Zeroing In
By Keith Loria
Radiology Today
Vol. 26 No. 7 P. 14

PET Radiotracer Could Be a Solution for Detecting Mold Infections

A recent study conducted by researchers at Johns Hopkins Children’s Center in Baltimore has demonstrated that advanced, noninvasive imaging techniques can effectively detect mold infections in humans. Funded by the National Institutes of Health, the study highlights that invasive mold infections are a significant cause of mortality among immunosuppressed individuals, including cancer patients, with death rates reaching as high as 85%.

The fungus Aspergillus, a common mold, poses a serious health threat to those with weakened immune systems, leading to infections that are increasingly prevalent. Early detection of these invasive mold infections is essential for improving survival rates, but diagnosis remains difficult and often necessitates invasive procedures such as biopsies.

“When infections occur in relatively inaccessible sites, minimally invasive clinical samples (blood, urine, stool, or cerebrospinal fluid) can often yield nondiagnostic results,” says Sanjay Jain, MD, director of the Center for Infection and Inflammation Imaging Research at Johns Hopkins Children’s Center. “Although major strides have been made in the development of new, clinically available biomarkers, namely galactomannan and 1,3-beta-D-glucan, they have limited sensitivity and specificity for invasive mold infections, and galactomannan is unable to reliably detect non-Aspergillus molds.”

Specifically, the overall sensitivity of serum/plasma galactomannan is ~50% to 60%. While galactomannan from the bronchoalveolar lavage (BAL) has a higher sensitivity (75% to 86%), BAL is an invasive process and cannot be performed in many cancer patients at risk for bleeding complications.

“Similarly, all clinically available imaging tools such as radiography, ultrasonography, CT, and MRI are nonspecific and reflect a combination of the infection and the host inflammatory response,” Jain says. “Currently available imaging tools rely on host responses to infection that could be significantly altered in an immunocompromised host, and none can reliably differentiate infectious sites from sterile inflammatory or cancerous lesions. Therefore, definitive diagnosis of invasive mold infections almost always requires invasive procedures such as BAL or deep tissue biopsy to obtain a relevant clinical sample, which is challenging, dangerous, and delays the diagnosis.”

Difficult to Diagnose
Jain and his team suggest that the use of 18F-fluorodeoxysorbitol (18F-FDS), a PET radiotracer, offers a promising solution. This innovative agent can be utilized to diagnose and monitor mold infections noninvasively, potentially transforming how these infections are detected and managed. For this study, Jain conducted the first-in-human studies using 18F-FDS PET scans to identify these infections.

“Our technology overcomes these issues and provides a rapid—same day— noninvasive method to detect, localize, and monitor invasive mold infection with high sensitivity and specificity,” he says.

Another advantage of using 18F-FDS PET is that it detects and localizes infection anywhere in the body with the same scan, which is important because a brain infection would be treated differently than a lung infection. It’s also highly sensitive and specific and detects a wide range of clinically relevant invasive mold infections, including antimicrobial resistant molds.

Invasive mold infections are an increasing global public health concern, affecting multiple organ systems (eg, lung, brain), and associated with mortality rates as high as 85%. “The rising incidence of invasive mold infections is attributed to the expanding population of immunosuppressed patients,” Jain says.

Aspergillus spp. remains the major cause of invasive mold infections in hematologic cancer patients and transplant recipients. However, other opportunistic molds, which are even more challenging to diagnose and treat, such as mucormycosis, are on the rise due to several reasons, including widespread Aspergillustargeted antifungal prophylaxis. In 2022, the World Health Organization categorized Aspergillus and Mucorales molds as “critical” and “high” priority group pathogens, respectively.

“Prompt identification of invasive mold infections and initiation of appropriate treatment is critical for improving patient outcomes,” Jain says.

Study Results
In the Johns Hopkins study, nine patients were prospectively enrolled—four with confirmed invasive mold infections (per European Organization for Research and Treatment of Cancer/Mycoses Study Group Education and Research Consortium criteria) and five with other pathologies serving as controls. Confirmatory diagnosis in the infected group was established through culture or molecular testing. The control group included patients with sterile inflammatory conditions or other diseases, such as breast cancer diagnosed via tissue biopsy. The median age was 59 years, with 44% female, and the cohort reflected the typical hospitalized patient population with multiple comorbidities.

All participants underwent whole body 18F-FDS PET/CT scans, which were well tolerated and showed pharmacokinetics consistent with prior studies—rapid renal and hepatobiliary elimination with low background activity in unaffected brain and lung tissues. To assess the imaging results, regions of interest were drawn at infection sites and unaffected tissue to calculate target-tonontarget ratios (TNT). The PET scans revealed significantly higher signals at sites of invasive mold infections, with a median TNT of 5.79, compared with 1.40 in controls with sterile or cancerous pathologies (P=0.008). Notably, while 18F-FDG PET detected the breast cancer lesion, 18F-FDS PET showed no signal there, confirming specificity. Using a TNT cutoff of 3.0, 18F-FDS PET accurately identified and localized pulmonary and brain mold infections—including those caused by Aspergillus, non-Aspergillus molds, and azole-resistant Aspergillus calidoustus—despite ongoing antifungal treatment. All control lesions had TNT ratios below three, demonstrating the method’s specificity.

