk assessment of PCa.
APTWI can noninvasively detect the expression of soluble proteins and polypeptides, and it can be used in the discrimination of malignant and benign tissue. Our study showed that all APTWI parameters were higher in PCa than in BL, with excellent diagnostic efficacy in diagnosing PCa. The results are consistent with previous research and suggest that the pathological changes associate with tight cellular arrangement and high polypeptide concentration, which correlate with high MTRasym values (3.5 ppm)[25–28]. On the contrary, normal and hypoplastic prostate tissue is composed of loose or dense cellular arrangement, smooth muscle, and fibrous tissue, with large extracellular spaces and gland cavities, so MTRasym values (3.5 ppm) were lower compared to its counterpart[29–31].
We also found moderate positive correlations between APTmax, APTmean, and GS, indicating that tumor cell density and protein expression increased gradually with elevated GS. However, Takayama et al.[32] found no correlation between APT and GS. The growth rate of PCa increases rapidly with the degree of malignancy, at which time gland function and soluble proteins may be destroyed, resulting in low MTRasym values (3.5ppm)[33]. However, three-dimensional acquisition with a higher SNR and a smaller ROI was used to obtain complete tumor information; therefore, it is unlikely that poor gland function affected our measurements. Furthermore, a weaker correlation was found for APTmin and GS, which may be due to the fact that the minimal proton exchange rate did not reflect the protein and peptide content of the whole tumor tissue[27].
There were significant differences in APT parameters among the different GG lesions, and protein and polypeptide levels increased gradually in PCa with increasing aggressiveness (reflected in the different GG subgroups), suggesting that APTmax and APTmean may be valuable biomarkers in predicting patient prognosis. Further intra-group analyses found that differences existed between GG1 and GG3, GG1 and GG4, GG2 and GG3, GG2 and GG4 lesions. The were no differences in APT parameters between GG1 and GG2 lesions because glands of two subgroups have similar differentiation degree, along with similar protein and polypeptide content. However, differences were not found between GG3 and GG4 lesions, perhaps because most of the GG4 lesions were complicated by necrosis and damaged proteins, which reduced MTRasym (3.5 ppm) values and required larger sample sizes for intra-group comparisons.
Based on the definition of csPCa (GS ≥ 3 + 4), ROC analysis of APTWI parameters was used to distinguish GG1 from GG ≥ 2 lesions to evaluate the risk of aggressive PCa, and APTmean showed the highest diagnostic efficacy (AUC = 0.843). Meanwhile, ADC and PSAD were used separately or in combination with other parameters in the ROC analysis, as they perform well in the diagnosis and risk assessment of PCa. Combined analysis showed that the diagnostic efficacy of APTmean+ADC + PSAD was higher than APTmean in evaluating the risk of aggressive PCa, indicating that the metabolic APT parameter can serve as an important biomarker for risk stratification, and along with laboratory indicators and ADC, it is an important criterion for PI-RADS v2.1 classification.
Limitations
This study had some limitations. Firstly, this was a retrospective study at a single center with a limited sample size. Future prospective multicenter trials with larger cohorts should be performed. Secondly, multiple factors, such as GS, PSA level, and TNM staging, determine the risk of aggressive PCa; however, this study only focused on the relationship between GS and APTWI as this was a preliminary study exploring the relationship between APTWI parameters and PCa aggressiveness. Thirdly, although visible necrosis or cystic areas were excluded during ROI selection, we could not eliminate the influence of tumor heterogeneity. Therefore, radiomics may be more objective and accurate in the interpretation of APTWI research in the future.