The Promise of Metastasis-Directed Therapy for Oligometastatic Prostate Cancer: Going Beneath the Surface with Molecular Imaging

Epidemiology, treatment patterns, and clinical outcomes in de novo oligometastatic hormone-sensitive prostate cancer

BACKGROUND

This study was conducted to better characterize the epidemiology, clinical outcomes, and current treatment patterns of de novo oligometastatic hormone-sensitive prostate cancer (omHSPC) in the United States Veterans Affairs Health Care System.

METHODS

In this observational retrospective cohort study, 400 de novo metastatic hormone-sensitive PC (mHSPC) patients diagnosed from January 2015 to December 2020 (follow-up through December 2021) were randomly selected. omHSPC was defined as five or less total metastases (excluding liver) by conventional imaging. Kaplan-Meier methods estimated overall survival (OS) and castration-resistant prostate cancer (CRPC)-free survival from mHSPC diagnosis date and a log-rank test compared these outcomes by oligometastatic status.

RESULTS

Twenty percent (79 of 400) of de novo mHSPC patients were oligometastatic. Most baseline characteristics were similar by oligometastatic status; however, men with non-omHSPC had higher median prostate-specific antigen at diagnosis (151.7) than omHSPC (44.1). First-line (1L) novel hormonal therapy was similar between groups (20%); 1L chemotherapy was lower in omHSPC (5%) versus non-omHSPC (14%). More omHSPC patients received metastasis-directed therapy/prostate radiation therapy (14%) versus non-omHSPC (2%). Median OS and CRPC-free survival (in months) were higher in omHSPC versus non-omHSPC (44.4; 95% confidence interval [CI], 33.9-not estimated vs. 26.2; 95% CI, 20.5-32.5, p = .0089 and 27.6; 95% CI, 22.1-37.2 vs. 15.3; 95% CI, 12.8-17.9, p = .0049), respectively.

CONCLUSIONS

Approximately 20% of de novo mHSPC were oligometastatic, and OS was significantly longer in omHSPC versus non-omHSPC. Although potentially "curative" therapy use was higher in omHSPC versus non-omHSPC, the percentages were still relatively low. Future studies are warranted given potential for prolonged responses with multimodal therapy inclusive of systemic and local therapies.

Machine learning predicts conventional imaging metastasis-free survival (MFS) for oligometastatic castration-sensitive prostate cancer (omCSPC) using prostate-specific membrane antigen (PSMA) PET radiomics

PURPOSE

This study investigated imaging biomarkers derived from PSMA-PET acquired pre- and post-metastasis-directed therapy (MDT) to predict 2-year metastasis-free survival (MFS), which provides valuable early response assessment to improve patient outcomes.

MATERIALS/METHODS

An international cohort of 117 oligometastatic castration-sensitive prostate cancer (omCSPC) patients, comprising 34 from John Hopkins Hospital (JHH) and 83 from Baskent University (BU), were treated with stereotactic ablative radiation therapy (SABR) MDT with both pre- and post-MDT PSMA-PET/CT scans acquired. PET radiomic features were analyzed from a CT-PET fusion defined gross tumor volume ((GTV) or zone 1), and a 5 mm expansion ring area outside the GTV (zone 2). A total of 1748 PET radiomic features were extracted from these zones. The six most significant features selected using the Chi2 method, along with five clinical parameters (age, Gleason score, number of total lesions, untreated lesions, and pre-MDT prostate-specific antigen (PSA)) were extracted as inputs to the models. Various machine learning models, including Random Forest, Decision Tree, Support Vector Machine, and Naïve Bayesian, were employed for 2-year MFS prediction and tested using leave-one-out and cross-institution validation.

