Improving diagnostic efficacy of primary prostate cancer with combined 99mTc-PSMA SPECT/CT and multiparametric-MRI and quantitative parameters

Purpose This prospective study aimed to evaluate the difference between 99mTc-PSMA single-photon emission computed tomography (SPECT)/CT and multiparametric magnetic resonance imaging (mpMRI) in the detection of primary prostate cancer (PCa). Materials and methods Fifty-six men with suspected PCa between October 2019 and November 2022 were prospectively enrolled in this study. The median age of the patients was 70 years (range, 29-87 years). Patients were divided into high-(Gleason score>7, n=31), medium- (Gleason score=7, n=6) and low-risk groups (Gleason score < 7, n=6). All patients underwent 99mTc-PSMA SPECT/CT and mpMRI at an average interval of 3 days (range, 1-7 days). The maximum standardized uptake value (SUVmax), the minimum apparent diffusion coefficient (ADCmin), and their ratio (SUVmax/ADCmin) were used as imaging parameters to distinguish benign from malignant prostatic lesions. Results Of the 56 patients, 12 were pathologically diagnosed with a benign disease, and 44 were diagnosed with PCa. 99mTc-PSMA SPECT/CT and mpMRI showed no significant difference in the detection of primary PCa (kappa =0.401, P=0.002), with sensitivities of 97.7% (43/44) and 90.9% (40/44), specificities of 75.0% (9/12) and 75.0% (9/12), and AUC of 97.4% and 95.1%, respectively. The AUC of SUVmax/ADCmin was better than those of SUVmax or ADCmin alone. When SUVmax/ADCmin in the prostatic lesion was >7.0×103, the lesion was more likely to be malignant. When SUVmax/ADCmin in the prostatic lesion is >27.0×103, the PCa patient may have lymph node and bone metastases. SUVmax was positively correlated with the Gleason score (r=0.61, P=0.008), whereas ADCmin was negatively correlated with the Gleason score (r=-0.35, P=0.023). SUVmax/ADCmin was positively correlated with the Gleason score (r=0.59, P=0.023). SUVmax/ADCmin was the main predictor of the high-risk group, with an optimal cut-off value of 15.0×103. Conclusions The combination of 99mTc-PSMA SPECT/CT and mpMRI can improve the diagnostic efficacy for PCa compared with either modality alone; SUVmax/ADCmin is a valuable differential diagnostic imaging parameter.


Introduction
Prostate cancer (PCa) is one of the most common malignancies in men (1).Early diagnosis and accurate grading of PCa are of great significance for formulating therapeutic strategies and improving prognosis (2).Multiparametric magnetic resonance imaging (mpMRI) is a well-established tool for the appraisal of primary PCa and has shown high affectability (3,4).Prostate biopsy remains the gold standard for PCa diagnosis.In addition to providing evidence for diagnosis, the pathological results can also provide the classification and grouping information of PCa.Ultrasoundguided puncture biopsy still has a high false-negative rate of 20-25%, and there are complications such as bleeding, infection, pain, and urinary retention (3).Therefore, it is important to explore a noninvasive preoperative diagnosis method for PCa to improve puncture accuracy and avoid unnecessary biopsy.Current strategies used to locally stage PCa and recognize the exact location of disease foci depend on the results of systematic or targeted biopsies and mpMRI.However, mpMRI has limited specificity (4)(5)(6).Although targeted mpMRI biopsies have significantly improved the identification of clinical PCa, there is still over a 30% chance of missing primary PCa in men (7).Therefore, additional complementary methods are required to better characterize and identify primary PCa.
Prostate-specific membrane antigen (PSMA) is a type II transmembrane protein that is overexpressed on the surface of 90% of PCa cells.Its expression positively correlates with the degree of malignancy (8).Published studies have demonstrated the superiority of 68 Ga/ 18 F-PSMA PET/CT or PET/MR in the detection of primary PCa.However, PET/CT or PET/MR is not widely available in less developed countries, and far fewer institutions have PET/CT or PET/MR devices than SPECT/CT devices.The limited production of 68 Ga from 68 Ge-68 Ga generator and 18 F from cyclotron, combined with the relatively short half-life of 68 Ga (67.71 min) and 18 F (109.8min), results in the need for multiple rounds of production per day to maintain patient use, limiting the number of patient tests per day.Although clinical SPECT system sensitivity and resolution are not as good as those of PET, the recent combination of SPECT and CT and the ability to quantify tissue radioactivity concentration in absolute units have resulted in a significant improvement in imaging quality.99m Tc, available from 99 Mo-99m Tc generators, is a nuclide routinely used in SPECT imaging, has good physical properties (half-life is 361.2 min), and is inexpensive and widely available.Thus, 99m Tcbased PSMA ligands are a cost-effective clinical alternative.Our previous study showed that 99m Tc-labelled PSMA molecular probe ( 99m Tc-HYNIC-Glu-Urea-A, herein referred to as 99m Tc-PSMA) single-photon emission computed tomography (SPECT)/CT can display bone metastases of PCa with high sensitivity and specificity (9), with only a small amount of radiation uptake in the intestinal tract and no significant radiation uptake in other major organs (10).However, to our knowledge, 99m Tc-PSMA SPECT/CT has rarely been reported for the diagnosis of primary PCa.In recent years, with the development of imaging technology, mpMRI including functional sequences such as diffusion weighted imaging(DWI) had been widely used in the diagnosis and preoperative localization of PCa (5).The ADC min from DWI reflects the degree of diffusion of water molecules in the tumor tissue.SUV max represents PSMA expression associated with the biological characteristics of tumors.Our study aimed to evaluate the difference between 99m Tc-PSMA SPECT/CT and mpMRI for the detection of primary PCa.

