Biological Dose Calculations for Particle Therapy in FLUKA

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1 U N I V E R S I T Y O F B E R G E N Department of Physics and Technology Biological Dose Calculations for Particle Therapy in FLUKA Tordis J. Dahle May 2016

2 Introduction About half of all cancer patients will receive radiotherapy The goal of radiotherapy: irradiate the tumor while sparing the surrounding healthy tissue Particle therapy is a promising alternative to conventional radiotherapy It has declared that particle therapy is to be established in Norway, location still undecided

3 Particle Therapy Radiotherapy with protons and heavier ions The dose can be delivered more accurately to the target, because of the «inverse depth-dose profile» Figure: Depth-dose distribution of photons and protons. From Levin et al. (2005)

4 Biological Dose Different types of radiation have different biological effectiveness To make optimal use of the physical and biological characteristics of particles, the biological dose has to be considered: D bio = RBE D phys The RBE (Relative Biological Effectiveness) may depend on many factors, including the LET or specific energy (z), physical dose and tissue type Linear energy transfer (LET): The energy transferred from charged particles, per unit length of their path, to the biological material in or near these paths Specific energy: Definition: z = ε/m, where ε is the energy imparted by the mass, m The specific energy is similar to the absorbed dose, but on a micrometer scale

5 Models for calculating the biological dose There are several models for calculating the biological dose in carbon therapy, which gives different results/dose Two of the models for calculating the biological dose: the microdosimetric kinetic model (MKM) the local effect model (LEM) The MKM is used clinically in Japan, and the LEM1 is used clinically in Europe

6 Project Overview FLUKA will be used for biological dose calculations in carbon therapy The primary objective: implementation of Japanese dose model (MKM) in FLUKA recalculation of treatment plans in FLUKA and direct comparison to the model currently used in Europe (LEM) The project is a collaboration with the National Centre of Oncological Hadrontherapy (CNAO) in Pavia, Italy Particle accelerator at CNAO

7 The Microdosimetric Kinetic Model (MKM) Been in use in Japan since 2011 The model was developed by Roland B. Hawkins (1994, 1996), and modified by Kase et al. (2006). Biological dose, according to the MKM, at a location x: D bio x = α r 2β + α r 2β 2 + α 0D x + βz 1Dmix x D x + βd x 2 β α r, β and α 0 are constants (Inaniwa et al. 2010) z 1Dmix is the dose-averaged saturation-corrected specific energy in a mixed-radiation field of SOBP C-ion beam D x is the absorbed dose In Japan, they use clinical dose (D clin ) when treating patients. This is the D bio multiplied by a scaling factor from clinical experience

8 Dose-averaged saturation-corrected specific energy A correction for overkill effect in a very high specific energy region (Kase et al. 2006) The figure is used at NIRS to find z 1Dmix, based on Ekin of ion Figure is made based on the Kiefer Chatterjee model and the MKM Figure: z 1D for mono-energetic ions of atomic number 1-6 as a function of their kinetic energy. From Inaniwa et al. (2010)

9 Local Effect Model (LEM) Fully integrated in the treatment planning systems used at GSI (Darmstadt), HIT (Heideberg) and CNAO (Pavia) Developed by M. Scholz and co-workers Implemented in FLUKA by Mairani et al. ( 2010) Main assumption of the LEM: Equal local dose should lead to equal local effects, independent on the radiation quality Figure: Comparison of the microscopic local dose distributions of C-ions and photons for the same macroscopic dose. From Friedrich et al. (2013)

10 FLUKA Beam and Geometry To compare LEM and MKM, a carbon-beam from the CNAO beam line hitting a water target was simulated The beam was planned according to CNAO TPS, and delivered in FLUKA using a source.f file made by A. Mairani at CNAO

11 FLUKA Scoring Fluence was scored with USRBIN in 2 mm bins Scored values were weighted with FLUSCW to get: absorbed dose, D, and D z 1Dmix (MKM) absorbed dose, D, and α- and β-weighted dose (LEM) (α and β are parameters from the linear-quadratic model in radiotherapy)

12 fluscw Fluscw is activated by option USERWEIGH The value returned by fluscw is multiplied by Yields obtained via USRYIELD Fluences calculated with USRBDX, USRTRACK, USRCOLL, USRBIN Currents calculated with USRBDX

13 FLUKA Scoring and Output MKM: A fluscw.f user routine was made to get dose, D, and D z 1Dmix : D = Φ LET, ρ - Φ is fluence, scored in FLUKA, - LET is found by getlet-function, - ρ is density of water z 1Dmix : found using the table connecting Ekin and z 1D Last part of fluscw-code: FLUSCW will be multiplied by the scored value The FLUKA output was given in a.txt file, which was converted into a.mat file The biological doses in LEM and FLUKA was then evaluated in Python, putting the values from FLUKA into equations for calculating biological dose

14 Preliminary Results MKM estimates a lower relative efficacy of carbon ions when compared to LEM

15 Further Work Recalculate patient treatment plans for comparisons In FLUKA: generate a MKMbased database for fast forward recalculations of plans Database will contain depth dose profiles and corresponding microdosimetric values (see Figure) Figure: Depth dose distribution and corresponding z* for c-ion beams. From Inaniwa et al. (2010)

16 Thank you! Department of Physics and Technology