Abstract
PurposeWe examined the influence of body mass index (BMI) and body fat distribution on rectal dose in patients treated with permanent seed brachytherapy for localized prostate cancer.Methods and materialsWe analyzed 213 patients treated with I125 seed brachytherapy for localized prostate cancer. BMI and rectal dosimetry data for all patients were available. Data on visceral and subcutaneous fat distribution at the level of the iliac crest (n = 140) as well as the distribution of periprostatic and subcutaneous fat at the symphysis pubis level were obtained (n = 117). Fat distribution was manually contoured on CT on day 30 after brachytherapy. The correlation between BMI, fat distribution and rectal dose (R100 (in cc), R150 (cc), D2 (Gy)) was analyzed using the Spearman correlation coefficient. Differences in rectal dose between tertiles of body fat distribution were calculated using nonparametric tests.ResultsPeriprostatic adipose was only weakly correlated with BMI (r = 0.0.245, p = 0.008) and only weakly correlated with the other fat measurements (r = 0.31-0.37, p < 0.001). On the other hand, BMI was correlated with all other fat measurements (≥0.58, p < 0.001). All the other fat measurements were strongly correlated with each other (r = 0.5-0.87, p < 0.001). Patients with an R100 of >1.3 cc (23% of patients) had less visceral fat (p = 0.004), less subcutaneous fat at the level of the iliac crest (p = 0.046) and a lower BMI (26.8 kg/m2 vs. 28.5 kg/m2, p = 0.02) than patients with an R100 of <1.3 cc. Results were very similar when comparing an R100 of >1.0 cc (34% of patients) across the tertiles. None of the tested linear regression models were predictive (max 12%) of dose to the rectum.ConclusionDose to the rectum is dependent on BMI and body fat distribution. Periprostatic fat does not influence rectal dose. Dose to the rectum remains difficult to predict and depends on many factors, one of which is body fat distribution.
Highlights
Several studies have shown a dose-volume dependence on rectal bleeding after permanent seed brachytherapy (BT) [1,2,3,4]
We hypothesized that directly measuring adipose tissue distribution, especially around the prostate, would more accurately predict the dose to the rectum than using body mass index (BMI) alone
Adipose tissue measurements were available for patients who were part of two prior studies in our center investigating the influence of adipose tissue distribution on prostate specific antigen (PSA) bounce after BT [11,12]
Summary
Several studies have shown a dose-volume dependence on rectal bleeding after permanent seed brachytherapy (BT) [1,2,3,4]. Several European societies have published guidelines regarding the rectal dose [5]. Little is known about the factors that might influence the dose to the rectum in BT. Some studies have shown that rectal dose decreases with the physician’s. BMI has often been used as a surrogate for adipose tissue. We hypothesized that directly measuring adipose tissue distribution, especially around the prostate, would more accurately predict the dose to the rectum than using BMI alone
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