Ethical approval
This study was approved by the ethics committee of Fujian Provincial Hospital (reference number, K2019-10-017) and conducted in compliance with the principles of the Declaration of Helsinki.Furthermore, informed consent was obtained from all participants and/or their legal guardians.

Sample size calculation
We conducted a prospective head-to-head observational study to analyze the diagnostic efficacy between 99m Tc-PSMA SPECT/CT and mpMRI in treatment-naive PCa.In this study, the sensitivity and specificity of 99m Tc-PSMA SPECT/CT and mpMRI in the diagnosis of PCa were assumed to be greater than 50% (H 0 = 50%).Referring to similar published literature on 68 Ga-PSMA PET/CT and mpMRI (11), a sensitivity and specificity value of 80% was assumed.PASS 11 software (Power Analysis and Sample Size, NCSS, LLC) was used to estimate the required sample size.Assuming a=0.05 (unilateral), b=0.1, and a 1:1 ratio between the groups, the calculations indicated that at least 46 patients needed to be included in the study.Consequently, 56 individuals were enrolled in this study.

Image analysis
9 9 m Tc-PSMA SPECT/CT and mpMRI images were independently read by two nuclear medicine physicians and two radiologists, respectively.The readers were blinded to the mpMRI and 99m Tc-PSMA SPECT/CT clinical reports and other readers' findings.99m Tc-PSMA SPECT/CT and mpMRI were performed on a workstation (Xeleris, General Electric, Waukesha, WI) and (syngo.via,Siemens Healthineers, respectively).The locations of lesions on mpMRI and 99m Tc-PSMA SPECT/CT images were compared, and lesions with the same locations on the two scans were selected as the primary lesion to extract parameters for analysis.

Diagnostic criteria for primary PCa
On mpMRI, combined with the reconstructed apparent diffusion coefficient (ADC) images, a lesion with a prostate imaging reporting and data system (PI-RADS) score > 3 was considered a positive lesion (PCa) (13).The lesions' region of interest (ROI) was delineated, and the lowest ADC (ADC min ) was calculated.On SPECT/CT, areas with higher imaging agent uptake than normal prostate tissue after excluding physiological uptake were considered positive lesions (PCa).For imaging-based quantification analysis, Q.Metrix software (Q.Metrix GE Healthcare) was used (14).Acquisition information, including camera sensitivity, activities in full and empty syringes, administration time, and scan time, was input into the system.The volume of interest (VOI) was delineated, and the NM was 0.4.The calculated maximum standardized uptake value (SUV) voxel volume was 3.2×10 -3 mL.VOI-related quantitative parameters were autom aticall y gen er ate d, and SUV m a x was used for quantitative analysis.

