title: Greater postimplant swelling in small-volume prostate glands: implications for dosimetry, treatment planning, and operating room technique.
abstract: OBJECTIVE
  Postimplant prostatic edema has been implicated in suboptimal permanent implants, and smaller prostates have been reported to have worse dosimetric coverage. In this study we compare the degree of postimplant edema between larger and smaller prostates and examine the effects of prostate size on the dose delivered to 90% of the prostate (D90).
  
  
  METHODS
  From September 2003 to February 2006, 105 hormone-naive patients underwent permanent prostate brachytherapy with (125)I Rapid Strand (Oncura Inc., Arlington Heights, IL). All patients underwent pelvic magnetic resonance imaging (MRI) within 3 weeks before implant, transrectal ultrasound at the time of implant, and both computed tomography and MRI 2.5 to 3 weeks after implant. Prostates were divided into 5 subgroups based on preimplant MRI volumes: less than 25 mL, 25 to 35 mL, 35 to 45 mL, 45 to 55 mL, and greater than 55 mL. Prostate swelling was assessed by use of preimplant and postimplant MRI volumes. Postimplant dosimetry was determined by MRI and compared between the subgroups.
  
  
  RESULTS
  All prostates showed postimplant swelling on MRI when compared with preimplant MRI, with a mean increase of 31% ± 31% (p < 0.0001). The greatest swelling was noted in small prostates (volume less than 25 mL), with a mean increase of 70% ± 36%. The degree of swelling in the group with a volume less than 25 mL was significantly larger than the degree of swelling in all other prostate subgroups (p < 0.003). Transrectal ultrasound significantly overestimates the prostate volume when compared with MRI by a mean of 15% ± 25% (p = 0.0006) and is more pronounced for smaller prostates. Although prostates with volumes less than 25 mL did not have significantly worse D90 compared with larger prostates, they had the largest percent of suboptimal implants by the standard ratio of D90 divided by the prescription dose.
  
  
  CONCLUSIONS
  Although small prostates have the greatest postimplant edema, planning ultrasound at the time of implant overestimates the volumes of smaller prostates to a greater degree than larger prostates, which may minimize the effects of edema on postimplant dosimetry.
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, USA.
authors.forename: Eugene
authors.initials: E
authors.lastname: Chung
authors.forename: Matthew H
authors.initials: MH
authors.lastname: Stenmark
authors.forename: Cheryl
authors.initials: C
authors.lastname: Evans
authors.forename: Vrinda
authors.initials: V
authors.lastname: Narayana
authors.forename: Patrick W
authors.initials: PW
authors.lastname: McLaughlin
mesh_terms: D001918:Brachytherapy / Q000009:adverse effects* / Q000295:instrumentation / Q000379:methods; D003952:Diagnostic Imaging / Q000379:methods; D004487:Edema / Q000209:etiology*; D006801:Humans; D007457:Iodine Radioisotopes / Q000627:therapeutic use; D008297:Male; D009929:Organ Size; D011467:Prostate / Q000473:pathology*; D011469:Prostatic Diseases / Q000209:etiology*; D011471:Prostatic Neoplasms / Q000473:pathology* / Q000532:radiotherapy*; D012189:Retrospective Studies
chemical_list: D007457:Iodine Radioisotopes