Potential of asphericity as a novel diagnostic parameter in the evaluation of patients with 68Ga-PSMA-HBED-CC PET-positive prostate cancer lesions

Background The aim of this study was to evaluate the diagnostic value of the asphericity (ASP) as a novel quantitative parameter, reflecting the spatial heterogeneity of tracer uptake, in the staging process of patients with 68Ga-PSMA-HBED-CC positron emission tomography (PET)-positive prostate cancer (PC). In this study, 37 patients (median age 72 years, range 52–82 years) with newly diagnosed PC, who received a 68Ga-PSMA-HBED-CC PET fused with computed tomography (68Ga-PSMA-PET/CT), a magnetic resonance imaging (MRI) of the prostate, and a core needle biopsy (within 74.2 ± 80.2 days) with an available Gleason score (GSc) were extracted from the local database. The ASP and the viable tumor volume (VTV) was calculated using the rover software (ABX GmbH, Radeberg, Germany), a segmentation tool for automated tumor volume delineation. Additionally, parameters including total lesion binding rate (TLB), maximum, mean and peak standardized uptake value (SUVmax/mean/peak), prostate-specific antigen (PSA), D’Amico classification, and prostate imaging reporting and data system (PI-RADS) were analyzed. Results The ASP mean differed significantly (p ≤ 0.05) between the different GSc groups: GSc 6–7: 11.9 ± 4.8%, GSc 8: 25.5 ± 4.8%, GSc 9–10: 33.3 ± 6.8%. A significant correlation between ASP and GSc (rho = 0.88; CI 0.78–0.94; p < 0.05) was measured. The ASP enabled an independent (p > 0.05) prediction of the GSc. A moderate correlation was measured between ASP and the D’Amico classification (rho = 0.6; CI 0.32–0.78; p < 0.05). The VTV showed a moderate correlation with the SUVmax (rho = 0.58; CI 0.32–0.76; p < 0.05) and the GSc (rho = 0.51; CI 0.23–0.72; p < 0.05). Conclusion The asphericity in 68Ga-PSMA-PET could represent a promising novel quantitative parameter for an improved non-invasive tumor staging of patients with PC.


Background
Prostate cancer (PC) is the most frequent cancer entity diagnosed in men in the western world with the second highest overall mortality [1]. PC is responsible for up to 8% of all cancer-related deaths in males, resulting in the fourth leading cause of cancer-related death in both sexes [1]. The prostate-specific antigen (PSA) blood level is currently the reference standard for PC screening of the population. With a risk reduction of 1.07 deaths per 1000 cases, PSA screening was shown to reduce PCassociated mortality up to 21% [2]. However, it is not fully elucidated to which extent PSA reduces the allcause PC-associated mortality [2]. Besides the positive effects of PSA screening, PSA testing can also be associated with an overdiagnosis of up to 23-42% [3,4]. While advances, especially in the non-invasive local staging of patients with primary PC, have been made using magnetic resonance imaging (MRI) in recent years, this technique is still associated with limitations regarding the grading of the primary tumor [5].
Most current studies, which investigate positron emission tomography (PET) tracer uptake in different malignancies, rely on the maximum standardized uptake value (SUVmax) as a quantitative parameter for the characterization of the binding/uptake of the tracer. However, it is increasingly recognized that not only the overall uptake or metabolism, but also the heterogeneity of the tracer uptake, plays a role in the characterization of malignancies [15][16][17][18]. The quantification of the heterogeneity of tracer uptake could represent a promising novel parameter for an improved characterization of tumor heterogeneiety and therefore the malignancy of tumors [19]. In this context, a novel parameter-the asphericity (ASP)-was recently introduced [15]. The ASP describes the non-spherical shape of a tumor, compared to a sphere with the same volume. Initially, the ASP was used to quantify the spatial irregularity of the metabolic tumor volume (MTV) in 18 F-FDG-PET [15,[18][19][20][21]. Most tumors are genetically and histopathologically heterogenic and further dedifferentiation and infiltration is often associated with poorer prognosis [15]. Previous studies demonstrated the potential of the ASP for an improved tumor staging [15,19].
The aim of this study was to test the diagnostic value of the ASP in the staging process of patients with 68 Ga-PSMA-HBED-CC-PET ( 68 Ga-PSMA-PET)positive PC.

