SPECT/CT bests planar imaging for prostate cancer

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Because the lymph drainage of tumors draining deeply in the abdomen, such as prostate carcinoma, is often complex, preoperative localization is mandatory. According to research published in the June issue of the Journal of Nuclear Medicine, SPECT/CT provides both superior detection and anatomic localization of sentinel nodes in prostate cancer.

"In our center, laparoscopic sentinel node lymphadenectomy is preferred because it is minimally invasive and has proven to be a reliable diagnostic tool," wrote the authors of the study, which was conducted in the departments of nuclear medicine and urology at the Netherlands Cancer Institute, Antoni van Leeuwenhoek Hospital in Amsterdam, The Netherlands.

However, they noted that although laparoscopic evaluation of sentinel nodes is useful for staging prostate cancer, preoperative localization of deep abdominal sentinel nodes with planar lymphoscintigraphy is difficult.

The extended pelvic lymphadenectomy area concerns the regions between the external iliac arteries, the obturator fossa, the pelvic wall, the common iliac arteries up to the ureteric crossing, and the internal iliac arteries just caudal of the superior vesical artery.

SPECT shows localization (C) of two sentinel nodes. Those nodes are localized with SPECT/CT, which clearly shows (D) one sentinel node in left presacral area and another in right iliac area. Image and caption courtesy of SNM.

"Because this modality (planar lymphoscintigraphy) is able to provide only two-dimensional information, exact anatomic localization is usually impossible," the authors observed.

The combination of SPECT and CT in a single device allows the nodal tracer uptake detected by SPECT to be fused with CT data. In turn, this provides a surgeon with better information on the anatomic location of the sentinel node.

The scientists evaluated the use of SPECT/CT in 46 patients with prostate cancer of intermediate prognosis.

The patients elected to be treated with external radiotherapy in the Netherlands Cancer Institute between June 2006 and September 2008, and a sentinel node lymphadenectomy took place before the radiotherapy. Prior to lymphadenectomy, the patients had routine planar imaging as well as a SPECT/CT exam performed.

Preoperative planar imaging and SPECT/CT were performed after injection of 99mTc-nanocolloid, both peri- and intra-tumorally. Planar imaging was performed 15 minutes after the injection sequence was complete and again at two hours. SPECT/CT (Symbia T, Siemens Healthcare) was conducted after the planar imaging was done, and image fusion was performed on an Osirix DICOM viewer.

Images were analyzed using 2D orthogonal reslicing in axial, sagittal and coronal directions. A 3D presentation, using volume rendering, was generated to localize sentinel nodes in relation to anatomic structures, the researchers reported.

The team reported that visualization rates for planar imaging were 91 percent and SPECT/CT was 98 percent. Planar imaging visualized a mean of 2.2 sentinel nodes per patient, while SPECT/CT visualized a mean of 4.3 sentinel nodes per patient.

"Sixteen patients (35 percent) had sentinel nodes outside the extended pelvic lymphadenectomy area (inguinal, near the aortoiliac junction, presacral, or near the umbilical ligament in the abdominal wall)," the authors reported. "In nine patients (56 percent), these sentinel nodes were detected only on SPECT/CT and would have been be missed if only planar imaging had been performed."

Overall, the utilization of SPECT/CT revealed additional sentinel nodes in 63 percent of all the prostate cancer patients, leading to excision of more sentinel nodes and thus complete staging. The researchers noted that the relevance of excising those nodes was demonstrated by the fact that in almost half of all tumor-positive patients in the study, a sentinel node that was missed on planar images showed tumor positivity.

"Sequential planar imaging will remain important for preoperatively identifying early-appearing lymph nodes as sentinel nodes," the authors observed. However, "because the introduction of SPECT/CT in sentinel node procedures for prostate cancer has markedly improved sentinel node detection and localization, we advise the routine use of this modality in malignancies with deep intraabdominal drainage."