Atten Half 1

Lauren Brandl 03/23/2019 DOS 522 Attenuation of a 45º Wedge Upon discussing various devices used during radiation treatments that may or may not purposefully diminish the strength of the radiation beam, I chose to analyze specifically the attenuation factor of a 45º enhanced dynamic wedge (EDW). I had recently created a plan for a patient set to undergo radiation treatment to the rectum and surrounding lymph nodes. The patient’s 3D plan used 3 fields to encompass the targeted volume. This included a left lateral (LLAT) field, a right lateral field, and a posterior to anterior field, with both left and right lateral fields utilizing a 45º EDW. In working with Kristen, a Physics resident at my clinical site, the Moores Cancer Center at University of California San Diego Health, we aimed to determine the attenuation factor of this wedge for the left lateral field during this patient’s radiation treatments. Wedges used during treatment were once physical, “hard” wedges made of steel, lead, or a combination of blocking material inserted into the head of the treatment machine to shield one area while pushing the dose to another.1 While blocking the dose, though, this material also had a tendency to scatter the radiation beam. Some of this scatter would inevitably reach the patient and deliver unwanted dose to superficial areas of the body near the wedge. This issue lead to the implementation of the EDW, which was simply multileaf collimator (MLC) motion. The MLCs are able to simulate the blocking of a physical wedge across a field by spending a greater amount of time closed and attenuating the beam where the thicker heel of the wedge would be and more time open where the thin toe of the wedge would have been. In this particular case, a 45º EDW was used, which would cause a shift in the isodose lines of a beam profile by that of 45º, due to differences in the length of time the field would be open or MLC blocked.1 To conduct our own measurement of the wedge factor that would later be used to calculate the monitor units (MU) needed to deliver the treatment, we anonymized the patient and set up a simulation in quality assurance (QA) mode. The reading was completed using a Farmer Chamber connected to an electrometer, which we were able to access in the treatment viewing room to record the readings. The session was conducted

on the same Varian 21 EX linear accelerator that was used for the patient’s treatments with the same beam energy of 15 megavolts (MV). At this energy of 15 MV, the linear accelerators at my clinical site are calibrated to deliver 1 centigray (cGy)/MU at 3 cm depth at 100 cm source to axis distance (SAD).2 Also taken into consideration during our setup was the depth at which the measurement should be taken. The reading was recorded at a 10 cm depth, given the definition of a wedge being defined as the angle of the isodose tilt at the intersection of the central axis at a depth of 10 cm.3 To emulate these parameters in our setup, we used a base of 10 centimeters of solid water, a material that mimics the properties of water but is simpler to manipulate than an entire water phantom. Kristen explained this base was necessary in reproduction of a clinical setting because in a human body, back-scatter from below will contribute to the total dose recorded at a point. By using a base of the same material we are able to replicate these scatter conditions that would not have been present if the reading was taken against the table. 2 In accordance with wedge definition, an additional 10 centimeters of solid water were placed above the chamber, and the chamber was fitted into a small hole halfway through the stack at the depth of 10 cm. The blocks were aligned to their marks at the laser point intersections, and the couch was moved so that the superior surface of the solid water block read 90 cm surface to source distance (SSD) placing the chamber at 100 cm SAD. Please see the photos below and captions beneath for a view of the setup and materials used.

Figure 1. Photo demonstrating laser alignment.

Figure 2. Photo demonstrating 10 cm depth of chamber placement.

Figure 3. Photo demonstrating 90 cm SSD.

Figure 4. Photo of Farmer Chamber used during testing.

Figure 5. Photo of electrometer used during testing.