Assessment of intraoperative gamma probe performance for sentinel lymph node mapping using a PMMA phantom

This study aimed to characterize the physical performance of an intraoperative gamma probe used in nuclear medicine services for radioguided sentinel lymph node surgery. A polymethyl methacrylate (PMMA) phantom representing the axillary region was developed based on the NEMA NU 3-2004 recommendations to simulate a clinical sentinel lymph node mapping scenario in breast cancer. Experimental tests included sensitivity in air, sensitivity in a scattering medium, evaluation of lateral shielding influence in air, spatial resolution in a scattering medium, and angular dependence at different depths. The probe was a CdZnTe semiconductor detector operated without an external collimator, and all measurements were performed using 99 ᵐTc. The results allowed identification of detector saturation regions, assessment of the influence of source activity on detector response, and evaluation of angular dependence under clinically relevant conditions. Measurement uncertainty was treated as part of the detector characterization. Repeatability was represented by Type A standard uncertainties from repeated count-rate measurements, whereas source-activity correction included Type B contributions associated with radioactive decay correction. For spatial resolution, the nominal FWHM values obtained from the response profiles were preserved and reported with approximate standard uncertainties estimated by first-order propagation of Type A count-rate uncertainties through the linear interpolation used to determine the half-maximum crossing positions. The study identified detector operating conditions associated with quasi-linear response, saturation, loss of spatial resolution with depth, and angular dependence under clinically relevant sentinel lymph node mapping conditions. Overall, detector response was strongly influenced by source activity, source-to-detector distance, source depth, lateral positioning, probe orientation, and collimator absence. The air-sensitivity results were interpreted in relation to the expected inverse-square behavior by evaluating the distance-normalized sensitivity S(D)D 2 , while linear fits were retained only as local empirical descriptors of selected operating regions. The proposed methodology provides a reproducible experimental framework for characterizing gamma probes and may support optimization of clinical protocols for radioguided surgery.