Assessing the impact of motion on treatment planning during stereotactic body radiotherapy of lung cancer

Cancer is a leading cause of death in Australia and approximately 52% of cancer patients will require radiotherapy at some stage in their treatment. In recent years, stereotactic radiotherapy has emerged as an increasingly common treatment modality for small lesions in various sites of the human body.  To facilitate the investigation into the effects of imaging small mobile lesions, a see-saw 4D-CT phantom was developed. This phantom was used to investigate phase-binning artifacts that can be present when assigning an insufficient number of phases to 4D-CT data. The interplay between a lesion’s size and its amplitude, and the effects this relationship has on 4D-CT data was also investigated. An upgrade to a commercially available respiratory motion phantom was also pursued in order to replicate patient motion recorded with the Varian RPM system. Monte Carlo methods were used to determine the impact of motion on PET data by incorporating a computational moving phantom (XCAT) with a full Monte Carlo model of a commercially available PET scanner. To assess the impact of motion on treatment planning and dose calculation, two treatment planning scenarios were simulated using Monte Carlo. The traditional method of calculating dose on an average intensity projection from 4D-CT was compared to 4D dose calculation, in which tumour motion data from 4D-CT is explicitly incorporated into the treatment plan. Monte Carlo methods are also employed to evaluate the degree of underdosage at the periphery of lung lesions arising from electronic disequilibrium associated with density changes.  It was found that small lesions typically seen in SBRT of lung cancer require extra care when considering treatment planning, motion mitigation, and treatment delivery. The upgraded QUASAR phantom allows for patient specific verification of SBRT/SABR treatment plans to be conducted and was found to replicate patient motion accurately. Respiratory analysis software presented in this work enables detailed statistics of a patient’s respiratory characteristics to be evaluated. The number of phase-bins required to mitigate banding artifacts in 4D-CT projections is quantified in a simple equation for sinusoidal motion. It was also found that for lesion with diameters greater than 2.0 cm and amplitudes less than 4.0 cm, ten phase-bins are adequate to negate all banding artifacts in projection images.  Experimental and Monte Carlo simulations of PET and 4D-PET revealed that motion greater than 1.0 cm resulted in a reduction in apparent activity that increased with motion amplitude. A Dose Reduction Factor (DRF) metric was developed using Monte Carlo simulation which is defined as the ratio of the average dose to the periphery of the lesion to the dose in the central portion. The mean DRF was found to be 0.97 and 0.92 for 6 MV and 15 MV photon beams respectively, for lesion sizes ranging from 10 – 50 mm. The dynamic scenario was simulated with 4D dose calculation methods of registering and adding the dose distributions in each phase-bin from 4D-CT. The dose-volume distributions compared well with 3D (AIP) methods if multiple beams were used and the amplitude of motion was less than 3.0 cm.