Session Item

In this study we quantify the accuracy of the day-to-day positioning in Ocular Proton Therapy using fiducials.

Treatment of an ocular patient with proton therapy consists of five stages: fiducial placement, simulation, treatment planning, dry run and treatment. Four Tantalum fiducials (2.5 mm diameter, 0.17 mm thickness) are used to localize the tumor perioperatively. During simulation, the patient is positioned in an upright position in a robotic chair and fixated with a bite block, a thermoplastic mask and strap to support the back of the head. The fiducials are located on two orthogonal x-ray images. Localization parameters, i.e. clip-tumor distances, fundus and ultrasound and MR images are used in the Eclipse Ocular Proton Planning system (Varian Medical Systems, Inc.) to create the treatment plan. Before treatment, the position verification is done by matching the reference fiducials images of the treatment plan to the x-ray images. The patient’s position is corrected by the robotic chair for translations only.

We investigated the day-to-day position variations based on the Tantalum fiducials position in the x-ray images just prior to the treatment. For this, analyses the four fiducials on the x-ray images are localized using ImageJ. The fiducial positions in the reference images created in the treatment planning system were extracted from the dicom file via a self-developed Python script. The day-to-day accuracy is calculated by comparing the center of gravity of the fiducials position of the x-ray images to the center of gravity of the fiducials found in the reference images. 60 consecutive patients were analyzed with a total of 240 fractions. From the collected data the mean deviation, random error and systematic error of the patient population are calculated.

The day-to-day statistics of the patient population are shown in table 1. As expected the mean deviation is around zero millimeters. The largest deviation is found in the longitudinal direction (AP-PA for the patient) both in mean deviation, random and systematic error and is caused by  tightening the strap at the back of the patient’s head. This is done manually for each fraction. Figure 1 shows the deviation in the patient positioning as a function of the percentage of the found deviations. Also this graphs shows larger deviations in the longitudinal direction.

Table 1: Day-to-day statistics of OPT patient population

Figure 1: Histogram of found positioning deviations in OPT. Image insert shows the patient’s reference axes.  

The found statistics will be used as an baseline accuracy in future work towards clipless treatment.