Letter to the editor to comment on “Diagnostic accuracy of 99mTc-HDP SPECT/CT and MRI for foot and ankle osteoarthritis (ProSPECT-trial): Study protocol”

To the Editor,

We read with interest the study protocol by van Hasselt et al., outlining the ProSPECT trial, which aims to compare the diagnostic accuracy of 99mTc-HDP SPECT/CT and MRI in evaluating symptomatic foot and ankle osteoarthritis (OA) . This study addresses an important clinical question, given the diagnostic limitations of conventional radiographs in these anatomically complex regions. However, several critical aspects of the study design and rationale merit closer examination.

The authors posit that SPECT/CT may improve the identification of symptomatic joints by correlating tracer uptake with pain generators. While plausible, this assumption is not robustly supported by prior evidence , and increased uptake may simply reflect mechanical loading or bone turnover rather than pain-specific pathology . This introduces potential diagnostic ambiguity, especially when no standardized metabolic threshold defines clinical significance.

A central concern lies in the choice of reference standard . Using pain relief following intra-articular injection as a surrogate for true joint pathology introduces subjectivity and potential misclassification . Pain is multifactorial, may stem from adjacent structures, and does not always localize accurately to imaging findings. As a result, false positives or negatives may affect both sensitivity and specificity estimates.

The study’s use of FOAMRIS to score only bony MRI features —excluding soft tissue structures such as synovitis, tenosynovitis, or capsulitis—may underrepresent MRI’s full diagnostic value . Given that soft tissue changes often drive symptoms in OA, this omission risks biasing results in favor of SPECT/CT, which is inherently bone-focused ^, .

Additionally, the radiation exposure from SPECT/CT (∼ 3.2 mSv) , while modest, must be weighed against MRI’s radiation-free advantage —a particularly relevant point in chronic disease surveillance and younger populations. The authors acknowledge this but stop short of discussing implications for long-term clinical adoption.

While the sample size of 369 is adequate, the single-center design may limit external validity. Furthermore, the protocol does not detail how diagnostic discordance between modalities will be managed or whether imaging findings will influence clinical decisions—a crucial omission for real-world applicability.

In summary, while the ProSPECT trial is timely and ambitious, methodological refinements—especially in defining diagnostic standards and capturing the full spectrum of pathology—will be essential to ensure its findings translate into clinical impact.