The aim of this study was to analyze the flow cytometric S-phase fraction (SPF) in rectal tumors before and after preoperative radiotherapy (15x2 Gy) and to compare the findings to the clinical outcome. Archival specimens from 84 cases, treated from 1980 to 1988 with S-phase data and complete follow-up were reviewed. There was no significant correlation between SPF and clinicopathological factors. The median SPF for the 26 diploid tumors before irradiation was 6.6%+/-3.1, compared to a significantly higher median for the 58 preirradiated aneuploid tumors (20.3%+/-6.1; p<0.0001). With a median follow-up of 6 years, there was a significant difference in the number of recurrences for aneuploid tumors with a pretreatment SPF < and >20.3 (51.7% vs. 20.7%; p=0.029), which also led to a significant difference in recurrence-free survival (p=0.05). For diploid tumors, a reduction in the percentage of cells in S-phase after radiation resulted in a borderline significant lower number of recurrences (p=0.06). It is concluded that pretreatment S-phase measurements may be of predictive value especially for aneuploid tumors. An alteration in SPF after radiotherapy may also be helpful in predicting outcome and planning therapy.

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