Dose to water versus dose to medium in proton beam therapy

Dose in radiation therapy is traditionally reported as the water-equivalent dose, or dose to water. Monte Carlo dose calculations report dose to medium and thus a methodology is needed to convert dose to medium into dose to water (or vice versa) for comparison of Monte Carlo results with results from planning systems. This paper describes the development of a formalism to convert dose to medium into dose to water for proton fields when simulating the dose with Monte Carlo techniques. The conversion is based on relative stopping power but also considers energy transferred via nuclear interactions. The influence of different interaction mechanisms of proton beams (electromagnetic versus nuclear) is demonstrated. Further, an approximate method for converting doses retroactively is presented. Based on the outlined formalism, five proton therapy patients with a total of 33 fields were analyzed. Dose distributions, dose volume histograms and absolute doses to assess the clinical significance of differences between dose to medium and dose to water are presented. We found that the difference between the two dose reporting definitions can be up to 10% for high CT numbers if analyzing the mean dose to the target. The difference is clinically insignificant for soft tissues. For the structures analyzed, the mean dose to water could be converted to dose to medium by applying a correction factor increasing linearly with increasing average CT number in the volume. We determined that an approximate conversion method, done retroactively with an energy-independent stopping power ratio and without considering nuclear interaction events separately (as compared to on-the-fly conversion during simulation), is sufficiently accurate to compute mean doses. It is insufficient, however, when analyzing the beam range. For proton beams stopping in bony anatomy, the predicted beam range can differ by 2-3 mm when comparing dose to tissue and dose to water.

[1]  J. Ziegler.  Stopping of energetic light ions in elemental matter , 1999 .

[2]  R K Tripathi,et al.  Accurate universal parameterization of absorption cross sections. , 1996, Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms.

[3]  D F Jackson,et al.  The relation between X-ray CT numbers and charged particle stopping powers and its significance for radiotherapy treatment planning. , 1983, Physics in medicine and biology.

[4]  Pedro Andreo,et al.  Stopping powers for the ion-chamber dosimetry of radiotherapeutic heavy-particle beams , 1992 .

[5]  F. Nüsslin,et al.  Comments on 'Converting absorbed dose to medium to absorbed dose to water for Monte Carlo based photon beam dose calculations'. , 2000, Physics in medicine and biology.

[6]  P Andreo,et al.  Monte Carlo calculated stopping-power ratios, water/air, for clinical proton dosimetry (50-250 MeV). , 1997, Physics in medicine and biology.

[7]  P. Keall.  Dm rather than Dw should be used in Monte Carlo treatment planning. Against the proposition. , 2002, Medical physics.

[8]  R. K. Bull,et al.  Stopping powers for electrons and positrons: ICRU Report 37; 271 pp.; 24 figures; U.S. $24.00. , 1986 .

[9]  O. Jäkel.  Ranges of ions in metals for use in particle treatment planning , 2006, Physics in Medicine and Biology.

[10]  M. Rosetti,et al.  Proton stopping powers averaged over beam energy spectra , 2000, Physics in medicine and biology.

[11]  Oliver Jäkel,et al.  The influence of metal artefacts on the range of ion beams , 2007, Physics in medicine and biology.

[12]  H Paganetti,et al.  Adaptation of GEANT4 to Monte Carlo dose calculations based on CT data. , 2004, Medical physics.

[13]  M. Rosetti,et al.  Effects of nuclear interactions on energy and stopping power in proton beam dosimetry , 1996 .

[14]  H Paganetti,et al.  Accurate Monte Carlo simulations for nozzle design, commissioning and quality assurance for a proton radiation therapy facility. , 2004, Medical physics.

[15]  H. Paganetti,et al.  Physics Settings for Using the Geant4 Toolkit in Proton Therapy , 2008, IEEE Transactions on Nuclear Science.

[16]  S Minohara,et al.  Relationship between CT number and electron density, scatter angle and nuclear reaction for hadron-therapy treatment planning. , 1998, Physics in medicine and biology.

[17]  H Paganetti,et al.  Nuclear interactions in proton therapy: dose and relative biological effect distributions originating from primary and secondary particles. , 2002, Physics in medicine and biology.

[18]  P. Keall,et al.  Clinical comparison of head and neck and prostate IMRT plans using absorbed dose to medium and absorbed dose to water , 2006, Physics in medicine and biology.

[19]  H Helen Liu,et al.  Dm rather than Dw should be used in Monte Carlo treatment planning. For the proposition. , 2002, Medical physics.

[20]  Frank Verhaegen,et al.  Assigning nonelastic nuclear interaction cross sections to Hounsfield units for Monte Carlo treatment planning of proton beams , 2005, Physics in medicine and biology.

[21]  Reinhard W. Schulte,et al.  Nuclear Data for Neutron and Proton Radiotherapy and for Radiation Protection , 2001 .

[22]  M. G. Lötter,et al.  The indirect use of CT numbers to establish material properties needed for Monte Carlo calculation of dose distributions in patients. , 1998, Medical physics.

[23]  Katia Parodi,et al.  PET/CT imaging for treatment verification after proton therapy: a study with plastic phantoms and metallic implants. , 2007, Medical physics.

[24]  松藤 成弘.  Relationship between CT number and electron density, scatter angle and nuclear reaction for hadrontherapy treatment planning , 1998 .

[25]  Katia Parodi,et al.  Clinical implementation of full Monte Carlo dose calculation in proton beam therapy , 2008, Physics in medicine and biology.

[26]  R. Mohan,et al.  Converting absorbed dose to medium to absorbed dose to water for Monte Carlo based photon beam dose calculations. , 2000, Physics in medicine and biology.

[27]  Joao Seco,et al.  Effects of Hounsfield number conversion on CT based proton Monte Carlo dose calculations. , 2007, Medical physics.

[28]  E. Pedroni,et al.  The calibration of CT Hounsfield units for radiotherapy treatment planning. , 1996, Physics in medicine and biology.

[29]  M Goitein,et al.  Radiobiological significance of beamline dependent proton energy distributions in a spread-out Bragg peak. , 2000, Medical physics.

[30]  F. Cucinotta,et al.  Universal Parameterization of Absorption Cross Sections , 1999 .

[31]  W. John,et al.  A Simple Method for Nucleon-Nucleon Cross Sections in a Nucleus , 1999 .

[32]  E Pedroni,et al.  The precision of proton range calculations in proton radiotherapy treatment planning: experimental verification of the relation between CT-HU and proton stopping power. , 1998, Physics in medicine and biology.

[33]  W. Kalender,et al.  Correlation between CT numbers and tissue parameters needed for Monte Carlo simulations of clinical dose distributions , 2000 .