A particle track-repeating algorithm has been developed for proton beam dose calculation for radiotherapy. Monoenergetic protons with 250 MeV kinetic energy were simulated in an infinite water phantom using the GEANT3 Monte Carlo code. The changes in location, angle and energy for every transport step and the energy deposition along the track were recorded for the primary protons and all secondary particles. When calculating dose for a patient with a realistic proton beam, the pre-generated particle tracks were repeated in the patient geometry consisting of air, soft tissue and bone. The medium and density for each dose scoring voxel in the patient geometry were derived from patient CT data. The starting point, at which a proton track was repeated, was determined according to the incident proton energy. Thus, any protons with kinetic energy less than 250 MeV can be simulated. Based on the direction of the incident proton, the tracks were first rotated and for the subsequent steps, the scattering angles were simply repeated for air and soft tissue but adjusted properly based on the scattering power for bone. The particle step lengths were adjusted based on the density for air and soft tissue and also on the stopping powers for bone while keeping the energy deposition unchanged in each step. The difference in nuclear interactions and secondary particle generation between water and these materials was ignored. The algorithm has been validated by comparing the dose distributions in uniform water and layered heterogeneous phantoms with those calculated using the GEANT3 code for 120, 150, 180 and 250 MeV proton beams. The differences between them were within 2%. The new algorithm was about 13 times faster than the GEANT3 Monte Carlo code for a uniform phantom geometry and over 700 times faster for a heterogeneous phantom geometry.

The purpose of this work is to model electron contamination in clinical photon beams and to commission the source model using measured data for Monte Carlo treatment planning. In this work, a planar source is used to represent the contaminant electrons at a plane above the upper jaws. The source size depends on the dimensions of the field size at the isocentre. The energy spectra of the contaminant electrons are predetermined using Monte Carlo simulations for photon beams from different clinical accelerators. A 'random creep' method is employed to derive the weight of the electron contamination source by matching Monte Carlo calculated monoenergetic photon and electron percent depth-dose (PDD) curves with measured PDD curves. We have integrated this electron contamination source into a previously developed multiple source model and validated the model for photon beams from Siemens PRIMUS accelerators. The EGS4 based Monte Carlo user code BEAM and MCSIM were used for linac head sinulation and dose calculation. The Monte Carlo calculated dose distributions were compared with measured data. Our results showed good agreement (less than 2% or 2 mm) for 6, 10 and 18 MV photon beams.

Modulated electron radiotherapy (MERT) may potentially be an effective modality for the treatment of shallow tumors, but dose calculation accuracy and delivery efficiency challenges remain. The purpose of this work is to investigate the dose accuracy of MERT delivery using a photon multileaf collimator (pMLC) on a Siemens Primus accelerator. A Monte Carlo (MC)-based inverse treatment planning system was developed for the 3D treatment planning process. Phase space data of 6, 9, 12 and 15 MeV electron beams were commissioned and used as the input source for MC dose calculations. A treatment plan was performed based on the 3D CT data of a heterogeneous 'breast phantom' that mimics a breast cancer patient, and delivered with 22 segments, each associated with a particular energy and Monitor Unit value. Film and ion chamber dosimetry was carefully performed for the conversion from measurement reading to dose, and the results were employed for plan verification using the heterogeneous breast phantom and a solid water phantom. Dose comparisons between measurements and calculations showed agreement within 2% or 1 mm. We conclude that our in-house MC treatment planning system is capable of performing treatment planning and accurate dose calculations for MERT using the pMLC to deliver radiation therapy to the intact breast.

In this work, we investigate a formalism for monitor unit (MU) calculation in Monte Carlo based treatment planning. By relating MU to dose measured under reference calibration conditions (central axis, depth of dose maximum in water, 10 cm x 10 cm field defined at 100 cm source-to-surface distance) our formalism determines the MU required for a treatment plan based on the prescription dose and Monte Carlo calculated dose distribution. Detailed descriptions and formulae are given for various clinical situations including conventional treatments and advanced techniques such as intensity-modulated radiotherapy (IMRT) and modulated electron radiotherapy (MERT). Analysis is made of the effects of source modelling, beam modifier simulation and patient dose calculation accuracy, all of which are important factors for absolute dose calculations using Monte Carlo simulations. We have tested the formalism through phantom measurements and the predicted MU values were consistent with measured values to within 2%. The formalism has been used for MU calculation and plan comparison for advanced treatment techniques such as MERT, extracranial stereotactic IMRT, MRI-based treatment planning and intensity-modulated laser-proton therapy studies. It is also used for absolute dose calculations using Monte Carlo simulations for treatment verification, which has become part of our comprehensive IMRT quality assurance programme.

Purpose: Image‐guided radiation therapy has been employed for cancer treatment to improve the tumor localization accuracy. Radiation therapy with proton beams requires more on this accuracy because the proton beam has larger uncertainty and dramatic dose variation along the beam direction. Among all the image modalities, magnetic‐resonance image (MRI) is the best for soft tissue delineation and real time motion monitoring. In this work, we investigated the behavior of the proton beam in magnetic field with Monte Carlo simulations. Methods: A proton Monte Carlo platform, TOPAS, was used for this investigation. Dose calculations were performed with this platform in a 30cmx30cmx30cm water phantom for both pencil and broad proton beams with different energies (120, 150 and 180MeV) in different magnetic fields (0.5T, 1T and 3T). The isodose distributions, dose profiles in lateral and beam direction were evaluated. The shifts of the Bragg peak in different magnetic fields for different proton energies were compared and the magnetic field effects on the characters of the dose distribution were analyzed. Results: Significant effects of magnetic field have been observed on the proton beam dose distributions, especially for magnetic field of 1T and up. The effects are more significant for higher energy proton beam because higher energy protons travel longer distance in the magnetic field. The Bragg peak shift in the lateral direction is about 38mm for 180MeV and 11mm for 120MeV proton beams in 3T magnetic field. The peak positions are retracted back for 6mm and 2mm, respectively. The effect on the beam penumbra and dose falloff at the distal edge of the Bragg peak is negligible. Conclusion: Though significant magnetic effects on dose distribution have been observed for proton beams, MRI guided proton therapy is feasible because the magnetic effects on dose is predictable and can be considered in patient dose calculation.