Purpose: Gastrointestinal (GI) symptoms pose a significant burden to patients receiving chemoradiation therapy (CRT) for anal cancer; however, the impact of symptoms from the patient perspective has not been quantified. This retrospective study examined and compared patient and clinician reports of acute GI toxicity during CRT.Materials and Methods: Patients treated with definitive RT using intensity-modulated radiation therapy for anal cancer between 9/09 and 11/12 were reviewed. Median RT dose was 56 Gy (range 45-56), and 76 patients (97%) received concurrent 5-fluorouracil-based chemotherapy. During RT, patients completed the 7-item Bowel Problem Scale (BPS) weekly. Clinicians assessed toxicity separately using CTCAE v. 3.0. Scores of BPS ≥ 3 and CTCAE ≥ 1 were considered to be clinically meaningful. Agreement of the two assessments was evaluated by Cohen's kappa coefficient.Results: Seventy-eight patients completed at least one BPS and had a corresponding clinician assessment. Patients reporting scores of ≥3 was highest at week 5 (n = 68) for diarrhea (44.1%), proctitis (57.4%), and mucus (48.4%), while urgency (47.6%), tenesmus (31.7%), and cramping (27%) were highest at week 4 (n = 63). Baseline bleeding scores (26.7%; score ≥3) improved during treatment (13.4% at week 5). "Poor" agreement was observed between patient- and clinician-reported proctitis (Cohen's k = 0.11; n = 58); however, there was "good" agreement for diarrhea (Cohen's k = 0.68; n = 58).Conclusions: Acute GI toxicity during definitive CRT for anal cancer was most significant during weeks 4-5, while rectal bleeding improved during treatment. Discrepancies in patient- and clinician-reported symptoms demonstrate the potential for patient-reported outcomes to be useful tools for anal cancer clinical assessments. [ABSTRACT FROM AUTHOR]