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Worth Attention rectal toxicity

Baseline lymphocyte count predicted severe lymphopenia better than dosimetry in rectal cancer

Severe lymphopenia occurred in 44%, while baseline and week-2 lymphocyte counts outperformed 600 dosimetric variables for risk prediction.

RadOncToday · ·1 min read

KEY POINTS

  • This retrospective single-institution study included 179 patients with non-metastatic rectal cancer treated with curative-intent long-course fluoropyrimidine-based chemoradiotherapy from 2018–2024. Treatment used 50.4 Gy in 28 fractions, with 45 Gy elective nodal irradiation.
  • Severe radiation-induced lymphopenia, defined as a nadir absolute lymphocyte count below 0.5 × 10⁹/L, occurred in 76 of 173 evaluable patients (44%). Median lymphocyte count fell approximately 71%, from 1.88 × 10⁹/L at baseline to a nadir of 0.53 × 10⁹/L around day 33, and only 19% had recovered to at least 1.0 × 10⁹/L by three months.
  • Baseline lymphocyte count was the strongest pretreatment predictor: severe lymphopenia occurred in 67% of the lowest baseline quartile versus 17% of the highest. Each 1 × 10⁹/L increase in baseline count was associated with substantially lower risk, with a Firth odds ratio of 0.27 (p<0.001).
  • A three-variable model combining baseline lymphocyte count, age and sex achieved a bootstrap-corrected C-index of 0.746. Female sex was independently associated with higher risk, with an adjusted odds ratio of 3.44 (95% CI 1.69–7.27).
  • The investigators screened 23 complete blood count parameters, six inflammatory markers and 600 dosimetric variables from 25 structures. None of the dosimetric variables improved prediction; commonly proposed pelvic bone marrow constraints were already met in 98% for V10, 100% for mean dose and 86% for V20, limiting their discriminatory value in this standardized treatment cohort.
  • A lymphocyte count at approximately day 14 substantially refined prediction, increasing cross-validated area under the curve from 0.737 to 0.827. Severe lymphopenia was not associated with pathological complete response, tumor regression or T-downstaging, and an apparent survival association disappeared after correcting for time-dependent bias, from HR 0.22 to 0.93.

CLINICAL TAKEAWAY

A routine pretreatment lymphocyte count, particularly when repeated around week 2, may identify rectal cancer patients most likely to develop profound lymphopenia more effectively than elaborate dosimetric profiling. The model appears to predict toxicity susceptibility rather than treatment efficacy and currently supports enhanced monitoring, not modification of oncologic treatment.

SOURCE

International Journal of Radiation Oncology, Biology, Physics