Evaluation of surgical gamma probe systems for sentinel lymph node detection
(Independent source: Hermann Wengenmair, Jürgen Kopp, Peter Heidenreich, Central Clinic, Augsburg, Germany)
Introduction
Intraoperative localization of sentinel lymph nodes opens up new possibilities in the treatment of lymphogenous metastasizing tumors [1]. The accuracy of the diagnostic statement depends on the clear pre- and intraoperative identification of the sentinel lymph node (SLN) [2,3,4]. Experience and training of the user as well as the quality of the probe system are the decisive factors. The appropriate strategy of measurement has to meet the anatomical and clinical situation as well as the performance of the probe system. Some authors [7,8] have already defined some quality criteria for
surgical gamma probes which in our opinion have to be complemented. The steadily increasing number of probe systems requires simple, expressive and reproducible control methods [5,6]. In extension of the already presented quality control proposals the goal of this work is to:
* define standard regulations to evaluate quality parameters
* establish minimal requirements for surgical gamma probe systems
* compare various commercial surgical gamma probe systems
* discuss the value of these quality parameters in various clinical applications
Quality Criteria
To evaluate the quality of a surgical gamma probe system we measured or discussed the following parameters [Fig. 1]:
* Spatial selectivity ( radial sensitivity distribution at defined distances)
* Spatial resolution ( lateral sensitivity distribution)
* Maximum sensitivity
* Quality of shielding (maximum penetration outside the measurement field)
* Energy resolution and spectral discrimination according to Tc(99m)
* Display of signal
Radial Sensitivity Distribution
The sensitivity distribution is evaluated equidistant to the measurement area (frontal radiation entrance window) dependant on the polar angle. Variations of the distribution with the distance are mainly due to the relative position of radiation entrance window and detector crystal. The radial sensitivity distribution at 3cm (nearfield) and 30cm distance (farfield) describes the width of the measurement cone out of which radiation is detected. The full width at half maximum (FWHM) of the distribution function is a good quality criterion for the detectability of lymph nodes in presence of non target radiation (injection depot, background). With a broad measurement cone the background signal can exceed the target signal of the lymph node, which then cannot be detected. A small cone mainly reduces background maintaining a constant target signal. [Fig. 2]
Therefore with increased background in the target area (e.g. mamma carcinoma, prostate carcinoma) a smaller FWHM of radial sensitivity distribution is desired.
Spatial Resolution
The spatial resolution (lateral sensitivity distribution) can be determined if a
surgical gamma probe is scanned laterally above a Tc99m point source)
Spatial resolution depends on the distance between source and probe crystal. For comparison and simplification we measured at a minimal distance of 1cm to the front end of the probe which is inside the variation of true lymph node depth. To separate neighboring lymph nodes and perform an adequate exact localization the FWHM of the lateral sensitivity distribution should be better than the typical distance between neighboring lymph nodes or a typical node diameter in the preparation region. Therefore we recommend a spatial resolution better than 25 mm for lymph nodes in the axilla, inguinal and illiacal region. Increased requirements have to be set up for lymph nodes that are close together (e.g. in the head-, neck- and supraclavicular region). Probes or probe/collimator combinations for these applications should have a FWHM of less than 15mm.
Sensitivity
The sensitivity of the probe was determined directly at the tip of the probe or collimator. In general the necessary sensitivity depends on radionuclide uptake, measurement geometry and time between injection and SLNE. The maximum radionuclide uptake of the sentinel lymph node typically varies between 0.01% and 1% with a median at around 0.1%. The SLNE is mostly performed at the day after Tc99m-Nanocolloid injection. Then about 0.05% to 0.005% of the administered activity can be found in the lymph node intraoperatively. Assuming a typical activity application of 80 MBq a spot of activity between 4 and 40 kBq has to be localized. The sensitivity of the measurement system therefore should be better than 5 cps/kBq.
Shielding
Out of constructional reasons the shielding of a surgical gamma probe mostly has a weak area. A high background source (e.g. injection spot at mamma- or prostate-carcinoma) in the direction of such a leakage can lead to false orientation [Fig. 4]. The lymph node should produce a higher signal than any background source. Assuming an uptake of 0.1% for a lymph node the leak sensitivity should not exceed 0.1% of the system sensitivity.
Energy Resolution
With the presence of scatter medium and high background activity compton photons produce an additional blurring of the spatial information. An energy discrimination that separates compton- and photopeak-signal is therefore important.
Display
All kinds of display have to be adapted to the special situation in an operation cabinet. An acoustic display should enable the user to visually concentrate to the operation field during measurement. Therefore a clear correlation between the acoustic tone and the measurement signal has to be available. For the quantitative results either a digital or analogue display is necessary which has to be clearly readable from at least 2m distance. To cope with the statistical variation and to influence the inertia of the display measurement interval respectively time constant should be adjustable.
Results
Based on the described method of measurement and our clinical experience we set up minimal requirements to system for intraoperative localization
[Tab. 1].
Up to now we tested 43 probe/collimator combinations with 12 measurement systems from 7 manufacturers.
Table 2 summarizes the assessments for the display and measurement systems,
table 3 shows the results for the different
surgical gamma probe and collimators. The results indicate marking differences in performance. These characteristics of a system have to be integrated into the performed method of detecting the SLN and the user has to be trained in the optimal strategy of measurement for the distinct system. These results also contain the improvements that could be achieved by the efforts of various companies.
* Tab. 1: Minimal requirements for an intraoperative probe system (pdf)
* Tab. 2: Summary of control unit characteristics (pdf)
* Tab. 3: Summary of probe characteristics (pdf)
Presence and future
The success of the SLNE-technique in melanoma and mamma carcinoma opened up new fields of application like SLNE at prostate carcinoma and various other tumors, radio-guided surgery of parathyroid adenoma etc.. Measurement approaches had to be adopted to the clinical demands and anatomical situations.
Tab. 4: Intraoperative measurement conditions for different gamma-probe applications
The quality parameters we described are of course of different validity in the assessment of a suitable probe/collimator-combination.
Tab. 5: Evaluation of quality control parameters
The plus sign indicates the importance of the marked parameter in the evaluation of the performance of a system for a certain application.
Continuous improvements and new developments are necessary to meet the upcoming requests:
Laparoscopic probes should have movable tips to measure in variable angulation. The probe and the scope should be put together so that viewing and measuring would be simultaneously possible in the viewing channel.
Sandwich detectors and new detector material could discriminate between electrons and gamma-rays and therefore could indicate the distance to a small target structure even in the presence of a high background and could optimize the intraoperative measurement of PET-nuclides.
* Tab. 4: Intraoperative
measurement conditions for different gamma-probe applications (pdf)
* Tab. 5: Evaluation of quality
control parameters (pdf)
References
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