RESULTS

Six radiomic features, including Total Energy, Entropy, and Standard Deviation from pre-PSMA-PET zone 1, Total Energy and Contrast from post-PSMA-PET zone 1, and Entropy from pre-PSMA-PET zone 2, along with five clinical parameters were selected for predicting 2-year MFS. In a leave-one-out test with all the patients, random forest achieved an accuracy of 80 % and an AUC of 0.82 in predicting 2-year MFS. In cross-institution validation, the model correctly predicted 2-year MFS events with an accuracy of 75 % and an AUC of 0.77 for patients from JHH, and an accuracy of 78 % and an AUC of 0.80 for BU patients, respectively.

CONCLUSION

Our study demonstrated the promise of using pre- and post-MDT PSMA-PET-based imaging biomarkers for MFS prediction for omCSPC patients.

Imaging and therapy in prostate cancer using prostate specific membrane antigen radioligands

Prostate specific membrane antigen (PSMA) directed PET imaging has rapidly transformed prostate cancer workup over the past decade and paved the way for a theranostic approach using 177Lu-labelled PSMA radioligand therapy (RLT). This review gives an overview of the underlying principles behind PSMA as a target; the current use of PSMA PET in prostate cancer imaging and benefits compared to conventional imaging; and therapeutic applications including optimisation of patient selection. It also explores the evidence base of PSMA PET for other indications not in routine clinical use and the future of PSMA-directed RLT.

Prostate-Specific Membrane Antigen PET Response Associates with Metastasis-Free Survival After Stereotactic Ablative Radiation in Oligometastatic Prostate Cancer

PURPOSE

Emerging data suggest that metastasis-directed therapy (MDT) improves outcomes in patients with oligometastatic castration-sensitive prostate cancer (omCSPC). Prostate-specific membrane antigen positron emission tomography (PSMA-PET) can detect occult metastatic disease, and PSMA response has been proposed as a biomarker for treatment response. Herein, we identify and validate a PSMA-PET biomarker for metastasis-free survival (MFS) following MDT in omCSPC.

METHODS AND MATERIALS

We performed an international multi-institutional retrospective study of patients with omCSPC, defined as ≤3 lesions, treated with metastasis-directed stereotactic ablative radiation who underwent PSMA-PET/computed tomography (CT) before and after (median, 6.2 months; range, 2.4-10.9 months) treatment. Pre- and post-MDT PSMA-PET/CT maximum standardized uptake value (SUVmax) was measured for all lesions, and PSMA response was defined as the percent change in SUVmax of the least responsive lesion. PSMA response was both evaluated as a continuous variable and dichotomized into PSMA responders, with a complete/partial response (at least a 30% reduction in SUVmax), and PSMA nonresponders, with stable/progressive disease (less than a 30% reduction in SUVmax). PSMA response was correlated with conventional imaging-defined metastasis-free survival (MFS) via Kaplan-Meier and Cox regression analysis.

RESULTS

A total of 131 patients with 261 treated metastases were included in the analysis, with a median follow-up of 29 months (IQR, 18.5-41.3 months). After stereotactic ablative radiation, 70.2% of patients were classified as PSMA responders. Multivariable analysis demonstrated that PSMA response as a continuous variable was associated with a significantly worse MFS (hazard ratio = 1.003; 95% CI, 1.001-1.006; P = .016). Patients classified as PSMA responders were found to have a significantly improved median MFS of 39.9 versus 12 months (P = .001) compared with PSMA nonresponders. Our study is limited as it is a retrospective review of a heterogenous population.

CONCLUSIONS

After stereotactic ablative radiation, PSMA-PET response appears to be a radiographic biomarker that correlates with MFS in omCSPC. This approach holds promise for guiding clinical management of omCSPC and should be validated in a prospective setting.

Review of Prospective Trials Assessing the Role of Stereotactic Body Radiation Therapy for Metastasis-directed Treatment in Oligometastatic Genitourinary Cancers

CONTEXT

Emerging evidence supports the use of stereotactic body radiation therapy (SBRT) as metastatic-directed therapy (MDT) for oligometastatic genitourinary cancers; however, the prospective data to guide its application as an alternative standard of care remain limited.

