Image-guided Focal Brachytherapy Utilizing Combined 18F-DCFPyl PET/CT

Overview

The Principal Investigator's (PI) working hypothesis is that the PI can utilize the high predictive value of 18F-DCFPyl PSMA to identify clinically significant tumors in patients who will undergo brachytherapy, as well as areas which are uninvolved or contain only clinically insignificant disease. In the PI's clinical trial, the uninvolved regions (as defined by combined PET-MR-biopsy data) will not be targeted and receive only fall-off dose, which we have shown to be associated with reductions in toxicity.

Current conventional prostate cancer (PCa) imaging modalities (computed tomography, bone scan, magnetic resonance imaging, ultrasound) have limited accuracy in the initial staging and for determining prognosis of PCa. Prostate-specific membrane antigen (PSMA) is a cell surface antigen which is highly expressed in PCa and correlates with prognostic factors such as Gleason score. High PSMA expression in prostate tumor has been significantly associated with lethality of disease, allowing specific identification of tumors most in need of treatment. Combined PET and computed tomography (PET-CT) imaging using small molecules targeting PSMA-expressing cells have been developed and tested clinically, and have shown superiority when compared with conventional imaging. An added advantage of PET compared to MRI is the ability to identify both distant metastatic disease as well as intraprostatic disease with one imaging modality. PSMA-radiotracers have continued to evolve since their initial development, with successive improvements in imaging and diagnostic characteristics. One such second-generation PSMA-binding compound, 18F-DCFPyl, has been developed and characterized at our institution, and offers superior imaging qualities compared to prior PSMA-based radiotracers. In realization of the toxicity of current therapies, there is substantial interest throughout the urologic oncology community in utilizing focal therapy to mitigate such toxicities. The rationale for focal therapy is based upon the recognition that whole gland treatment is associated with unacceptable toxicity rates, while concurrently it is also realized that patient morbidity and mortality is due to the progression of major foci of high-grade disease, i.e. the index lesion. Planning studies have shown that focal brachytherapy is feasible and results in significant reductions of dose to critical structures. In a historic cohort of patients treated at Johns Hopkins, the investigators have demonstrated that a modest reduction in dose results in clinically meaningful reductions in urinary toxicity. Al-Qaiseh et al. found that focal plans resulted in \>50% reductions in dose to urethra and rectum. However, focal plans were highly sensitive to seed positioning errors, and focal targeting made seed positioning more critical. This highlights the key utility and importance of the investigators' iRUF system (integrated Registered Fluoroscopy and Ultrasound) in delivering focal therapy. The investigators have developed a system of true dynamic intraoperative dosimetry which utilizes fluoroscopy for seed cloud reconstruction and fusion to transrectal ultrasound imaging. The investigators previously confirmed this method in a pilot trial of 6 patients with encouraging results. Further refinement of the system was followed by a Phase II clinical trial of this integrated platform on a larger group of patients. The investigators confirmed the primary endpoint to compare intraoperative dosimetric predicted values using iRUF method vs standard ultrasound-based seed tracking. The iRUF Phase II cohort had statistically significant improvements in prostate coverage parameters, as well as lower rates of rectal doses exceeding prescribed tolerance limits when compared to a historical group of patients. Importantly, there was no trend toward higher prostate V200 doses, indicating that excellent coverage did not come at the expense of excessive dose within prostate. This study will test the combination of PSMA-imaging with iRUF dynamic dosimetry to treat prostate cancer with a focal approach.