The added value of 68 Ga-PSMA PET/CT in anatomical staging of prostatic carcinoma in correlation with the histopathological zonal staging

Background Prostate cancer is well known as the commonest cancer in men and the second leading cause of cancer-related death. CT, MRI and bone scintigraphy are considered the commonly widely used imaging diagnostic tools for detection, staging and follow-up of prostate cancer. Prostate-specific membrane antigen (PSMA) is a membrane glycoprotein, that can be concentrated in prostate cancer cells up to 100 times higher than in normal cells. PSMA-tar-geted imaging modalities have now proven their efficacy in diagnosis, staging and follow-up of prostate cancer. The use of 68 Ga PSMA PET-CT has efficiently improved the detection of loco-regional and metastatic disease. 68 Ga PSMA PET-CT also has an effective role in the primary diagnosis, staging, and detecting biochemical recurrence after curative treatment and in metastasis-targeted therapy. This work aims to review the role of 68 Ga PSMA PET-CT in anatomical staging of prostate cancer in correlation with histopathological staging. Results Zonal correlation between 68 Ga PSMA findings and biopsy results showed sensitivity ranging between 76.9 and 90.6% and specificity ranging from 85.7 to 100%. There was high significant correlation between the SUVmax uptake and the biopsy results, between the SUVmax uptake and the local staging as well as between the Gleason score and 68 Ga PSMA PET/CT findings. Conclusions 68 Ga PSMA PET/CT is a highly promising imaging modality with an effective role in detection of prostate cancer showing high sensitivity and specificity in prediction of zonal histopathological results and loco-regional Gleason score staging with significant positive correlation between the SUV uptake results, Gleason score and the PSA levels.


Background
Prostate cancer is well known as one of the most common cancer in men and second main leading cause of men cancer-related death.CT, MRI and bone scintigraphy are considered the most commonly used imaging modalities for diagnosis, staging and for post-therapy follow-up of prostate cancer.Prostate-specific membrane antigen (PSMA) is a membrane glycoprotein, that expresses significant higher concentration values on prostate cancer cells compared to normal prostate cells [1]. 68Ga PSMA targeting imaging agents are now widely available and being commonly used due to their high efficient diagnostic accuracy. 68Ga PSMA PET has been combined with multi-slice CT, for proper and better diagnostic and prognostic performance [2].Recently, a multidisciplinary panel of healthcare providers and prostate cancer imaging experts developed an appropriate criteria for 68 Ga PSMA imaging, including patients with newly diagnosed intermediate, high risk prostate cancer, castration-resistant prostate cancer (CRPC) and biochemical recurrence [3].

68
Ga PSMA-PET has also been studied to detect and localize prostatic carcinoma.Some studies have been conducted to evaluate the relation between different pathological zones 68 Ga PSMA uptake and the histopathological subtypes [4].
PET/CT imaging modality has already proved its accuracy and efficiency in prostate cancer detection and characterization and also has shown accurate relation between anatomical imaging and differential zonal histopathology [5].
This work aims to review the added value of 68 Ga PSMA PET-CT in anatomical prostate cancer staging in correlation with histopathological staging.

Methods
A retrospective study that was conducted at Ain Shams University Hospitals from March 2023 till September 2023.40 patients were included in the study with age range between 55 and 84 years old with mean age of 69.55 years old (Table 1).
The main source of data for this study was the PET/CT scans and PACS archiving system of Ain Shams University hospitals.

• Ethical Considerations
-An informed consent was obtained from all patients after explaining the procedure details of the study.-The study has been initiated after the approval of the research ethical committee, faculty of medicine and Ain Shams University.-The patient privacy and confidentiality of data were guaranteed during the various phases of the study.
• Study Tools and procedure Clinical history and imaging findings of the included patients were retrospectively obtained from the picture archiving and communications system (PACS) of nuclear medicine unit in Ain Shams University.

-Inclusion Criteria
All patients were referred for 68 Ga PSMA PET/ CT scan with pathologically proved prostatic carcinoma, for initial staging.

