Can the radiation dose to CT-enlarged but FDG-PET-negative inguinal lymph nodes in anal cancer be reduced?

PURPOSE: To investigate whether a dose reduction to CT-enlarged but FDG-PET-negative (([(18)F]-fluoro-2-deoxy-D-glucose positron emission tomography) inguinal lymph nodes in radiochemotherapy of anal cancer is safe. PATIENTS AND METHODS: 39 sequential patients with anal cancer (mean age 59 years [range: 37-86 years], median follow-up 26 months [range: 3-51 months]) receiving pretherapeutic FDG-PET were included. All patients were treated with combined radiochemotherapy including elective radiation of the inguinal lymph nodes with 36 Gy. In case of involvement (FDG-PET positivity defined as normalized SUV [standard uptake value] above Delta > 2.5 higher than blood pool), radiation dose was increased up to 50-54 Gy. Planning CT and PET results were compared for detectability and localization of lymph nodes. In addition, local control and freedom from metastases were analyzed regarding the lymph node status as determined by FDG-PET. RESULTS: In the planning CTs, a total of 162 inguinal lymph nodes were detected with 16 in nine patients being suspicious. Only three of these lymph nodes in three patients were PET-positive receiving 50.4-54 Gy, whereas all other patients only received elective inguinal nodal irradiation. No recurrence in inguinal lymph nodes occurred, especially not in patients with CT-enlarged inguinal lymph nodes and elective irradiation only. Patients with PET-positive nodal disease had a higher risk of developing distant metastases (p = 0.045). CONCLUSION: Reduction of the irradiation dose to CT-enlarged but PET-negative inguinal lymph nodes in anal cancer seems not to result in increased failure rates.