Departmental Oral Examination (Thesis Title: "Assessment of spatially inhomogeneous radiation dose response in the parotid gland")

Event Date and Time: 
Fri, 2017-01-06 16:00 - 18:00
Henn 309
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 

Cancers treated with radiotherapy must be adequately irradiated to suppress growth at the site of origin. To both achieve doses high enough to attain `local control' and inhibit growth of secondary cancers, surrounding normal tissues are selectively co-irradiated. Current clinical practice for head-and-neck cancers involve irradiation of salivary glands. Threshold doses that minimize adverse induced toxicities are currently based on whole-organ mean dose. Modern radiation delivery techniques are able to sculpt the dose profile to accommodate sub-organ irradiation, but knowledge of the relative importance of sub-organ structures remains unknown. As tissue-sparing techniques improve, assessment of the normal tissue irradiation toxicity risk becomes increasingly important.

Loss of salivary function and xerostomia (subjective dry mouth) are the most common normal tissue complications in head-and-neck cancer patients. Radiotherapy-induced dysfunction and xerostomia can, in severe cases, drastically reduce oral hygiene and health and may negatively impact the ability to eat, speak, sleep, or swallow. These pervasive toxicities negatively detract from overall quality of life and can be permanent, presenting a lasting negative psychological impact.

The purpose of this work is to characterize and quantify the relative importance of spatial regions within the major salivary glands on late salivary function (i.e., `regional effects'). The ultimate aim is to improve knowledge of complication risk. Various studies have shown effects that indicate salivary gland mean dose models may be insufficient to fully characterize toxicity risk. Broad regional effects have been noted in rat parotid, and it has recently been claimed that a localized `critical region' has been located in human parotid glands. Furthermore, a morphological dependence on the dose profile has been noted, but only for subjective xerostomia. Clinical trials involving specific lobe and sub-structure sparing are underway, yet comprehensive quantification of the relative importance of sub-organ structures remains unknown.

To this end, the association between radiation dose delivered to subsegments of the parotid and whole mouth salivary flow measurements is assessed. Two independent analysis procedures are developed that are capable of quantifying the relative importance of sub-segments. Using these procedures, evidence is found that subsegments are inhomogeneously important for maintenance of late salivary flow, with the caudal-anterior aspects having greatest importance. A secondary imaging protocol is developed which may help pinpoint specific tissues or functional units responsible within these regions.

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