A theoretical evaluation of transmission dosimetry in 3D conformal radiotherapy.

Abstract

Two-dimensional transmission dosimetry in radiotherapy has been discussed in the literature for some time as being a potential method for in vivo dosimetry. However, it still remains to become a wide spread practice in radiotherapy clinics. This is most likely due to the variety in radiotherapy treatment sites and the challenges they would present in terms of detection and interpretation at the transmitted dose level. Thus, the full potential and limitations of applying transmission dosimetry in the presence of dosimetry errors still need to be demonstrated.

This thesis is a theoretical evaluation of transmission dosimetry using the Pinnacle3 treatment planning system. The accuracy of predicting reliable and accurate absolute transmitted dose maps using the planning system dose algorithm for comparison with measured transmitted dose maps was initially investigated. The resolution in the dose calculations at the transmitted level was then evaluated for rectilinear and curved homogeneous phantoms and rectilinear inhomogeneous phantoms, followed by studies combining both surface curvature and heterogeneities using anthropomorphic phantoms. In order to perform transmitted dose calculations at clinically relevant beam focus-to-transmitted dose plane distances using clinical patient CT data it was first necessary to extend the CT volume. Finally, the thesis explored the efficacy of applying transmission dosimetry in the clinic by simulating realistic dosimetry errors in the planning system using patient treatment plans for a prostate, head and neck, and breast CRT (Conformal Radiotherapy) treatment. Any differences at the transmitted dose level were interpreted and quantified using the gamma formalism. To determine whether the transmitted dose alone was a sufficient indicator of the dosimetry errors, the magnitude in transmission dose differences were compared with those predicted at the midplane of the patient. Dose-Volume Histograms (DVHs) were also used to evaluate the clinical significance of the dose delivery errors on the target volume and surrounding healthy tissue structures.