Four dimensional dose calculations and planning strategies for dynamic tumour tracking treatments
The objective of this thesis was to develop four-dimensional (4D) dose calculation methods and 4D treatment planning strategies for creating and assessing treatment plans for real-time dynamic tumour tracking (DTT) on the Vero4DRT (Vero) linear accelerator. Currently DTT treatments are created, optimized, and evaluated on a single breathing phase computed tomography (CT) image despite treatment occurring while the patient breathes freely. This treatment planning protocol could put organs near the target at risk of being over-dosed on other breathing phases. This thesis focuses on organ-at-risk (OAR) safety during DTT treatments and outlines treatment planning and dose calculation protocols that can be implemented to reduce dose to OARs during DTT.
The first aim was to model the beam’s panning/tilting geometry during DTT for dose calculations. After optimizing a plan on an end-exhale CT image following current clinical protocols, the plan was re-calculated on other breathing phases of a patient’s four-dimensional computed tomography (4DCT) data set while modelling panning/tilting within the treatment planning system (TPS). These dose distributions were accumulated onto a reference phase to create 4D dose distributions using 2 breathing phases (exhale and inhale) and 10 breathing phases from the patient’s 4DCT. Subsequently, 2- and 10-phase 4D dose calculations that model panning/tilting were performed using MC as secondary dose verification of the TPS’ 4D dose calculations. These 4D dose calculations indicated that some OARs below their dose limit on the original planning CT image would exceed their dose limit during a DTT treatment. Additionally, MC dose calculations had improved accuracy in the calculated dose to OARs receiving primarily beam penumbra dose.
In the final work for this thesis, two 4D treatment planning strategies were developed. These strategies incorporate anatomical or dosimetric information from other breathing phases when creating/optimizing a plan to spare OARs, and the dose to OARs is then confirmed with a 4D dose calculation after implementing the strategy. Both strategies were shown to spare OARs after a 4D dose calculation. The implementation of 4D dose calculations and 4D planning strategies are necessary for ensuring OARs will not exceed their dose limits during DTT treatments, and this thesis demonstrates methods that are clinically feasible as well as accurate.