-Exclusion criteria
Patients who underwent any intervention or previously treated.
Any patient without pathological diagnosis of cancer prostate.
• Patient preparation for PET/CT scans -Patients were instructed to fast from all types of food for a minimum of 4-6 h prior to the examination.-Patients were asked to be well hydrated before the study, and during the uptake time, then they were asked to empty their urinary bladder before the study.-Patients were asked to bring all related previous investigations, biopsy reports, and PSA level.-Serum creatinine was done within at least one month.It has to be within normal level (1.5 mg / dl) for IV contras administration.-The study was done in supine position with both arms elevated above head and scan on the whole body from the skull base down to the mid-thighs.-Then followed by diagnostic CT transmission scan using similar parameters.-A diagnostic contrast-enhanced CT scan was done initially covering the identical transverse field of view using the following parameters: 350 mA, 120 kV, 0.5-tube rotation time, and 5-mm slice thickness.-PET scan should start from the mid-thigh to skull base to exploit the reduced 68 Ga-PSMA ligand uptake in the urinary bladder and with several bed positions [5][6][7] were performed each with approximately 15-cm axial field of view per bed position with 4-mm in-plane spatial resolution and covered the same field of view of the CT.The time of acquisition emission data was about 2-4 min for each bed position and in time range between 13 and 17 min.-A workstation with fusion software (PHILIPS WORKSTATION) was used to view all CT, PET, and PET/CT images, and they were reconstructed in multi-planar reformation and viewed in different planes for all as well as "3D maximum intensity projection (MIP) images" PET image in a video mode.-A radiological teamwork, including three nuclear medicine radiologists, reviewed and interpreted the PET, CT, and fused PET/CT images.-Prostate was divided into six zones (similar to the pathological report), right basal (RB) zone, left basal (LB) zone, right mid (RM) zone, left mid (LM) zone, right apical (RA) and left apical (LA) zones.-Each zone was radiologically assessed both visually for PSMA uptake and quantitatively by SUV max.-These radiological findings were correlated with the Gleason score in the histopathology report.

Results
• Forty patients with pathologically proven cancer prostate were referred to 68 Ga PSMA PET /CT for initial staging before initiating any treatment.• The ages of the included patients in this study ranged from 55 to 84 years, with mean age of 69.55 years (Table 2).• Prostate was divided into six zones (similar to the pathological report), and each zone was radiologically assessed visually for PSMA uptake and quantitatively by SUV max.

Diagnostic performance of 68 Ga PSMA PET/CT for local staging
Relation between 68 Ga PSMA PET/CT results and biopsy results (Tables 3 and 6) • Zonal correlation between PSMA findings and biopsy results showed sensitivity ranging between 76.9 and 90.6%, specificity ranging between 85.7 and 100%, PPV ranging from 95.2 to 100% and NPV ranging from 50 to 81.3% in detection of prostatic cancer lesion in 68 Ga PSMA PET/CT studies in different prostatic zones.• According to TNM staging [5], two patients (5%) were staged as T0, fifteen patients (37.5%) were staged as T2, eighteen patients (45%) were staged as T3, and five patients (12.5%) were staged as T4 (Table 5).

Relation between SUVmax uptake and biopsy results (Table 7)
There was highly significant correlation between the SUVmax uptake and the biopsy findings where the median SUVmax uptake for cancer prostate lesion was 20.065, while the median SUVmax of normal prostatic tissue was 1.6 with P value < 0.001.

Relation between the PET/CT findings, SUVmax and local staging (Tables 8 and 9)
• There was highly significant correlation between the SUVmax uptake and the local staging where the SUVmax uptake was higher when the local staging was high with P value ranging from 0.01 to < 0.001.• Mild significant relationship was seen between the PET/CT findings and the local staging with P value ranging from 0.03 to < 0.001.

Relation between 68 Ga PSMA PET/CT findings and Gleason score (Table 10)
There was highly significant correlation between the Gleason score and 68 Ga PSMA PET/CT findings with P value 0.003 where high PSMA sensitivity was found with mean Gleason score of 7. Few false negative results noted with very high Gleason score at 8.

Relation between SUVmax uptake and Gleason score (Table 10)
These was significant correlation between the Gleason score and SUVmax results with P value 0.03.

Relation between PSA level and Gleason score (Table 11)
These was significant correlation between the Gleason score and PSA levels with P value 0.02.

Discussion
The main aim of this study was to determine the correlation between 68 Ga-PSMA expression in the primary prostate cancer patients including SUV and the PSA value as well as Gleason score in those cases.