Study population
This retrospective study was approved by the local ethics review board. The local database was screened for patients, who received a 68 Ga-PSMA-PET combined with computed tomography (CT) and a 3 T MRI of the prostate within 110 days for staging of suspected primary PC. MRI was used as reference standard for the definition of the primary tumor and to evaluate our delineation process. The proximity of the 68 Ga-PSMA PET/CT to the MRI was a requirement to ensure that the lesion which was evaluated in 68 Ga-PSMA PET reflects the primary PC lesion. We extracted 691 patients from our imaging database, who underwent 68 Ga-PSMA-PET/CT in-between Oct. 01, 2013 and Feb. 01, 2017. MRI data of the prostate was available in 169 cases, excluding 522 cases without a MRI of the prostate. In our institution, 3 T MRI has established itself as the routine MR examination for the evaluation of patients with prostate cancer. Twenty-six cases were excluded as only 1.5 T MRI datasets were available. Ninety patients were excluded as the delay between 68 Ga-PSMA-PET/CT and 3 T MRI was more than 110 days. All PSMA-positive lesions had to be confirmed through core needle biopsy, which was not available for this study in 16 patients. The final cohort of 37 patients had a mean age of 71.3 ± 7.5 years and received both scans within 50.2 ± 32.5 days. Core needle biopsy and 68 Ga-PSMA-PET/CT were performed within 74.2 ± 80.2 days. A detailed overview regarding the patient characteristics are summarized in Table 1.

Imaging protocol
PET/CT imaging was performed 75.4 ± 27.5 min after intravenous injection of 122.4 ± 19.7 MBq of [ 68 Ga]-PSMA-HBED-CC. A 3D acquisition mode was used on a Gemini TF 16 Astonish PET/CT scanner (Philips Medical Systems) [25]. Default parameter settings were used in the system software to reconstruct the transaxial slices (144 × 144 voxels, 4 mm 3 ). Immediately before the PET scan, a low-dose CT was acquired for attenuation correction (120 kVp, 30 mAs) and anatomical mapping.

Asphericity
Definition of ASP: ASP is defined as a marker for non-spherical tumor volumes. A sphere has the smallest possible surface S for a given volume V for which ASP = 0 by definition [15]. For non-spherical lesions, ASP > 0 provides a quantitative measure for deviation of spherical shape. For example, an ASP of 30% means a 30% larger surface than a sphere with the same volume [19]. A detailed description of the definition was published earlier [15].

Imaging analysis
The program Visage 7.1 (Visage Imaging) was used prior to the delineation process for evaluation of the 68 Ga-PSMA-PET/CT and the MRI data. In case of multiple foci or lesions with unclear borders in PET, a simultaneous evaluation of the MRI data in Visage 7.1 supported the detection/delineation of the primary PC lesion. Following the detection of the primary lesion within the prostate using the rover software (ABX GmbH, Radeberg, Germany), a 3D mask was placed around the volume of interest (VOI). Rover uses a specific algorithm that delineates the tumor automatically. This is achieved by adaptive thresholding and taking the background signal of the surrounding tissue into account [20]. In some cases, the strong radiotracer signal from the bladder was interfering with a fully automatic delineation of the tumor. Therefore, the tumor delineation was inspected in axial, coronal, and sagittal planes in all cases and the VOIs were corrected manually, if necessary. Computed parameters of the VOIs included the ASP, SUVmax, SUVmean, SUVpeak and the viable tumor volume (VTV). Figures 1 and 2 show examples of the delineation of the tumor using the rover software in patients with different Gleason scores (GSc).

Viable tumor volume and tumor lesion binding rate
Accumulation of 68 Ga-PSMA-HBED-CC in PC cells depends on the PSMA expression on the cell surface. PSMA is highly overexpressed in PC cells, resulting in a strong signal in PET [26]. The accumulation of 68 Ga-PSMA-HBED-CC is reduced in non-diseased prostate tissue, expressing lower PSMA levels on their cell surface. This leads to a delineation of a VTV, at the location at which 68 Ga-PSMA-HBED-CC internalization is highly active. Derived from the "total lesion glycolysis" used in 18 F-FDG-PET/CT, a further parameter used in this study was the "total lesion binding" rate (TLB). The TLB was defined as the product of the SUVmean and the VTV of a PC lesion.