Diagnostic criteria for PCa metastases
On mpMRI, ①lymph node metastases: round, short-axis diameter>8 mm, uneven signals in lymph nodes on T2WI, irregular boundaries, and evident enhancement on dynamic contrast-enhanced (DCE) (15); ② bone metastases: low signal intensity on T1WI and T2WI, limited diffusion on DWI, and early enhancement after contrast agent injection on DEC (16,17).On SPECT/CT, ①lymph node and bone metastases: uptake than normal tissue(lesion SUV max ≥liver SUV max ) after excluding lacrimal glands, salivary glands, kidneys, bladder and intestines physiological uptake.The SUV max of all focal SPECT-positive sites was determined based on the ROI basis (17).In the quantification analysis, the size of each SPECT-positive bone and lymph node correlated with the SUV max .

Validation of findings
Prostate needle biopsies were performed in all participants.We used a protocol for transperineal MRI/PSMA-ultrasound fusion targeted and systematic biopsy.In brief, the image-guide (cognitive guidance, MRI/US and PSMA/US) technique was used.Targeted and systematic biopsies were performed in the same session.The number of biopsy cores was as follows: 3-4 cores for targeted biopsy and 10-12 cores for systematic biopsy.If the biopsy results were positive, patients with surgical indications underwent radical prostatectomy, and the pathological results were based on the gross specimen.For patients without surgical indications, pathological results were based on biopsy results.If the needle biopsy results are negative and the clinical symptoms are highly indicative of PCa, the patient's serum PSA value and imaging (mpMRI, 99m Tc-PSMA SPECT/CT) should be followed up for 3-6 months.If the disease does not progress, PCa could be excluded.If the disease progresses, an additional needle biopsy should be performed (12).Not all bone and lymph node lesions showed positive pathological results.Thus, the validated method reported in previous studies was used (9,17).All patients were followed up for at least 6 months (or until death).Serum PSA levels were reviewed every 3 months for all patients.The subsequent therapeutic schedule options depended on the patient's condition, including radical prostatectomy, local radiation therapy, and chemotherapy.Future imaging modalities were selected according to their respective clinical needs and were not bound by a specific protocol.Patients who met at least one of the following conditions were metastases: ① Response to therapy (hormone therapy and radiation) and subsequent serum PSA decline were confirmed by follow-up examination (MRI, CT, PET, etc.); ② two or more imaging examinations recommended metastases, and ③ PSA≥100 ng/mL, suggesting distant metastases (18).

Statistical analysis
Data analysis was performed using SPSS 19.0 software (statistical product and service solutions, Chicago, Illinois).McNemar's test was used to compare the cancer detection concordance rates between 99m Tc-PSMA SPECT/CT and mpMRI.The Mann-Whitney U test was used to compare the differences in quantitative diagnostic parameters among the different groups.Receiver operating characteristic (ROC) analysis was performed to evaluate the sensitivity, specificity, area under the ROC curve (AUC), and a cut-off value of each parameter.The Kruskal-Wallis test was used to compare the differences in quantitative diagnostic parameters among different tumor size groups.The correlation between the Gleason Score and SUV max , ADC min , and SUV max / ADC min was evaluated using Spearman correlation analysis.Logistic regression analysis was used to calculate predictors of the Gleason score.P<0.05 was considered statistically significant.

Overall results
Among the 56 participants, 44 (78.5%) were diagnosed with PCa, and 12 (21.5%)with prostate hyperplasia (BPH).A flowchart illustrating the participant inclusion procedure is shown in Figure 1.Among the 44 patients with PCa, one (2%) had neuroendocrine carcinoma, and 43 (98%) had adenocarcinoma.The surgical indications were judged by the urological surgeon according to the clinical status of the patient (2).The 23 patients with PCa diagnosed by puncture underwent robot-assisted laparoscopic radical prostatectomy (RP); postoperative pathology results were consistent with those of puncture in 11 (11/23,47.8%)patients.12 (12/23,52.2%)patients with PCa experienced pathological upgrading.The Gleason score of patients who underwent surgery was based on the surgical specimen, and the Gleason score of patients who did not undergo surgery was based on the puncture specimen.Among 44 patients with PCa, 23 (52.3%) had metastases.

