In the current study, we present the first-in-man usage of intraoperative 68Ga-PSMA CLI in primary PCa. The results from the first patients enabled optimisation of the imaging protocol, and demonstrated the technical ability to correctly identify PSMs. The use of optical filters improves the ability to visually distinguish between PSM and NSM on CLI, a finding that has not been reported before. Importantly, the performed workflow was considered clinically feasible and safe for surgical staff in terms of radiation exposure levels.
These initial results show that by using an administered activity of ≥ 65 MBq 68Ga-PSMA, a 150-s acquisition time with 8 × 8 pixel binning, and specimen imaging with and without 550 nm shortpass filter, a PSM can be successfully detected. Compared with the published 18F-FDG CLI results in breast cancer, the current protocol uses a 3 times lower activity and a 2 times shorter acquisition time which is possible due to the higher tracer uptake and Cerenkov yield of 68Ga-PSMA [18]. Despite the longer duration of prostate cancer surgery, the lower injected activity and shorter half-life resulted in a roughly 2 times lower staff exposure. The radiation exposure to personnel, a maximum of 0.016 mSv per procedure, was within the International Commission on Radiological Protection limits [27]. Based on these values, a single scrub nurse can perform a minimum of 62 CLI procedures before exceeding the limits; a surgeon can perform 200 CLI procedures. All 6 sides of the prostate could be assessed with the CLI within approximately 20 min, which is well within the time window acceptable for intraoperative use. Although CLI may still delay surgery to some extent, the procedure is approximately twice as fast than a typical NeuroSAFE procedure that is only performed on the posterolateral surface of the prostate. Furthermore, CLI enables whole-specimen assessment in the operating theatre without the need for a dedicated pathology department, contrarily to NeuroSAFE. Next, when a lymph node dissection is performed as well, the surgery can continue with the dissection while CLI imaging of the prostate occurs.
In 3 out of 5 patients, the histopathological PSMs and NSMs were correctly detected on CLI. In two patients, a hotspot on the Cerenkov image from the intact prostate was obtained from a tumour deposit at < 0.1 mm from the inked surface. These false-positive results can be explained by the physical properties of 68Ga. The CLI signal is not produced in one spatial location, but instead along the positron trajectory in tissue (± 2.8 mm on average for 68Ga, which is the same as the PET range) [28]. Although the use of an optical filter restricts the detected signal to enable visualisation of activity on the surface [29], it remains hard to quantify from what depth this activity actually originates. As identified in this first-in-man study, depth estimation is a key challenge of CLI and an important limitation compared with NeuroSAFE. The spatial accuracy that is achieved with histopathological evaluation is far greater than will ever be achieved using CLI. This aspect is further accomplished by the fact that it is difficult to estimate the real amount of activity present in the prostate after administration based on the Cerenkov images alone. In the future, the amount of activity present in the excised prostate specimen could be measured using dose calibrators.
A quantitative approach could help to discriminate between benign and tumour based on a threshold value, rather than on visual interpretation alone, since visual assessment is inherently prone to subjective window-level settings, and may result in erroneous identification of hotspots. In this study, each prostate was cleaved to enable visualisation and quantification of benign and cancerous tissue in the apex. This resulted in a mean TBR > 2, showing that the technology is able to sufficiently differentiate between tumour and benign tissue. Still, the process of cleaving is not preferred in the eventual work up, as it is time intensive and requires additional training. Our ongoing study will explore the possibility to define a TBR based on preoperative PET/CT-scans to recognize and to quantify potential areas that have a higher risk for PSMs on Cerenkov images based on radiance values. However, in the current feasibility study, the numbers were too small to make quantitative conclusions from the radiance levels. For quantification, the time between injection and imaging, administered activity, can be used to normalise the data. Additional variables that influence the CLI signal that are more challenging to account for are the PSMA expression of the tumour, and the fact that CLI uses optical imaging which means that scattering and attenuation from superficial tissue and blood can alter the Cerenkov signal intensity.
Though the current histopathological definition of a PSM is 0 mm (i.e. tumour on ink) [24], it is suggested that CLI could still provide valuable information on margin status, not only to the surgeon but also to inform the pathologist about areas at risk to guide more detailed histopathological evaluation. Another possibility is to guide the NeuroSAFE procedure itself by indicating suspicious areas, so that only those areas could be sampled for NeuroSAFE assessment, thereby reducing the risk of sampling errors and the duration of the NeuroSAFE procedure.
The current feasibility study has several limitations. Firstly, this feasibility study only addresses a small selective population. The patients were selected based on a high chance of a PSM, thus not representative for the general population that will undergo RARP. Still, the included patients are the ones that could benefit the most from intraoperative margin assessment [29]. Secondly, the administered activity varied between the patients. The 68Ga-PSMA was produced locally at specific time slots each day due to the fact that this is a generator product. Thus, the actual time of surgery needs to correspond to the 68Ga-PSMA production time. Batches for multiple patients are retrieved from one production; therefore, once the syringe is prepared for the study patient, and surgery is delayed, the activity cannot be adjusted accordingly. This is further complicated by the relatively short half-life of 68Ga (68 min). To account for lower 68Ga-PSMA activity levels due to delays in the start of surgery, the exposure time could be increased, thereby increasing TBR and ultimately aiding the visual identification of PSM on CLI. Still, variation in activity present in the prostate is normal considering a different PSMA expression between patients, and even within one prostatic lesion. This could be correlated to the SUV from preoperative PET/CT scans.