Dose delivery accuracy of visually gated SBRT : a stress test for clinical acceptance

Abstract

This thesis focused on determining the targeting and dose accuracy of four-dimensional radiotherapy. When treating tumors within anatomy affected by respiratory motion, it is crucial to account for the continuous changes in tumor position and shape to ensure precise dose delivery and minimize damage to surrounding healthy tissues.

A dynamic thorax phantom was employed with multiple dynamic motion patterns along with static control counterparts to quantify the effect of motion management. A visually gated respiratory motion management technique, involving an infrared camera tracking a marker block, was utilized to manage the dynamic motions of the phantom. The measurements were carried out at Helsinki University Hospital with dosimetric film placed within a custom-built insert for the dynamic thorax phantom. Custom dose interpolation software was programmed to interpolate the equivalent dose distribution from the radiotherapy treatment plan to the dosimetric film, and to calculate the dose difference and gamma analysis between the evaluated dose (measured film dose) and the reference dose (interpolated dose from the radiotherapy plan).

Key findings were that i) dose delivery inaccuracies can mostly be attributed to the underlying gating technique, where the target is irradiated when it is inside a user-selectable gating window, and ii) the accuracy (i.e., gamma passing rate) of four-dimensional radiotherapy/'dynamic & gated' delivery is less than six percentage points lower than three-dimensional conformal radiotherapy/'static & continuous' delivery, meaning that the visual gating method used has high accuracy.