68
Ga Prostate-specific membrane antigen (PSMA) PET has been recently emerging as a new promising imaging modality for evaluation of prostate cancer, with higher  sensitivity and sensitivity compared to other conventional imaging modalities [6].Prostate-specific membrane antigen (PSMA) is a transmembrane glycoprotein that showed significant overexpression in prostate cancer cells in which radiolabeled small molecules can avidly bind to the active prostate cancer cells, resulting in significant high tumor-to-background contrast images [7].
Current study-based evidences demonstrate higher sensitivity and specificity parameters of 68 Ga PSMA PET /CT compared to the conventional imaging, for better morphological and functional evaluation of prostate cancer lesions [8].
In this study, there was high significant correlation between the Gleason score and 68 Ga PSMA PET/CT findings with P value 0.003 where high PSMA sensitivity found with mean Gleason score of 7 and this was in line with other studies (Fig. 1).
This finding is matching with other various studies that have similar conclusion; In an another study conducted by Altun et.al. [9] including 98 patient and found that total PSA level was significantly higher in Gleason score > 7 than in the group with Gleason level ≤ 7 (p = 0.001).In addition, PSMA SUVmax levels of the group with a Gleason score of > 7 revealed to be significantly higher than the group with a Gleason score of ≤ 7 (p = 0.03).
Rahbar et al. [10] prospectively evaluated 68 Ga-PSMA PET /CT in six patients who underwent prostatectomy for Gleason ≥ 3 + 4 tumors and found that 68 Ga-PSMA PET had a sensitivity and specificity of 92% for detecting prostate cancer lesions.
Fendler et al. [11] also prospectively evaluated 68 Ga-PSMA PET in 21 patients with biopsy-proven prostate carcinoma in which the prostate was sectioned into sixsegmental model with a sensitivity and specificity of 67% and 92%, respectively.False-negative results were noted in 6 out of 12 segments with a Gleason score of 6, 12 out of 27 segments with a Gleason score of 7, 4 out of 19 segments with a GS of 8, 10 out of 41 segments with a Gleason score of 9, and only one segment with a Gleason score of 10.
Determining the accuracy of an imaging modality to detect prostate cancer requires appropriate imaging/histopathologic concordance as the distribution of prostate cancer within the gland can be heterogeneous and the  deformation can occur during surgery and histopathologic work up.Zamboglou et al. [12] have conducted a prospective study of 8 men who went 68 Ga-PSMA PET/CT scans prior to prostatectomy and revealed a statistically significant correlation of 68 Ga-PSMA PET/CT with histopathological results.
Another prospective study conducted by Courtney et al. [13] for 302 patients with pretherapy to prostate cancer and found out a significant sensitivity and specificity of 68 Ga-PSMA PET (85% and 98%, respectively) in loco-regional prostate cancer staging.
Luiting et al. [14] also reviewed retrospectively the diagnostic performance of 68 Ga-PSMA PET/CT for primary prostate cancer staging, where patient-based sensitivity and specificity were 4-100% and 90-95%, lesion-based sensitivity and specificity ranges were 50-58% and 96-100%, respectively.That was concordant with the results in this study that proved significant relationship between the PET/CT findings and the local staging with P value ranging from 0.03 to < 0.001.
Those findings are correlated with our study results, in which there was highly significant correlation between the SUVmax uptake and the histopathological results where the median SUVmax uptake for cancer prostate lesion was 20.065, while the median SUVmax of normal prostatic tissue was 1.6 with P value < 0.001 (Fig. 2).
Another study conducted by Meissner et al. [15] stated SUVmax cut-off value was 4.0.
This discrepancy in SUV cut-offs values maybe attributed to multiple variations such as scanners, tracers, reconstruction parameters and others.
Demerci et al. [16] reported in their study the correlation coefficients between PSA values and SUV max that ranged between 0.071 and 0.57.And the correlation coefficients between SUV max and Gleason scores ranged between 0.096 and 0.5.Meißner et al. [17] conducted a cohort study on 37 patients and reported significant correlation between Gleason score and SUV uptake and moderate correlation between tumor volume and Gleason scores.
Christian et al. [18] combined in their study GS, SUV max and PSA values, where the median SUV max was higher in patients with PSA levels ≥ 10 ng/ml.
Few false negative results reported in our study with Gleason score of 8 and some other false negative results also elicited with very low Gleason score (Fig. 3).
This was matching with a study conducted by JIRI et al. [19], that revealed that poorly differentiated or anaplastic tumor cell subtypes can express relatively lower Gleason score of 5 only have a weak correlation with 68 Ga-PSMA accumulation.
This could be explained as the most aggressive tumors subtypes have highly altered cells that stop expressing PSMA in high concentration and hence show low 68 Ga-PSMA accumulation.

