[¹⁸F]-FDG PET/CT in the Initial Staging of Squamous Cell Cancer of the Anal Canal: Results of a Prospective Multicenter Registry

In patients with squamous cell carcinoma of the anal canal (ACC), disease stage influences treatment plans and determines prognosis. Our purpose was to determine the impact of PET on the initial staging of patients with presumed stages II-IV ACC and to assess the association of disease stage per conventional workup (CW) and PET imaging to patient outcomes. <b>Methods:</b> In this multicenter registry, patients with CW stages II-IV ACC or equivocal findings for a specific stage were included. Demographic data and stage according to the American Joint Committee on Cancer (AJCC) version 7 as determined by CW and PET were recorded and compared with overall survival (OS). For patients from 1 of the participating institutions, CW and PET stage according to AJCC versions 7-9 were compared with progression-free survival (PFS) and OS. <b>Results:</b> There were 813 patients included. PET upstaged 150 of 531 patients (28.2%) and downstaged 84 of 531 patients (15.8%) and assigned a specific stage to 200 of 232 patients (86.2%) with equivocal findings on CW. Stage IV on PET was predictive of significantly poorer OS (<i>P</i> = 0.005). For the 136 patients with staging according to AJCC versions 7-9, CW stages I-IV per versions 7-9 were not predictive of OS (<i>P</i> = 0.684, 0.329, and 0.083, respectively) or PFS (<i>P</i> = 0.622, 0.606, and 0.115, respectively). However, PET stages I-IV per versions 7-9 were associated with OS (<i>P</i> = 0.037, 0.003, 0.003, respectively) and PFS (<i>P</i> = 0.004, <0.001, <0.001, respectively), with version 9 best discriminating PFS for stages II and III. <b>Conclusion:</b> In patients with presumed stages II-IV ACC, PET stage differs in up to 44% from CW. PET assigns a specific stage in most patients with equivocal staging on CW. The PET-derived stage was predictive of PFS and OS. Because of its superior prognostication, PET should be used routinely to stage patients with ACC clinical stage II or above.