Date Approved

8-9-2021

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

Allied Health Sciences

First Advisor

Lauren Rydquist

Academic Year

2020/2021

Abstract

The purpose of this study was to evaluate the effect of phantom size and level of roll on the percent change in position and percent of planned treatment volume (PTV) receiving prescription dose of breast phantoms. Due to their non-rigid physical properties, breasts being treated for radiation therapy are vulnerable to positional variability following a roll adjustment. Cone Beam Computed Tomography (CBCT) scans are commonly used to verify anatomical breast position, but they are not always reacquired after a roll correction is made. Treating a volume at a different position has the potential to compromise the intended dose distribution. Factors like roll amount and breast size can contribute to the extent of positional change. A quantitative assessment of these factors on breast phantoms may be helpful for clinics who are determining whether to use large roll corrections for patients receiving breast cancer radiation treatment. It can also assist clinics in deciding the amount of roll that should require re-imaging prior to radiation treatment. The study is a prospective analysis using 3 fabricated breast phantoms with volumes corresponding to 34D, 34E and 34F cups. Each phantom was scanned at a 0-degree baseline position. The phantoms were then scanned 5 times each with 3, 6 and 10 degrees of roll added. MIM was used to correct the orientation of each angled scan to match that of the baseline scan. Contours for the phantom externals of each angled scan were created on the orientation-adjusted data to isolate the settling motion for ease of statistical analysis. Differences in position were calculated with Raystation region of interest (ROI) algebra tool. The primary outcome measure was the percent change in the position. Secondly, a three dimensional (3D) parallel opposed photon plan was created for the phantoms on the baseline scan. The scan was recalculated for each angled scan’s phantom external contours. The change in percent of PTV volume covered by prescription dose was recorded as the secondary outcome measure. The results did not indicate statistically significant differences in the percent change in phantom position or percent change in PTV receiving prescription dose based on different sized phantoms. The results did, however, indicate statistically significant higher percent change in phantom position with larger roll angles. Statistically significant changes in PTV receiving prescription dose were only found between 3 and 6 degrees of roll. Nevertheless, there was only a 0.99% difference between medians which is not clinically significant. The amount of roll applied to a breast phantom has a significant impact on percent change of position. This may indicate that applying roll to a breast cancer patient can change the position of the affected breast. Whether applied manually or using a 6 degree of freedom (DOF) couch, re-taking CBCTs may be beneficial to ensure optimal breast position prior to radiation treatment. Even though this study did not conclude significant differences in percent change of PTV receiving prescription dose using a 3D tangent photon plan, further research should be conducted to explore the dosimetric impacts for proton plans.

Comments

Grand Valley State University

University of Toledo

Download

Included in

Anatomy Commons

Share

COinS