OBJECTIVE

To review prospective trials that assess the role of SBRT for patients with genitourinary cancers within a modern framework of oligometastatic disease (OMD) and to highlight clinical scenarios where SBRT may offer a benefit to patients with metastatic cancer.

EVIDENCE ACQUISITION

We performed a critical review of PubMed and clinicaltrials.gov in April 2022 according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement, combined with expert input to identify prospective studies investigating the role of SBRT for oligometastatic prostate, renal, or bladder cancer.

EVIDENCE SYNTHESIS

The most commonly studied application of SBRT has been for metachronous oligorecurrent hormone-sensitive prostate cancer (HSPC). Further prospective study is needed to define the role of SBRT in delaying time to next therapy or inducing synergy with other systemic therapies.

CONCLUSIONS

SBRT has been associated with high rates of local control and minimal risk of toxicity with multiple trials assessing an MDT-alone approach for oligorecurrent HSPC. From a tumor-agnostic perspective, the clinical benefit of SBRT for OMD has been associated with the ability to extend overall survival. As methods of cancer detection and treatment evolve, expansion of studies that prospectively evaluate SBRT MDT, stratifying by tumor histology and oligometastatic state, is needed to inform optimal patient selection and treatment strategy.

PATIENT SUMMARY

We review outcomes from prospective trials assessing the role of stereotactic body radiation therapy (SBRT) for oligometastatic genitourinary cancers, which have predominantly investigated SBRT for oligorecurrent prostate cancer. Much work remains to define how SBRT alone compares with other standard of care treatments for prostate cancer or the role of SBRT in tumor control or delaying time to next therapy in oligometastatic renal and bladder cancer.

Oligometastatic Prostate Cancer: Current Status and Future Challenges

In accordance with the spectrum theory of metastatic disease, an oligometastatic clinical state has been proposed as an intermediary step along the natural history of cancer with few (typically 1-3) metastatic lesions identifiable on imaging that may be amenable to metastasis-directed therapy. Effective therapy of oligometastatic disease is anticipated to impact cancer evolution by delaying progression and improving patient outcome at a minimal or acceptable cost of toxicity. There has been increasing recognition of oligometastatic disease in prostate cancer with the advent of new-generation imaging agents, most notably the recently approved PET radiotracers based on targeting prostate-specific membrane antigen. Early clinical trials with metastasis-directed therapy of oligometastases have provided evidence for delaying the employment of systematic therapy and improving outcome in selected patients. Despite these encouraging results, much needs to be investigated and learned about the underlying biology of the oligometastatic state along the evolutionary clinical course of prostate cancer, the identification of relevant imaging and nonimaging predictive and prognostic biomarkers, and the development of treatment strategies to optimize short-term and long-term patient outcome. We provide a review of the current status and the lingering challenges of this rapidly evolving clinical space in prostate cancer.

Defining Oligometastatic Disease in the New Era of PSMA-PET Imaging for Primary Staging of Prostate Cancer

Oligometastatic prostate cancer has traditionally been defined in the literature as a limited number of metastatic lesions (either to soft tissue or bone), typically based on findings seen on CT, MRI, and skeletal scintigraphy. Although definitions have varied among research studies, many important clinical trials have documented effective treatments and prognostication in patients with oligometastatic prostate cancer. In current clinical practice, prostate-specific membrane antigen (PSMA)-PET/CT is increasingly utilized for the initial staging of high-risk patients and, in many cases, detecting metastases that would have otherwise been undetected with conventional staging imaging. Thus, patients with presumed localized and/or oligometastatic prostate cancer undergo stage migration based on more novel molecular imaging. As a result, it is challenging to apply the data from the era before widespread PET utilization to current clinical practice and to relate current trials using PSMA-PET/CT for disease detection to older studies using conventional staging imaging alone. This manuscript aims to review the definition of oligometastatic prostate cancer, summarize important studies utilizing both PSMA-PET/CT and conventional anatomic imaging, discuss the concept of stage migration, and discuss current problems and challenges with the current definition of oligometastatic disease.