Combination of dosimetry for targeted radionuclide and external beam therapies

Abstract

In this thesis, issues relevant to the combined use and dosimetry of external beam radiotherapy (EBT) and targeted radionuclide therapy (TRT) are addressed. Methodology was developed to allow a combination of dose information from these two modalities, employing the biologically effective dose (BED) to address the inequivalence of biological effect. It was found that use of this combined modality radiotherapy (CMRT) could result in increased conformality of treatments and reduced doses to normal tissues compared with dose delivery by EBT alone. A model was developed to investigate the radiobiological effects that may arise if the two therapies were delivered concurrently. It was found that under most clinical conditions, the relative temporal delivery of these two therapies is unlikely to significantly influence the overall radiobiological effect to the tumour at the cellular level. Synergistic effects may be more significant in normal tissues and for tumours with low values of . The accuracy with which single photon emission computed tomography (SPECT) can be quantified for TRT dosimetry applied to CMRT was assessed via an phantom imaging study. Good agreement was found between the modeled and experimental data, but recovery of the object by deconvolution was found to be unsuccessful due to the limited spatial resolution of the system and noise content in the image. may therefore be unsuitable for this application. The incorporation of intensity modulated radiotherapy (IMRT) into CMRT was investigated, exploring both physically and biologically based methods of prescribing a nonuniform EBT dose distribution according to the distribution of TRT uptake. This approach was found to be feasible with IMRT providing sufficient dose modulation. The nonuniform EBT prescription can often be reduced to the simpler uniform case if planned as a component of CMRT and the effects of delivery errors are taken into account. In general, it has been found that a combination of EBT and TRT allows normal tissue doses to be reduced compared with dose delivery by EBT alone, and more satisfactory doses in the target to be reached. Methodology developed is applicable both to biologically based radiotherapy planning and to the wider concepts of combined modality therapies.