Difference between tumor size and quantitative parameters
The 44 prostatic lesions detected were grouped according to their maximum tumor diameter: G1 (7/44, maximum diameter < 1.0 cm), G2 (23/44, maximum diameter: 1.0 cm-3.0 cm), and G3 (14/44, maximum diameter > 3.0 cm).The Kruska-Wallis test was used to compare the differences in quantitative diagnostic parameters among different tumor size groups.There were differences in SUV max /ADC min among the tumor size groups; the larger the tumor size, the larger the SUV ma x /ADC m in value (Figure 4).

Relationship between Gleason score and quantitative parameters
Gleason scoring is unsuitable for treating neuroendocrine PCa (19).Therefore, 43 patients with PCa were enrolled in this cohort study.Spearman correlation analysis was used, and the results revealed that ADC min showed a weak negative correlation with Gleason score (r=-0.35,P=0.023), whereas SUV max (r=0.61,P=0.008) and SUV max /ADC min (r=0.59,P=0.023) showed a moderate positive correlation with Gleason score (Figure 5).Based on the Gleason score, the patients were divided into high- Receiver operating curve (ROC) for 99m Tc-PSMA SPECT/CT, mpMRI, and 99m Tc-PSMA SPECT/CT+ mpMRI for detection of primary prostate cancer (n=56).99m Tc-PSMA SPECT/CT, 99m Tc-labelled prostate-specific membrane antigen molecular probe single photon emission computed tomography; mpMRI, multiparametric magnetic resonance imaging; SUV max , maximum standardized uptake value; ADC min , the minimum apparent diffusion coefficient.