Gleason score
All 37 patients underwent core needle biopsy of the prostate for a histopathological characterization of tumor malignancy. The GSc takes into account that malignancy signs such as cell size, nucleus size, nucleus to cytosol ratio, abnormal mitosis, and necrosis affect the grading. The final score is an addition of the most common found tumor grade and the highest found tumor grade for all tissue samples [27].

TNM and D'Amico classification
TNM is a clinical used score for staging, for determination of prognosis and treatment planning. T-Stage describes the tumor size and tumor infiltration in surrounding tissue. N-stage indicates the presence of lymph node metastases; M-stage describes the presence of remote metastases outside the prostate [28]. The risk stratification for progression of PC can be measured with the D' Amico classification tool. It takes PSA blood level, clinical tumor size (through endorectal examination), and GSc (core needle biopsy) into account. It classifies patients according to low, intermediate, or high This table summarizes the main characteristics of the patients investigated in this study. This included the age of the patients, the PI-RADS and Gleason score, PSA blood level, and the viable tumor volume as well as the time difference between PSMA-PET and MRI, between Gleason score and PSMA-PET, between PSA blood level sampling date and PSMA-PET. Data are given in means, standard deviations, medians, and ranges. Abbreviations: GSc Gleason score, PSA prostate-specific antigen blood level, PSMA prostate-specific membrane antigen, PET positron emission tomography, MRI magnetic resonance imaging, PI-RADS prostate imaging reporting and data system, VTV viable tumor volume risk for early metastasis and increased tumor aggressiveness [29].

Prostate imaging reporting and data system
Prostate imaging reporting and data system (PI-RADS) is a score used for standardized reporting of clinical findings in magnetic resonance imaging examinations of the prostate. In 2014, an update towards version 2 was released. MRI sequences included in PI-RADS v2 evaluation, which was used for all patients in this study, are high-resolution T2-weighted imaging, diffusion-weighted imaging, and dynamic contrast-enhanced imaging sequences. For each sequence, a score of 0-5 indicates the probability of a clinical significant PC lesion leading from improbable to highly suspicious in score 5. Every lesion is scored in the three sequences resulting in three sub-scores. The resulting final PI-RADS score is a summarized score [30,31].

Standardized uptake value
The SUV is a degree of tracer uptake in a specific region of interest (ROI) or VOI. It is calculated as the product of the activity concentration (Bq/g) and the patient's weight (g) divided through the applied dose (Bq). SUVmax, SUVmean, and SUVpeak can be calculated for every ROI or VOI [32]. In addition to maximum and mean SUV, SUVpeak was computed as the mean value of a 3D sphere with a diameter of approximately 1.2 cm centered at the VOI maximum. All parameters were computed using the rover software.

Statistical analysis
Descriptive statistics, correlations, and scatter plots were computed using MedCalc Statistical Software version 17.2 (MedCalc Software bvba, Ostend, Belgium; http://www.medcalc.org; 2017). All univariate correlations including ordinal variables were tested using Spearman's rank correlation method. Pearson's correlation method was used for metric variables. A p value < 0.05 was considered statistically significant. The polytomous universal model implemented in the statistic software IBM SPSS (version 24) was used for the ordinal logistic regression. Ordinal regression models the propensity of the first ranked state against all higher ranked states. This is repeated for the second and ongoing ranked states resulting in k-1 intercept parameters for k ordinal levels. We included the independent variables ASP, VTV, and TLB in the model. Ordinal regression is preferable, when the outcome consists of several discrete but ordered states instead of the assumption of a continuous dependent variable in linear regression. The group-based analysis for GSc was tested using Bonferroni corrected t-tests in SPSS. ASP probability was visualized using R software (Version 3.2.5, Vienna, Austria, http://www.R-project.org).
Variables are reported as mean as well as standard deviation (SD).

