Objective: It is well known that the periphery of lung tumours is under-dosed in radiotherapy as a result of electronic disequilibrium at the interface of lung and tumour tissue. Clinical trials often employ dose calculation algorithms which poorly approximate the dose to peripheral regions of tumour volumes. The aim of this study was to develop a set of systematic under-dosage estimates corresponding to various clinical parameters.
Methods: High resolution Monte Carlo radiation transport calculations were undertaken for a systematic set of generic lung tumours irradiated with an external photon beam. Varied parameters include beam energy, field size, tumour size and distance to chest wall. Calculations were undertaken using both EGSnrc and GEANT4. A ‘Dose Reduction Factor’ is defined which describes the dose to the peripheral ‘shell’ of the tumour, as relevant for multiple-field and arc therapy.
Results: For a 6 MV beam, under-dosage is typically between 2 and 5 % for the different arrangements investigated, and for a 15 MV beam it is between 5 and 8 % (relative to the central dose). Good agreement between EGSnrc and GEANT4 was demonstrated. Comparisons with pencil beam convolution calculations indicate that the treatment planning system does not identify this under-dosage.
Conclusions: A systematic set of data has been obtained that characterises the extent of peripheral under-dosage in lung tumours for the retrospective evaluation of clinical trial data. The data presented is also informative for clinics using less sophisticated planning algorithms, particularly when dose is being prescribed to covering isodoses.