title: Preclinical investigations towards the first spacer gel application in prostate cancer treatment during particle therapy at HIT.
abstract: BACKGROUND
  The application of spacer gel represents a promising approach to reliably spare the rectal frontal wall during particle therapy (IJROBP 76:1251-1258, 2010). In order to qualify the spacer gel for the clinical use in particle therapy, a variety of measurements were performed in order to ensure the biological compatibility of the gel, its physical stability during and after the irradiation, and a proper definition of the gel in terms of the Hounsfield Unit (HU) values for the treatment planning system. The potential for the use of the spacer gel for particle therapy monitoring with off-line Positron Emission Tomography (PET) was also investigated.
  
  
  RESULTS
  The spacer gel implanted to the prostate patient in direct neighbourhood to the clinical target volume does not interfere with the particle therapy treatment planning procedure applied at Heidelberg Ion Beam Therapy Centre (HIT). The performed measurements show that Bragg-peak position of the particles can be properly predicted on the basis of computed tomography imaging with the treatment planning system used at HIT (measured water equivalent path length of 1.011 ±0.011 (2σ), measured Hounsfield Unit of 28.9 ±6.1 (2σ)). The spacer gel samples remain physically unchanged after irradiation with a dose exceeding the therapeutic dose level. The independently measured Bragg-Peak position does not change within the time interval of 10 weeks.
  
  
  CONCLUSIONS
  As a result of the presented experiments, the first clinical application of spacer gel implant during prostate cancer treatment with carbon ions and protons was possible at HIT in 2012. The reported pre-clinical investigations demonstrate that use of spacer gel is safe in particle therapy in presence of therapy target motion and patient positioning induced particle range variations. The spacer gel injected between prostate and rectum enlarge the distance between both organs, which is expected to clinically significantly decrease the undesirable exposure of the most critical organ at risk, i.e. rectal frontal wall. Further research on the composition of spacer gel material might lead to additional clinical benefits by validation of particle therapy of prostate via post-therapeutic PET-imaging or by patient positioning based on the gel as a radio-opaque marker.
authors.affiliation: Heidelberg Ion Beam Therapy Center and Department of Radiation Oncology, Heidelberg University Clinic, Im Neuenheimer Feld 400, Heidelberg 69120, Germany. antoni.rucinski@gmail.com
authors.forename: Antoni
authors.initials: A
authors.lastname: Ruciński
authors.forename: Julia
authors.initials: J
authors.lastname: Bauer
authors.forename: Patrick
authors.initials: P
authors.lastname: Campbell
authors.forename: Stephan
authors.initials: S
authors.lastname: Brons
authors.forename: Daniel
authors.initials: D
authors.lastname: Unholtz
authors.forename: Gregor
authors.initials: G
authors.lastname: Habl
authors.forename: Klaus
authors.initials: K
authors.lastname: Herfarth
authors.forename: Jürgen
authors.initials: J
authors.lastname: Debus
authors.forename: Christoph
authors.initials: C
authors.lastname: Bert
authors.forename: Katia
authors.initials: K
authors.lastname: Parodi
authors.forename: Oliver
authors.initials: O
authors.lastname: Jäkel
authors.forename: Thomas
authors.initials: T
authors.lastname: Haberer
mesh_terms: D005782:Gels / Q000528:radiation effects*; D063193:Heavy Ion Radiotherapy / Q000009:adverse effects / Q000379:methods*; D006801:Humans; D008297:Male; D019047:Phantoms, Imaging; D011092:Polyethylene Glycols / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D019736:Prostheses and Implants; D061766:Proton Therapy / Q000009:adverse effects / Q000379:methods*; D011832:Radiation Injuries / Q000517:prevention & control*; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000379:methods
chemical_list: D005782:Gels; D011092:Polyethylene Glycols