Association of asphericity with histopathology
Patients in this study demonstrated GSc ranging from 6 to 10 with a median of 8  Fig. 3 demonstrates the subgroup analysis. The scatter plot shows the regression with the associated 95% confidence interval. The correlation was shown to be significant (R 2 = 0.84, p < 0.05).
Probability of Gleason score based on asphericity  Table 2.  Table 2.
Association of prostate-specific antigen blood level with Gleason score, D'Amico classification, N-stage, and PI-RADS score  Prognostic estimation of Gleason scores using asphericity, viable tumor volume, and total lesion binding rate on multivariable analysis On multivariable analysis regarding independent association of ASP, VTV, and TLB with the GSc, the ASP and the VTV represented independent predictive parameters (0.71; CI 0.35-1.06 and 0.36; CI 0.03-0.69). The TLB did not represent an independent parameter (− 0.02; CI − 0.04-0.004). All variables are summarized in Table 3.
Association of maximum, peak and mean standardized uptake values and prostate-specific antigen blood level with asphericity, viable tumor volume, and total lesion binding rate

Discussion
This study demonstrated that the ASP in 68 Ga-PSMA-PET could represent a promising quantitative parameter for an improved non-invasive T-staging of patients with PC. In the investigated patient collective, patient groups with different GSc could be discriminated based on the quantitative assessment of the ASP of the local PC. On multivariable analysis, it was demonstrated that the ASP was independently associated with the GSc.

Asphericity for the evaluation of tumor heterogeneity
Aggressiveness of tumor behavior, therapy response and overall patient survival is known to be associated with the heterogeneity of the tumor [33,34]. The parameter ASP enables the quantification of the associated spatial irregularities in PET datasets. Previous studies already investigated the prognostic value of the ASP in certain types of head and neck cancers and non-small-cell lung cancer (NSCLC) [15,19,21,35]. The ASP was found to be an independent predictor of outcome in head and neck cancer patients undergoing pre-therapeutic 18 F-FDG-PET/CT. ASP measurements of the 18 F-FDG uptake also improved the prediction of tumor progression. These previous studies reported a moderate correlation between ASP and MTV, while no correlation between ASP and SUVmax were measured [19,35]. Additionally, a recent study demonstrated a significant association between progression-free survival and overall survival based on the assessment of the ASP [15]. Comparable results were published for NSCLC, in which the ASP provided a higher prognostic value for Spearman's rank correlation method. This table summarizes the associations between the ASP, VTV, TLB, SUVmax, SUVmean, SUVpeak and PSA in the left column and the GSc, D'Amico classification, N-stage and the PI-RADS score in the upper row using Spearman's rank correlation method. The closest significant (p < 0.05) correlations were measured between ASP and GSc followed by a moderate correlation towards the D'Amico classification. A significant (p < 0.05) weak correlation was seen between VTV and the GSc. Statistically significant correlations (p < 0.05) are highlighted in italics. Abbreviations: ASP asphericity, VTV viable tumor volume, TLB total lesion binding rate, SUVmax maximum standardized uptake value, SUVmean mean standardized uptake value, SUVpeak peak standardized uptake value, PSA prostate-specific antigen blood level, GSc Gleason score, PI-RADS prostate imaging reporting and data system On multivariable analysis using ordinal logistic regression, correlation of the ASP to GSc was independent against VTV and TLB. A comparable weaker effect was found for VTV as well. Abbreviations: ASP asphericity, VTV viable tumor volume, TLB total lesion binding rate progression-free survival and overall survival in NSCLC patients compared to SUVmax, MTV, and another parameter of spatial heterogeneity called solidity [19]. Moderate associations were found between ASP and MTV; no correlation was measured between SUVmax and ASP [19]. Additionally, correlations of ASP with histopathology and with the expression of the tumor proliferation markers KI-67 and epidermal growth factor receptor (EGFR) in NSCLC were found [35]. These previous studies introduced the ASP as a promising novel parameter for the non-invasive characterization of tumors. Additionally, the ASP could represent a strong predictive parameter regarding the overall survival in these patient collectives [15,19].
Evaluation and local staging of prostate cancer by positron emission tomography Various radiotracers have been tested for the local staging of PC. One of these tracers is 18 F-FDG. Its accumulation is based on an increased glucose metabolism of cancer cells due to an overexpression of hexokinase. 18 F-FDG uptake was shown to be increased in benign prostate tissue, including prostate hyperplasia, as well as in PC cells [11]. Sensitivities of up to 64% for detection of primary PC were reported [36]. The limited performance of 18 F-FDG for the primary PC diagnosis could be associated with the relatively low metabolic rate of PC and the lack of patient selection in these previous studies [6,[8][9][10]37].
A different tracer that demonstrated potential for the detection of PC is 18 F-C. The use of choline-based tracers is dependent on phosphorylcholine turnover in PC cells. Most studies, however, reported limited sensitivities, especially for the primary diagnosis of PC [6,7,11]. A further tracer that was evaluated in this context is 11 C-acetate. Its uptake is a result of an increased lipid synthesis in tumor cells [38]. Even though its uptake is not limited to PC cells, this radiotracer was shown to be superior to 18 F-FDG for the detection of PC lesions [39]. 11 C-MET and 18 F-FDHT have also been evaluated for the staging of PC. 11 C-MET targets the increased amino-acid transport of methionine for protein synthesis in cancer cells. 18 F-FDHT targeting is based on an overexpression of the androgen receptor. Limitations of these tracers include the lack of studies regarding their diagnostic value [6].
These previous studies demonstrated that novel more specific tracers and in vivo parameters are needed for an improved in vivo detection and characterization of PC.

