The effect of obesity on rectal dosimetry after permanent prostate brachytherapy.
OBJECTIVE
Men with higher body mass index (BMI) tend to have more fatty tissue in prostate-rectum interface, which may reduce the rectal wall dose by the inverse square law. We hypothesized that men with higher BMI will have a lower dose to the rectal wall and less rectal toxicity after permanent prostate implant.


METHODS
Prospectively collected data on rectal dosimetry/toxicity and BMI of 407 patients who underwent iodine-125 ((125)I) prostate implant were analyzed. Postimplant dosimetry used CT-MRI fusion on Day 30. Rectal wall was contoured on all slices where seeds were seen. The volume of rectal wall receiving the prescribed dose (RV(100) in cm(3)) and the dose to 1cc of rectal wall (RD(1cc)) were reported. Other factors evaluated for association with rectal dosimetry and toxicity included age, diabetes, hypertension, smoking, use of neoadjuvant hormones, T stage, baseline prostate volume, 1 month prostate edema, seed type and activity, and prostate dosimetry factors (the isodose enclosing 90% of the prostate volume [D(90)], the percentage of the prostate volume enclosed by the prescription [V(100)], and the percentage of the prostate volume enclosed by the 150% isodose [V(150)]). Rectal toxicity was reported as per Radiation Therapy Oncology Group criteria.


RESULTS
BMIs ranged from 15.9 to 46.8 (mean+/-standard deviation [SD]: 27.8+/-4.2). The mean+/-SD values for RV(100) and RD(1cc) were 0.79+/-0.49cm(3) and 128.2+/-27.8Gy, respectively. There was a significant negative association of BMI with RV(100) (p=0.007) and RD(1cc) (p=0.01) on univariate analysis. The mean RV(100) and RD(1cc) for men with higher BMI (>27.8) were lower compared with their slimmer counterparts (0.70 vs. 0.86cm(3) and 123.4 vs. 132.4Gy, respectively). On multivariate analysis for RV(100) and RD(1cc), BMI remained significant (p-values 0.004 and 0.01, respectively) along with prostate volume and V(150), suggesting that anatomic factors are important in rectal dosimetry in prostate brachytherapy. Overall the incidence of Radiation Therapy Oncology Group acute rectal toxicity was 12% (Grade 2, 1.3%) and chronic 6% (Grade 2, 0.5%). No Grade 3 toxicity occurred. None of the factors evaluated were predictive for rectal toxicity.


CONCLUSIONS
Men with a lower BMI received a higher rectal wall dose compared with those with higher BMI. This did not, however, translate into greater rectal toxicity.
Patil
Nikhilesh
N
Department of Radiation Oncology, Princess Margaret Hospital, University Health Network, Toronto, Canada.
Crook
Juanita
J
Department of Radiation Oncology, Princess Margaret Hospital, University Health Network, Toronto, Canada. Electronic address: juanita.crook@rmp.uhn.on.ca.
Saibishkumar
Elantholi P
EP
Department of Radiation Oncology, Princess Margaret Hospital, University Health Network, Toronto, Canada.
Aneja
Manipdip
M
Department of Radiation Medicine, Princess Margaret Hospital, University Health Network, Toronto, Canada.
Borg
Jette
J
Department of Radiation Physics, Princess Margaret Hospital, University Health Network, Toronto, Canada.
Pond
Greg
G
Department of Biostatistics, Princess Margaret Hospital, University Health Network, Toronto, Canada.
Ma
Clement
C
Department of Biostatistics, Princess Margaret Hospital, University Health Network, Toronto, Canada.
D015992:Body Mass Index; D001918:Brachytherapy / Q000379:methods*; D004307:Dose-Response Relationship, Radiation; D019160:Endosonography; D005500:Follow-Up Studies; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D009765:Obesity / Q000150:complications*; D011446:Prospective Studies; D011471:Prostatic Neoplasms / Q000150:complications / Q000175:diagnosis / Q000532:radiotherapy*; D011832:Radiation Injuries / Q000517:prevention & control; D012007:Rectum / Q000000981:diagnostic imaging / Q000473:pathology / Q000528:radiation effects*; D013997:Time Factors; D016896:Treatment Outcome