Foot skin depots of ¹⁸F-fluorodeoxyglucose do not enable PET/CT lymphography of the lower extremity lymphatic system in man

Background

Sentinel lymph node biopsy plays a key role in the staging of malignant melanoma [1] and breast cancer [2]. Conventionally, sentinel lymph nodes are localized by γ-camera imaging after injection of ^99mTc-labeled colloids. During surgery, γ-probe counting and peritumoral blue dye injections guide sentinel lymph node resection [3]. However, these techniques have received critique for being time-consuming and having poor spatial resolution [4].

In a recent murine study, it was shown that intradermal injection of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) into the tail skin enables detailed imaging of the lymphatic system and quantification of lymph node function by means of combined positron-emission tomography/computed tomography (PET/CT) [4]. In that study, it was suggested that these findings might apply to man. If so, it may improve the sentinel node technique through increased spatial resolution, shorter acquisition times, and improved anatomical localization.

However, in man, a likely path for the removal of intradermal depots of ¹⁸F-FDG is through diffusion to systemic capillaries. Activity in the blood from local veins will increase faster and to a larger extent compared to that in the blood from distant veins, and PET/CT imaging will show activity in the local veins. Contrarily, if ¹⁸F-FDG is drained primarily via the lymphatic system, a delay is expected before ¹⁸F-FDG activity increases in venous blood due to the transit time through the lymphatic system, and PET/CT imaging is expected to visualize lymphatic collectors and lymph nodes.

The aim of the present study was to elucidate whether foot skin depots of ¹⁸F-FDG allow for PET/CT lymphography of the lower extremities in man.

Ethical approval

The Science Ethics Committees for the Capital Region of Denmark approved the study (protocol number: H-2-2012-162). Written informed consent was obtained from all subjects.

Subjects

Four subjects, three men and one woman, aged 33 to 62 years, participated as healthy volunteers. None of the subjects suffered from any known disease or took any medications. Small lower leg superficial venous ectasias were allowed in the female subject.

¹⁸F-FDG-blood activity

Each subject was placed comfortably in supine position on the scanner table. The great saphenous veins were catheterized about 5 cm proximal to the ankle joints, and a medial cubital vein was catheterized as well (20 gauge, BD Venflon™ Pro, Becton Dickinson Infusion Therapy AB, Helsingborg, Sweden).

Depots, each consisting of approximately 5-MBq ¹⁸F-FDG in 0.1-mL isotonic saline, were injected into the skin of the first toe interstitium on the dorsum of the feet using a 1-mL syringe and a 27-gauge needle. On one foot, the depot was injected subcutaneously and on the other intradermally. Intradermal injection was verified by the elevation of a wheal at the injection site. Careful aspiration before subcutaneous injection avoided accidental intravenous injection. Different depot placements were used to test whether this affects the drainage route (veins vs. lymphatics).

Blood samples (2 mL/sample) were drawn simultaneously from each catheter before depot injection and once every minute for 15 min thereafter. Samples were transferred to pre-weighed plastic tubes, and sample activity was counted in a well counter with decay correction (Wallac 1480 Wizard® 3”, PerkinElmer, Waltham, MA, USA). Finally, the samples were weighed (precision 1/100 g), and measured sample activity was corrected for sample weight.

One subject deviated slightly from the described protocol on the following points: (1) catheterization of the great saphenous veins only succeeded unilaterally (intradermal depot); (2) depot activities were approximately 1 MBq/depot.

¹⁸F-FDG PET/CT imaging

Immediately following the last blood sample, approximately 20 min after depot injection, without moving the subject on the scanner table, the PET/CT acquisition was started (Gemini TF®, Philips Healthcare, Best, The Netherlands). A low-dose CT (20 mAs/slice) was performed from the ankles to the anterior superior iliac spine and followed by two identical PET scans (1 min/bed position, axial coverage 18 cm, 50% overlap) of the same region. Protocol duration was approximately 30 min. PET images were reconstructed with the standard vendor-supplied ‘WholeBody’ protocol including corrections for decay, scatter, and attenuation and utilizing time-of-flight information.

Results

The results for each subject are shown in Figures 1, 2, 3, and 4. Fused PET/CT images revealed ¹⁸F-FDG accumulation corresponding to both superficial and deep veins of the lower extremities in all subjects. Venous drainage routes showed both intra- and interindividual variation. Drainage pattern was not linked to depot placement. Neither lymphatic collectors nor lymph nodes were visualized in any subject.

Time-activity curves showed that activity in great saphenous vein blood increased faster and to a greater extent compared to medial cubital vein blood in all subjects. Great saphenous vein blood activity exhibited similar patterns for intradermal and subcutaneous depots.

Discussion

In mice, ¹⁸F-FDG positron lymphography provides detailed visualization of lymphatic collectors and lymph nodes as well as quantification of lymph node function after intradermal injection into the tail skin. However, we show that this finding cannot be extrapolated to humans as both intradermal and subcutaneous depots of ¹⁸F-FDG cause significant and immediate activity accumulation in local venous blood. Both fused PET/CT imaging and time-activity curves showed this.

In man, transcapillary transport of glucose is governed by diffusion through the interendothelial junctional space [5]. A skin depot of ¹⁸F-FDG gives rise to a large local concentration gradient between the interstitium and capillary blood facilitating passive washout of the depot to the systemic capillaries. Our results underline that important physiological differences exist in ¹⁸F-FDG transport between mouse tail skin and human lower extremity skin. One mechanism, which can explain the species difference, is that the mouse tail skin is more tightly bound than the skin on the human foot. This results in a higher hydrostatic pressure in the ¹⁸F-FDG depot in the mouse tail. It is well described that a high interstitial pressure promotes lymphatic drainage [6]. Another possible mechanism for lymph node uptake of ¹⁸F-FDG in the study by Thorek et al. is an inflammatory reaction to the co-injection of isosulfan blue with ¹⁸F-FDG. Although not common, it is well described that isosulfan blue injections in sentinel lymph node identification can cause skin reactions [7].

In lymphoscintigraphy, ^99mTc is coupled to larger molecules such as human colloidal albumin to avoid capillary washout and thus facilitate drainage by the lymphatic system [8]. We suggest that a similar principle needs to be applied in PET/CT lymphography in order to make the method feasible in man.

Conclusion

Lower extremity PET/CT lymphography using ¹⁸F-FDG skin depots in the feet is not possible in man due to significant tracer washout to systemic capillaries.