ESTRO 2022 - Abstract Book

S1269

Abstract book

ESTRO 2022

The geometric validation tests showed a good correspondence between the warped and ground truth contours. The average difference in distance was found to be on the order of the thickness of a CT slice ( mm ). A quadratic function was calculated as the best fit for the changes in parotid gland volume. In comparison, all other structures were best parameterised with a linear fit (high and low dose CTV, body). Structures can therefore be differentiated depending on their rate of volume change during treatment: constant or variable. Figure 1 shows the (a) individual and (b) mean fractional parotid gland volume changes. Figure 2 shows the average volume changes at the end of treatment (day 35), compared to the start of treatment, for all structures. Both figures indicate a broad patient variability within the population, especially in the final parotid gland volumes.

Conclusion H&N cancer patients are subject to large anatomical changes during fractionated radiotherapy. For most patient structures the volume decreases during treatment. The rate of volume change over treatment however, is structure and patient dependent. Therefore, complex models are needed to account for the patient specific variability.

PO-1493 Comparison of CBCT Hounsfield Unit to density conversion methods for treatment plan evaluation