Pitfalls of 68 Ga-PSMA PET/CT
The main pitfall in the current study is the relative limited number of sample volume due to some factors like patient non-compliance or financial obstacle factor.Some PSMA pitfalls are related to its expression in multiple benign conditions, mainly in inflammatory pathologies.
The pitfall of overlapping between malignant prostatic lesions and benign prostate lesions.Some benign lesions can mimic prostate cancer cells and their metastatic disease, including inflammatory lymph nodes, various benign osseous conditions, including Paget's disease, hemangiomas and fractures.

Conclusions
Overall, the current study could conclude that the accuracy of 68 Ga-PSMA PET/CT is superior in sensitivity and specificity to that of conventional imaging in prostate cancer diagnostic workup.The Gleason score and PSA level were significantly correlated with SUV uptake of 68 Ga-PSMA PET/CT imagings.
Based on those findings, we recommend the promising use of 68 Ga-PSMA PET/CT for the primary staging of prostate cancer, especially in tumors with GS > 7 or patients with PSA level ≥ 20.065 ng/ml; however, false negative results can occur with very low or very high Gleason scores due to non-differentiated cells.
68 Ga-PSMA PET/CT is recommended for many other aspects; metastasis workup in primary prostate cancer and for the detection of biochemical recurrent lesions after radical resection or radiotherapy status.
The superior diagnostic accuracy of 68 Ga-PSMA PET/CT for primary staging of prostate cancer proved to have a positive impact on clinical management outcome in those patients.

Fig. 1 A
Fig. 1 A 72-year-old male patient, presented with newly diagnosed prostatic carcinoma and referred for initial staging.His PSA level is 9.4 ng/ml.and Gleason score: 4 + 3 = 7.A MIP image of the PET/CT, B Axial PET/CT fused images and C Sagittal PET/CT fused images showing prostatic bilobar moderate 68GAa PSMA-avid focal lesions occupying the peripheral zones of mid-gland and apex.The lesions show SUVmax of 8.9

Fig. 2 A
Fig. 2 A 74-year-old male patient, presented with newly diagnosed prostatic carcinoma and referred for initial staging.His PSA level is 53.5 ng/ ml. and Gleason score: 4 + 4 = 8.A MIP image of the PET/CT showing B Axial PET/CT fused images and C Sagittal PET/CT fused images showing: Markedly 68Ga PSMA avid prostatic mass occupying the peripheral zones of the whole gland and extending to the transitional zones (white arrow) achieving 36 SUVmax.Multiple enlarged PSMA avid regional and distant retroperitoneal lymph nodes (Blue arrow).Few 68Ga PSMA avid bony deposits noted mainly at right femoral head and right humeral head (Yellow arrow)

Fig. 3 A
Fig. 3 A 65-year-old male patient, presented with newly diagnosed prostatic carcinoma and referred for initial staging.His PSA level is 14.6 ng/ml.and Gleason score: 3 + 3 = 6.A MIP image of the PET/CT, B Axial PET/CT fused images and C Sagittal PET/CT fused images showing low grade 68Ga PSMA focal lesion occupying the right posterior peripheral zone of base, mid-gland and apex.The lesion shows SUVmax of 5.6

Table 1
Age distribution of studied patients (N = 40)

Table 2
PSA level

Table 3
PSMA PET/CT results

Table 4
Biopsy results

Table 6
Relation between PSMA PET/CT results and biopsy results

Table 7
Relation between SUV max and biopsy results

Table 8
Relation between SUV max and histopathological local staging

Table 9
Relation between PSMA PET/CT results and histopathological local staging

Table 10
Relation between Gleason score and PSMA *Student t test

Table 11
Correlation between Gleason score and both PSA level and SUVmax