Master thesis defense by Morta Marcinkute

In the presence of skin cancer, one of the approaches for treatment is superficial radiotherapy. Particular difficulties may present when a cancerous tumor is located at the facial area. This area possesses high curvature structures which can be responsible for an underdosage effect within the radiation field, due to a lack of dose scattering upwards the tumor. The main purpose of this thesis is to investigate whether the use of a bolus can compensate for this lost amount of the prescribed radiation dose, and so reduce the underdosage effect.

The compensatory bolus is created by performing surface scans of the head phantom with the Structure Sensor accessory and utilizing 3D printing technology. The experience of implementing 3D printing technology in clinical practice is discussed in this thesis, providing the basis of using this technology in the bolus manufacturing process instead of conventional methods.

To evaluate the impact of the compensatory bolus on dose distribution inside the head phantom, the approach of Monte Carlo (MC) simulations has been selected. The EGSnrc code system, with its two user codes, called BEAMnrc and DOSXYZnrc, is used. The model of the Xstrahl 100 kV x-ray treatment unit is built into MC code systems. The performance of the created MC model is verified by comparing the simulated PDD curves in the slab phantom with the experimentally obtained ones. A satisfactory agreement between the simulated and experimental PDD curves is reached within the discrepancy of 2 %, in the clinically relevant region.

At the final stage of this project, MC simulations are performed for two sets of CT images - the head phantom with the 3D printed compensatory bolus and the head phantom with no bolus. The difference of simulated dose distribution is compared between these two head phantoms when applicators with various aperture sizes are used. The presence of the bolus resulted in a shielding effect on the head phantom. The deposited dose in the underlying bone structure inside the head phantom is reduced by up to around 60 % when applicators with diameter sizes of 4 cm and 6 cm are used.

A general increase in the deposited dose at the surface of the head phantom with the compensatory bolus is observed compared to the case where the bolus is absent. However, taking into the consideration the statistical uncertainties of the performed simulations, this increase in the deposited dose is no larger than 1.5 %. Therefore, a well-defined and significant impact of the compensatory bolus on dose distribution at the surface of the head phantom is not observed, and its positive influence on the reduction of the underdosage effect cannot be classified.