A significant discovery from the study is that mold infections in humans exhibit approximately 50 times greater biomass compared with similarly sized bacterial lesions. This finding has important implications for developing targeted imaging techniques to accurately diagnose these infections. It also sheds light on why other imaging methods that work well in animal models may not be as effective in human patients.

“This technology detects a wide range of clinically relevant invasive mold infections, including Aspergillus, non-Aspergillus (galactomannan-negative) molds, or antimicrobial resistant molds (eg, azole-resistant Aspergillus calidoustus),” Jain says. “In fact, we tested 30 randomly selected clinical isolates, including Aspergillus spp., Rhizopus spp., Mucor spp., and other clinically relevant molds (Lichtheimia corymbifera, Syncephalastrum racemosum, Cunninghamella bertholletiae, Cladophialophora bantiana [C. bantiana], and Fusarium solani), along with azole-resistant molds, which all demonstrated robust 18F-FDS uptake.”

Jain emphasizes that this advanced imaging technology addresses a critical gap in diagnosing invasive mold infections. “Since imaging is already a standard part of the diagnostic process for suspected cases, integrating 18F-FDS PET scans into current clinical workflows is straightforward,” he says. This advancement has the potential to significantly improve patient outcomes, as mold infections have a high mortality rate (85%), and early detection, enabled by 18F-FDS PET, would enable institution of effective treatments earlier, bringing mortality to <30%.

The study also unearthed some unexpected findings. Jain notes that in one case, 18F-FDS PET was able to localize known pulmonary invasive mold infection due to C. bantiana (a non-Aspergillus mold) in a patient with a negative galactomannan test. Here, 18F-FDS PET also identified a previously undiagnosed cerebral involvement with C. bantiana that was not detected on brain MRI performed for clinical reasons.

“C. bantiana has a high propensity for dissemination to the brain and is associated with high mortality due to delays in diagnosis (median of 115 days),” he says. “Early and accurate identification of cerebral infection has implications for treatment (the need for an antifungal agent with central nervous system penetration) and for prognostication.”

This example highlights many potential advantages of 18F-FDS PET, including its utility in diagnosing and localizing infections due to diverse clinically important molds not otherwise detected by currently available biomarkers and a level of sensitivity superior to the best currently available imaging tools.

Potential Uses
Many polymerase chain reaction-based tests and fungal cultures, which use clinical samples obtained via invasive procedures, provide species- or genus-level identification and can also detect mixed mold infections, though Jain notes these are reported rarely in immunocompromised patients. “Although 18F-FDS PET cannot distinguish aspergillosis from other mold infections (eg, mucormycosis), an approach combining 18F-FDS PET with galactomannan testing could aid in distinguishing between the two infections,” he says. “Further, due to the need for invasive procedures, many mucormycosis cases remain undiagnosed or the diagnoses are significantly delayed. Therefore, 18F-FDS PET could not only help in the early diagnosis but, when positive, also justify the need for an earlier invasive procedure as well as highlight the best sites to sample the infection, due to its high sensitivity and specificity.”

The technology can be used in human studies, Jain says, and hopes to see it utilized more in clinical studies to generate enough data for a future FDA-approval. “Importantly, 18F-FDS can be easily synthesized from commercially available 18FFDG, the most widely utilized PET tracer globally, by a simple one-step, kit-based method, without the need for specialized radiosynthesis and purification facilities, allowing on-demand synthesis and global availability of 18F-FDS,” he says.

“Rapid and accurate diagnosis of invasive mold infections is critical for early interventions and rational use of antifungals,” Jain says. “We therefore believe that our studies are a major advance and present a powerful, novel, noninvasive, clinical diagnostic tool to rapidly detect and localize invasive mold infections anywhere in the body. This technology fills a major gap in the diagnostic pathway for patients and will help to improve patient outcomes for a disease associated with high mortality.”

Looking ahead, Jain and the Johns Hopkins Children’s Center plan to conduct a larger clinical trial to validate the findings from the clinical study. As the technology evolves, there’s belief it could bring portable or faster versions for emergency and field use or integration with AI to aid diagnosis.

“There have been major advancements in PET technologies, eg, total-body PET scanners enable high-sensitivity imaging in a health care setting, and portable scanners are being developed that could be deployed in the field,” Jain says. “We also anticipate that AI-based technologies would be integrated into the analysis and interpretation of 18F-FDS PET scans, substantially improving workflows and accuracy.”

— Keith Loria is a freelance writer based in Oakton, Virginia. He is a frequent contributor to Radiology Today.