title: Factors predicting an increased dose to the penile bulb in permanent seed prostate brachytherapy.
abstract: OBJECTIVE
  Erectile dysfunction following permanent seed brachytherapy for prostate cancer may be related to the dose to the penile bulb. We investigated anatomic and dosimetric factors that might contribute to an increased dose to the penile bulb.
  
  
  METHODS
  One-month CT and MR images were examined for 50 consecutive patients treated with exclusive (125)I permanent seed prostate brachytherapy to a prescribed dose of 145 Gy. Implants were preplanned by transrectal ultrasound (TRUS). Postimplant dosimetry was performed at 1 month using an MRI-CT fusion. Spearman's correlation was used to establish a correlation between dosimetric parameters, anatomical factors, and the dose to the penile bulb.
  
  
  RESULTS
  Penile bulb volumes ranged from 1.2-8.5 cc (median, 3.9 cc). The distance from the penile bulb to the prostate apex ranged from 5-33 mm (median, 15.5 mm). D50 of the penile bulb ranged from 13-121 Gy. The range for the V45 (65 Gy) was 0-87%; only 3% of patients had >2 cc covered by this isodose and in 16% of patients the V45 covered more than 50% of the penile bulb. About one-third of the patients received a dose to the bulb that would put them at a high risk of erectile dysfunction after external beam radiation, if the dose were radiobiologically equivalent. There was a significant inverse correlation between the distance between the prostate apex and the penile bulb, and the dosimetric parameters of the bulb: r = -0.548, -0.656, p = < 0.01. The further caudal the apex was from the symphysis, the closer it was to the penile bulb (r = -0.564, p = <0.01). We could not find a correlation between the dose to the prostate or its apex and the dose to the penile bulb.
  
  
  CONCLUSIONS
  When the prostate apex is close to the penile bulb, care should be taken to limit the dose to the penile bulb, if possible. This may reduce the incidence of erectile dysfunction and urinary toxicity after permanent seed prostate brachytherapy.
authors.affiliation: Department of Radiation Oncology, Princess Margaret Hospital, 610 University Avenue, Toronto, Ontario M5G 2M9, Canada.
authors.forename: Daniel
authors.initials: D
authors.lastname: Taussky
authors.forename: Masoom
authors.initials: M
authors.lastname: Haider
authors.forename: Michael
authors.initials: M
authors.lastname: McLean
authors.forename: Ivan
authors.initials: I
authors.lastname: Yeung
authors.forename: Theresa
authors.initials: T
authors.lastname: Williams
authors.forename: Shannon
authors.initials: S
authors.lastname: Pearson
authors.forename: Gina
authors.initials: G
authors.lastname: Lockwood
authors.forename: Juanita
authors.initials: J
authors.lastname: Crook
mesh_terms: D001918:Brachytherapy; D006801:Humans; D008297:Male; D010413:Penis / Q000528:radiation effects*; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011834:Radiation Monitoring; D011879:Radiotherapy Dosage