Diagnostic performance of 18F-FDG PET/CT using point spread function reconstruction on initial staging of rectal cancer: a comparison study with conventional PET/CT and pelvic MRI

Abstract

Background: Accurate staging is crucial for treatment selection and prognosis prediction in patients with rectal cancer. Point spread function (PSF) reconstruction can improve spatial resolution and signal-to-noise ratio of PET imaging. The aim of this study was to evaluate the effectiveness of ¹⁸F-FDG PET/CT with PSF reconstruction for initial staging in rectal cancer compared with conventional PET/CT and pelvic MRI.

Methods: A total of 59 patients with rectal cancer underwent preoperative ¹⁸F-FDG PET/CT and pelvic MRI. The maximum standardized uptake value (SUVmax) and lesion to background (L/B) ratio of possible metastatic lymph nodes, and metabolic tumor volumes (MTVs) of primary tumors were calculated. For N and T (T1-2 vs T3-4) staging, sensitivities, specificities, positive predictive values, negative predictive values, and accuracies were compared between conventional PET/CT [reconstructed with ordered subset expectation maximization (OSEM)], PSF-PET/CT (reconstructed with OSEM+PSF), and pelvic MRI. Histopathologic analysis was the reference standard.

Results: For N staging, PSF-PET/CT provided higher sensitivity (78.6%) than conventional PET/CT (64.3%), and pelvic MRI (57.1%), and all techniques showed high specificity (PSF-PET: 95.4%, conventional PET: 96.7%, pelvic MRI: 93.5%). SUVmax and L/B ratio were significantly higher in PSF-PET/CT than conventional-PET/CT (p < 0.001). The accuracy for T staging in PSF-PET/CT (69.4%) was not significantly different to conventional PET/CT (73.5%) and pelvic MRI (73.5%). MTVs of PSF and conventional PET showed a significant difference among T stages (p < 0.001), with higher values in advanced stages. In M staging, both PSF and conventional PET/CT diagnosed all distant metastases correctly.

Conclusions: PSF-PET/CT produced images with higher lesion-to-background contrast than conventional PET/CT, which allowed improved detection of lymph node metastasis without compromising specificity, and showed comparable diagnostic value to MRI in local staging. PSF-PET/CT is likely to have a great value for initial staging in rectal cancer.

Keywords: 18F-FDG pet/ct; Pelvic MRI; Point spread function (PSF); Rectal cancer; Staging.

Fig. 1

Relationship between quantitative values obtained from conventional PET and point spread function (PSF) -PET, evaluated using linear regression analysis for maximum standardized uptake value (SUVmax) (a), and lesion to background (L/B) ratio (b)

Fig. 2

A 83-year-old woman with rectal cancer. The rectal cancer (arrowhead) can be seen from ¹⁸F-FDG uptake on both PSF-PET/CT (a) and conventional PET/CT (b) [images are scaled to the same maximum value]. A obturator lymph node (arrow) showed as ¹⁸F-FDG avid compared to the surrounding tissue on PSF-PET/CT, and therefore considered a positive result. This lymph node was obscure on conventional PET/CT, and thus regarded as negative. This lymph node was 5 mm in diameter, and did not show mixed signal intensity nor irregular contour on the high resolution T2-weighted image (c), and therefore considered negative on MRI also. Subsequently, this lymph node was pathologically confirmed as containing metastasis

Fig. 3

Mean metabolic tumor volumes (MTVs) of each T stage for point spread function (PSF)-PET (a), and for conventional PET (b). Error bars show 95% confidence intervals. Mean MTVs of PSF and conventional PET were 3.0 ± 4.4, 3.2 ± 4.8 for T1, 12.1 ± 10.7, 11.9 ± 7.6 for T2, 32.9 ± 37.6, 31.1 ± 36.1 for T3, and 87.6 ± 124.4, 89.8 ± 125.8 for T4, respectively. Receiver operating characteristic curves (ROC) analysis (c) for discriminating T3-4 stage using MTVs of PSF and conventional PET