Image-guided adaptive radiotherapy for nasopharyngeal carcinoma

Abstract

Nasopharyngeal carcinoma (NPC) is an endemic malignant disease in

Southern China. Intensity-modulated radiotherapy (IMRT) has been employed as a standard treatment for NPC because it delivers highly conformal dose

distribution to target volumes and spares organs at risk (OARs). The success of radiotherapy depends on the accurate delivery of the planned doses throughout the treatment. This can be achieved with the help of advanced image-guided adaptive radiotherapy (IGART) such as kilovoltage (kV) cone beam computed tomography (CBCT) which can reduce the geometric setup uncertainty, monitor the intra-course anatomic and dosimetric changes and adjust the treatment plan.

The aim of this thesis is to study the role of repeat imaging for NPC and the radiation dose from CBCT to patients. The objectives of this thesis are to evaluate the volumetric and dosimetric changes during a course of IMRT for loco-regionally advanced NPC patients with the contribution of repeat computed tomography (CT) and magnetic resonance imaging (MRI) scans; to quantify the absorbed dose, effective dose and the estimation of the additional risk of inducing fatal cancers from CBCT for NPC patients undergoing IMRT; and to compare the image quality of different head protocols.

Nineteen loco-regionally advanced NPC patients treated with IMRT were recruited prospectively. Repeat CT and MRI were acquired at 30 and 50 Gy

intervals. Recontouring of target volumes and OARs was based on the fused CT-MRI images. Hybrid plans with recontouring were generated. The volumetric and dosimetric changes were assessed by comparing the hybrid plans with the original plan. There was volume reduction of target volumes and parotid glands over the course of IMRT. Relative to the original plan, the hybrid plans demonstrated significantly higher dose to the target volumes with greater dose inhomogeneity, higher maximum doses to the spinal cord and brainstem, and higher medium doses to the parotid glands.

The image quality and dosimetry on the Varian CBCT system between software Versions 1.4.13 (“new” protocol) and 1.4.11 (“old” protocol) were studied. A calibrated Farmer-type ionization chamber and a standard cylindrical Perspex CT dosimetry head phantom were used to measure the weighted CBCT dose index (CBCTDIw) of the Varian CBCT system. The absorbed dose of different organs was measured in a female anthropomorphic phantom with thermoluminescent dosimeters (TLD) and the total effective dose was estimated according to ICRP Publication 103. The dosimetry and image quality were studied for head-and-neck region and comparison was made between the new and old protocols. The values of the CBCTDIw, absorbed dose, effective dose of the new head protocol were much lower than the old head protocol in each imaging group. The additional fatal cancer risk from daily CBCT might be up to 1.6%. In conclusion, replanning with repeat imaging at 30 Gy is essential to keep a satisfactory dose to the target volumes and avoid overdosing the OARs for NPC patients. The new Varian CBCT provides volumetric information for image guidance with acceptable image quality and lower radiation dose. This CBCT gives a better standard for NPC patient daily setup verification.