title: Long-term intra-fractional motion of the prostate using hydrogel spacer during Cyberknife® treatment for prostate cancer--a case report.
abstract: BACKGROUND
  There is a trend towards hypofractionated stereotactic radiotherapy (RT) in prostate cancer to apply high single doses in a few fractions. Using the Cyberknife® robotic system multiple non-coplanar fields are usually given with a treatment time of one hour or more. We planned to evaluate organ motion in this setting injecting a hydrogel spacer to protect the anterior rectal wall during treatment.
  
  
  METHODS
  A 66 years old man with low risk prostate cancer was planned for robotic hypofractionated stereotactic RT. After implantation of fiducial markers and a hydrogel spacer a total dose of 36.25 Gy in 5 fractions was given to the planning target volume (clinical target volume + 3 mm). After each beam the corresponding data reporting on the intra-fractional movement were pre-processed, the generated log-files extracted and the data analysed according to different directions: left -right (LR); anterior - posterior (AP); inferior -superior (IS). Clinical assessments were prospectively done before RT start, one week after the end of treatment as well as 1, 6 and 12 months afterwards. Symptoms were documented using Common Toxicity and Adverse Events Criteria 4.0.
  
  
  RESULTS
  Tolerability of marker and hydrogel implantation was excellent. A total of 284 non-coplanar fields were used per fraction. The total treatment time for all fields per fraction lasted more than 60 minutes. The detected and corrected movements over all 5 fractions were in a range of +/- 4 mm in all directions (LR: mean 0,238 - SD 0,798; AP: mean 0,450 - SD 1,690; and IS: mean 0,908 - SD 1,518). V36Gy for the rectum was 0.062 ccm. After RT, grade 1-2 intestinal toxicity and grade 1 genitourinarytoxicity occurred, but resolved completely after 10 days. On 1-, 6- and 12-months follow-up the patient was free of any symptoms with only slight decrease of erectile function (grade 1). There was a continuous PSA decline.
  
  
  CONCLUSIONS
  Prostate movement was relatively low (+/- 4 mm) even during fraction times of more than 60 minutes. The hydrogel spacer might serve as a kind of stabilisator for the prostate, but this should be analysed in a larger cohort of patients.
authors.forename: Marcin
authors.initials: M
authors.lastname: Sumila
authors.forename: Andreas
authors.initials: A
authors.lastname: Mack
authors.forename: Uwe
authors.initials: U
authors.lastname: Schneider
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Storelli
authors.forename: Jürgen
authors.initials: J
authors.lastname: Curschmann
authors.affiliation: Institute of Radiotherapy, Klinik Hirslanden, Witellikerstrasse 40, Zürich CH-8032, Switzerland. guenther.gruber@hirslanden.ch.
authors.forename: Günther
authors.initials: G
authors.lastname: Gruber
mesh_terms: D000368:Aged; D019583:Dose Fractionation, Radiation; D006801:Humans; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate; D008297:Male; D009038:Motion; D058958:Organs at Risk; D011471:Prostatic Neoplasms / Q000601:surgery*; D019736:Prostheses and Implants; D011832:Radiation Injuries / Q000517:prevention & control; D016634:Radiosurgery / Q000295:instrumentation*; D011880:Radiotherapy Planning, Computer-Assisted
chemical_list: D020136:Hydrogel, Polyethylene Glycol Dimethacrylate