68
Ga-PSMA-PET for the staging of prostate cancer lesions PSMA is significantly over-expressed in PC cells and overexpression increases with more advanced tumor stages [26]. Binding of 68 Ga-PSMA-HBED-CC leads to receptor internalization and tracer accumulation. It is important to mention that PSMA avid tissue can be found throughout the body since it is a zinc-dependent exopeptidase with glutamate carboxypeptidase activity [24]. A recent study demonstrated promising sensitivity, specificity, and accuracy rates of 65.9, 98.9, and 88.5% for the detection of highrisk PC using 68 Ga-PSMA-HBED-CC. However, a reliable detection can be challenging, as up to 8.4% of all primary tumors showed no tracer accumulation [13,40]. Another recent study investigated the intensity of tracer accumulation in 90 patients using 68 Ga-PSMA-HBED-CC. It was demonstrated that the SUVmax of primary PC was significantly higher in GSc > 7 compared to GSc < 7. Additionally, it was shown that a PSA value > 10 ng/ml was an associated with a significantly higher tracer uptake compared to a PSA value < 10 ng/ml [41]. Other studies focused on the diagnostic accuracy of recurrent PC using 68 Ga-PSMA-HBED-CC. These studies reported sensitivities and specificities up to 80 and 97% for the detection of recurrent PC [42][43][44][45]. PSA blood levels in biochemical recurrence correlated with positive findings, even in patients with low PSA levels (< 1 ng/ml) [14,43,46].
Potential of asphericity as a novel diagnostic parameter in the staging process of patients with 68 Ga-PSMA-PET-positive prostate cancer lesions To the best of our knowledge this was the first study which combined 68 Ga-PSMA-PET with the evaluation of The current study demonstrated that the ASP derived from 68 Ga-PSMA-PET enables a distinction between patient groups with different GSc. Additionally, a correlation of the ASP with the GSc, based on core needle biopsy, was found. The findings in our studies are in line with previous studies, in which a correlation between the ASP and the histopathological staging of NSCLC was demonstrated [35]. In the current study, no significant correlation between SUVmax and ASP was measured [15,19,35]. Furthermore, no significant correlation was found between SUVmax and GSc. In contrast to previous studies, this study did not demonstrate a correlation of ASP with the PET tumor volume. This could be explained by smaller VTV of PC lesions in comparison to lesions of NSCLC and head and neck cancer. Our study did not show statistically significant associations of the ASP to the PI-RADS score.
Further prospective studies and a higher number of patients are now warranted to investigate the potential of the ASP in the staging process of PC patients.
This study is limited by its retrospective study design. Only a relatively small patient cohort with clustered GSc was investigated. In case of multiple lesions, PC lesion selection on PET was based on the evaluation of the MRI data. If the strong radiotracer signal from the bladder was interfering with an automatic delineation of the tumor, the tumor delineation was inspected in axial, coronal, and sagittal planes and VOIs were corrected manually, if necessary. Although several viable automated algorithms have been described, the VTV is presently still determined by manual delineation in a high number of institutions [26,[47][48][49][50][51][52][53][54]. Manual delineation is prone to intra-and interobserver variability as well as to potentially size-and backgrounddependent bias if fixed absolute or relative thresholds are used.

Conclusions
The ASP in 68 Ga-PSMA-PET could represent a promising parameter for an improved non-invasive T-staging of patients with PC. Further prospective studies are now warranted to investigate the potential of the ASP in the staging process of PC patients.