Radiochromic film as a tool to study and validate a commercial Monte Carlo dose calculation algorithm for electron radiotherapy

摘要

Radiotherapy is a type of cancer treatment to suppress tumour growth by employing ionising radiation. External beam radiation is the most common form of radiotherapy and uses ionising radiation such as photon and electron beams. Megavoltage electron beams can be used to treat relatively superficial tumours of less than 5 cm depth such as skin, scalp, chest walls, breast, and nodal boost. In the radiotherapy process, the treatment planning system plays an important role in simulating dose distributions in a patient that would be delivered by the treatment machine. Conventional planning algorithms are based on relatively simple beam models: beam profiles are measured in homogeneous media, and modified to approximate changes in field shape and media. Predicting dose becomes difficult with complex treatment geometries such as beam obliquity, small field sizes and tissue inhomogeneities. In these complex patient geometries, the Monte Carlo method is known to be the most accurate to calculate dose distributions. Recently an electron Monte Carlo (eMC) dose calculation algorithm has been incorporated in XiO treatment planning systems (Elekta/CMS XiO). This algorithm is based on the Voxel Monte Carlo (VMC) code which allows fast dose calculations in voxelised heterogeneous media. This study aims to evaluate XiO eMC’s capability to accurately predict dose distribution. Simulations with the eMC module were performed for 6, 12 and 18 MeV beams in different beam setups. In addition to standard large uniform fields, the effects of oblique incidence, small field size and inhomogeneous media were investigated in this thesis. A new measuring method was developed for high resolution, absolute dose measurement of these non-standard irradiations using radiochromic film. A portable holder was designed and constructed to hold films vertically in a reproducible setup submerged in a water phantom. The film measurements were performed and compared to the calculated XiO eMC 2D dose distributions using the gamma dose analysis and distance-to-agreement tool from the SNC Patient software (Sun Nuclear Corporation, Melbourne, FL). All 2D comparisons of film and XiO eMC were done by comparing absolute dose per monitor unit (MU). The experimental film method was verified with ionisation chamber measurements and the total uncertainty for the film measurement method was determined to be ±3% (2σ). In water beyond Dmax, XiO electron Monte Carlo and EBT3 film agree to within the measurement uncertainties i.e. ± 3% or ± 2 mm for all standard, oblique and circular fields for measurements. Differences between XiO eMC and EBT3 film of up to 9% occur close to the surface for the oblique and circular fields. For the inhomogeneous media, agreement between XiO and film was also within the calculation and measurement uncertainties in the water and lung like regions. Within the rib phantom, XiO was up to 10% higher than film. By comparison, agreement between XiO and film within the denser skull phantom is within the uncertainties. This method has extended the standard set of commissioning measurements to include clinically relevant conditions that focus on specific geometries that are difficult or impossible to measure using ionisation chambers. Radiochromic film in water proved to be a convenient high spatial resolution method to verify electron dose distribution in non-standard conditions including inhomogeneous media.