Discussion
The labeling method for 99m Tc-HYNIC-Glu-Urea-A( 99m Tc-PSMA) is simple and has high radiochemical purity (10).Previous studies have demonstrated the high diagnostic efficacy of 99m Tc-PSMA SPECT/CT in detecting recurrent biochemical lesions after radical prostatectomy and bone metastases of PCa (9,20).With the introduction of imaging technology, the diagnosis and initial management of localized PCa are increasingly dependent on imaging findings.99m Tc-PSMA SPECT/CT is predominantly used in the primary PCa staging of regional and distant diseases.However, little is known about the value of 99m Tc-PSMA SPECT/ CT for the primary detection of lesions within the prostate.To our knowledge, this is the first comparison between 99m Tc-PSMA SPECT/CT and mpMRI for primary PCa lesions.
In our cohort study, 99m Tc-PSMA SPECT/CT and mpMRI had limited specificity for detecting primary PCa lesions.SUV max and ADC min are important quantitative parameters in SPECT/CT and mpMRI, respectively.Typical PCa foci showed localized highuptake foci on 99m Tc-PSMA SPECT/CT and low-signal foci on mpMRI ADC maps (Figure 6).Previous studies on PCa detection by PET have shown that there may be a certain degree of correlation between mpMRI and PET parameters in the same PCa lesion; that is, SUV max and ADC min were negatively correlated (21,22).Therefore, it is essential to study whether combining these two imaging techniques can further improve the diagnostic efficacy of PCa.In our study, SUV max was positively correlated with the Gleason score, while ADC min was negatively correlated.Based on the above results, we combined the two parameters and used the Tc-PSMA SPECT/CT, 99m Tc-labelled prostate-specific membrane antigen molecular probe single photon emission computed tomography; mpMRI, multiparametric magnetic resonance imaging.Comparison of SUV max between groups: △ compared to the prostate cancer group, P=0.005; ▴ compared with the no metastasis subgroup, P=0.007; * compared between the Gleason score subgroup, P < 0.001.Comparison of ADC min between groups: □ compared to the prostate cancer group, P=0.002; ♦ compared with the no metastasis subgroup, P=0.009; # compared between the Gleason score subgroup, P=0.012.Comparison of SUV max /ADC min between groups: ▽ compared to the prostate cancer group, P=0.003; • compared with the no metastasis subgroup, P=0.003; ★ compared between the Gleason score subgroup, P < 0.001.  in prostatic lesions could be used to predict primary PCa and lymph node and bone metastases.This may be the higher the uptake of PSMA in prostatic lesions, the higher the malignancy of the lesions, resulting in an increased risk of bone or lymph node metastasis (25).However, approximately 10% of patients with primary PCa have low PSMA expression (26).Some falsenegative results were obtained in the clinical setting when SUV max was used separately.The ratio SUV max /ADC min synthesizes the expression of PSMA and the degree of diffusion of water molecules (27).The present study found that the diagnostic efficacy of SUV max /ADC min was better than that of SUV max or ADC min alone.SUV max /ADC min may be used as a predictive parameter for PCa, helping to distinguish benign and malignant lesions of PCa and determine whether there were metastases.The result was also consistent with previous  (28).In our study, the larger the prostatic lesion size, the higher the SUV max /ADC min.When SUV max /ADC min in the prostatic lesion was >7.0×10 3 , the lesion was more likely to be malignant.When SUV max /ADC min in the prostatic lesion is >27.0×10 3 , the patient with PCa may have lymph node and bone metastases.Hence, we postulated that SUV max combined with ADC min (SUV max /ADC min ) might decrease bias and improve diagnostic accuracy.The prognosis of PCa is closely related to the Gleason score grading system (29).The Gleason score is a critical indicator of the pathological results of prostate biopsy.In the previous study on PSMA, Kasperzyk et al. evaluated the expression of PSMA in PCa tissues by immunohistochemical staining, and found that Gleason score in the group with high PSMA expression was significantly higher than that in the group with low PSMA expression (30).Uprimny et al. retrospectively analyzed the 68 Ga-PSMA-11 PET/CT examination data of 90 patients with PCa confirmed by prostate biopsy, and found that SUV max was significantly positively correlated with Gleason score (31).In our study, patients with PCa were divided into high-,medium-and low-risk groups with a Gleason score of 7 as the cut-off value.Our results showed that SUV max /ADC min was the main predictor of the high-risk group, with an optimal cut-off value of 15.0×10 3 .This suggests that SUV max /ADC min ratio is a useful imaging parameter for evaluating tumor biology and prognosis, which may significantly impact the selection of therapeutic strategies.This study had some limitations.Among the 44 patients diagnosed with PCa, 21 did not undergo RP, and gross specimens could not be obtained; only puncture biopsy could be used as the final pathological result.The pathological grading of puncture lesions may differ from actual grading.Furthermore, this was a single-center study with a small sample size, and the conclusions should be verified in a large-scale sample cohort.We did not evaluate the role of SUV max /ADC min in predicting prognosis at follow-up.However, this exploratory study is still valuable as the first clinical quantitative application of 99m Tc-PSMA SPECT/CT combined with mpMRI in PCa lesions.In a future study, we aim to develop a novel analytical approach based on a radiomics quantitative model derived from 99m Tc-PSMA SPECT/CT and mpMRI for noninvasive prediction of intraprostatic lesions in patients with PCa and prognosis.

Conclusion
In this prospective study, our results revealed that combined 99m Tc-PSMA SPECT/CT and mpMRI had a higher diagnostic accuracy for detecting treatment-naive PCa than either modality alone.In addition, SUV max /ADC min is a promising molecular imaging parameter for diagnosing PCa and evaluating its biological behavior.

4 5
FIGURE 4 Box plot of different parameters and tumor size.(A) Differences among SUV max and tumor size.(B) Differences among ADC min and tumor size.(C) Differences among SUV max /ADC min and tumor size.SUV max , maximum standardized uptake value; ADC min , the minimum apparent diffusion coefficient.

TABLE 1
Patient characteristics.

TABLE 2 99m
Tc-PSMA SPECT/CT and mpMRI in the diagnosis of primary prostate cancer.

TABLE 3
Difference between the three diagnostic parameters among different groups.

TABLE 4
Multivariate logistic regression analysis of related factors of Gleason score., confidence interval; OR, odds ratio; PSA, prostate specific antigen, SUV max /ADC min , maximum standardized uptake value/minimum apparent diffusion coefficient. CI