Development of a particle source model for a synergy linear accelerator to be used in Monte Carlo radiation dose calculations for cancer therapy

English: In oncology patients are treated for cancer with various methods such as surgery, chemo therapy and radiation therapy. Accurate radiation treatment planning and dose delivery to the tumour is necessary for the successful outcome of cancer treatment. In order to achieve this goal accurate radiation dose calculation codes must be utilized. EGSnrc based Monte Carlo (MC) codes such as BEAMnrc and DOSXYZnrc have been developed for just this purpose. The problem that arises in using these MC codes is that they lack suitable x-ray beam source models. These models must be accurate in order to replicate the true clinical x-ray beam emanating from the linear accelerator. One such machine for which radiation source data must be derived is currently being used at the Oncology department in Universitas Hospital Annex. It is desirable to model this linear accelerator in order to perform MC based dose calculations for radiation treatment. The use of MC based dose calculations is certainly not new in the radiation physics environment. Various authors have studied the replication of radiation beam characteristics using source models to simulate the phase-space parameters of particles produced by the linear accelerator. These parameters include the charge, energy, direction, and position of each particle as it crosses a certain reference plane below the linear accelerator. An accurate source model should be able to re-generate particles with the exact set of above-mentioned parameters as would be produced by the real linear accelerator. Sources can be very simple such as a single point from which the particles are radiating with a single invariant energy spectrum. Studies have shown that these beam models can yield accurate beam data over relatively small field sizes and is not general enough to use over a whole range of clinically useful field sizes. A graphical user interface (GUI) was developed that can assist in the construction of the source model. The source model can describe energy and fluence distributions for photons and electrons as separate point sources each with their own SSD. The accuracy of the model was validated by comparing simulated profiles with measured data for an Elekta Synergy linear accelerator. The modified Schiff formula was used to derive the bremsstrahlung spectra emanating from the target. The x-ray fluence Gaussian distribution consisted of the primary fluence from the target, which was modified by the primary collimator, secondary collimators as well as the multileaf collimators. The truncation and beam scatter caused by the face of the collimators were modelled with error functions. Exponential functions were used to model off-axis collimator transmission. Profiles and percentage depth dose curves were obtained with the source for square field sizes of 1 × 1 cm2 up to a 40 × 40 cm2. Offset fields for 10 × 10 cm2, 15 × 15 cm2 and 20 × 20 cm2, rectangular fields as well as wedged fields were included. Irregular field shapes were simulated to evaluate the source model‘s capability of reproducing complex treatment fields. Film dose verification was done in an anthropomorphic Rando® phantom and compared with the MC source model for 6 MV x-ray beams. A criterion of 2% / 2 mm was used to compare MC data and measured data. This study demonstrated that a diversity of field sizes and percentage depth dose curves can be modelled within 2% / 2 mm. The model can replicate irregular field sizes used for complex treatments. Minor discrepancies were found for the relative dose comparisons between the MC and film data for the anthropomorphic phantom.