Objective: To quantify incidence of, and risk factors for, progression to and spontaneous regression of high-grade anal squamous intraepithelial lesions (ASILs).
Design: Retrospective review of patients at St Vincent's Hospital Anal Cancer Screening Clinic during a period when high-grade ASILs were not routinely treated (2004–2011).
Methods: All patients who had an anal Papanicolaou smear or high-resolution anoscopy were included, except for patients with previous anal cancer. High-grade anal intraepithelial neoplasia (HGAIN) was defined as a composite of histologically confirmed grade 2 or 3 anal intraepithelial neoplasia (AIN2/3) and/or high-grade squamous intraepithelial lesion on anal cytology. Analyses were repeated restricting to histologically confirmed AIN3.
Results: There were 574 patients: median age 45 years (interquartile range, IQR 36–51), 99.3% male and 73.0% HIV-infected [median HIV duration was 13.8 years (IQR 6.4–19.8), median CD4+ T-lymphocyte count was 500 cells/μl (IQR 357–662), 83.5% had undetectable plasma HIV viral load]. Median follow-up was 1.1 years (IQR 0.26–2.76). Progression rate to HGAIN was 7.4/100 person-years (95% confidence interval, CI 4.73–11.63). No risk factor for progression to HGAIN was identified; progression to AIN3 was more likely with increasing age (Ptrend = 0.004) and in those who were HIV-infected [hazard ratio 2.8 (95% CI 1.18–6.68) versus HIV-uninfected; P = 0.019], particularly in those whose nadir CD4+ T-lymphocyte count was less than 200 cells/μl (Ptrend = 0.003). In 101 patients with HGAIN, 24 (23.8%) patients had spontaneous regression [rate 23.5/100 person-years (95% CI 15.73–35.02)], mostly to AIN1. Regression was less likely in older patients (Ptrend = 0.048). Two patients with HGAIN developed anal cancer.
Conclusion: High-grade ASILs frequently spontaneously regress. Longer-term, prospective studies are required to determine whether these regressions are sustained.