Parameters Impacting on Late Rectal Toxicity in 3D Conformal Radiotherapy for Prostate Cancer (original) (raw)
2009, International Journal of Radiation Oncology*Biology*Physics
kinetics after radiation treatment was an exponential decay or not. The PSA data were fitted to exponential decay curves and exponential decay behavior was assigned if the mathematical fits were close enough to the data. This proximity criteria was based on residual differences and coefficients of correlation. Chi Square tests (two sided) were used to compare patients with an exponential decay vs. a non-exponential decay. A general linear modelling for repeated measures was used to analyze the influence of various factors on 7 PSA measurements: pre-EBRT PSA and the following 6 post-EBRT measurements. Spearman rank correlation was used to identify an association between different pre-EBRT factors and the PSA nadir. Results: Median follow-up after for all patients was 163.14 weeks (range 92-265). Patients with an exponential decay (n = 56) were significantly older (median 72y) vs. patients with a non-exponential decay (n = 11, median 65y, p = 0.025). Patients exhibiting a mathematically sound exponential decay also had a significantly higher number of positive biopsies (3 vs. 2, p = 0.013). PSA nadir (0.42 vs. 0.39, p = 0.324) and time to nadir (122.1 vs. 124.1, p = 0.428) did not differ between both groups. Linear modeling showed that prostate volume and PSA density (both p\0.001) as well as BMI (p = 0.026) and age (p = 0.019) all had a significant influence on all 7 PSA measurements. The PSA nadir was positively correlated with PSA density (r=0.315, p = 0.009) but not with PSA half-life, EBRT dose, pre EBRT PSA, Gleason or BMI (p = 0.109-0.988). We also found that the longer the time to nadir, the smaller the value of the PSA nadir (r=-0.421, p\0.001). Conclusions: These observations suggest that an exponential decay behavior does not have an influence on the PSA nadir, a surrogate marker for disease free survival. Our data suggest that the PSA post-EBRT is influenced by factors linked to the tumor-load but also patient-specific factors. These findings could help in understanding PSA bounce and eventually in models for cellular death.
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