Atten Half 2

1 Once the setup was complete, we prepared the machine and took a total of 6 readings. The first three were completed with an open field of the left lateral beam of the patient’s treatment plan, followed by 3 more of the same field this time utilizing the wedge. It should be noted that the MLC block used to surround the target volume in the original treatment plan was removed for these measurements. The readings given by the electrometer in nanoCoulombs (nC) following dose delivery may be viewed in the table below. The results were then averaged for both the wedged and open field.

Fields Open Wedged

Trail 1 35.46 24.33

Charge (nC) Trial 2 35.5 24.35

Trial 3 35.51 24.35

Average 35.49 24.34

Using this data, we were then able to calculate our wedge factor. The equation to find the attenuation factor was simple, since we were only looking to find the ratio demonstrating difference in transmission between the wedged and open field.1 The equation and calculation may be seen below.

Wedgeattn = 24.34 / 35.49

Wedgeattn = 0.686

Now that this value was determined, it could be used in the equation to find the appropriate number of MUs used during the actual treatment for the left lateral beam. The equation developed by Task Group 71 to calculate the correct MU value for SAD treatments may be seen below.4,5

In order to find the prescribed dose, I had to first look at the original treatment and view the prescription. The total prescribed dose was 4500 cGy, delivered in increments of 180 centigray for 25 fractions. Because I was looking specifically at the LLAT field, I had to take into consideration the field weighting and the amount of dose being delivered by each beam. The

2 LLAT weight was 23.9%, therefore the amount delivered by the LLAT beam was .239 x 180 cGy, or 43.02 cGy. Dref was simply our calibration conditions and equaled 1. In the plan report I was able to find the collimator scatter factor (Sc), the phantom scatter factor (Sp), the tissue maximum ratio (TMR) values, which were 1.047, 0.990, and 0.675, respectively. The plan was normalized so that 100% of the dose would cover 95% of the target volume. This is not the same as the isodose line (IDL) definition used in the equation, but following Kristen’s advice I accounted for the normalization in the equation with a value of 0.95.2 Because the treatment was delivered at 100 cm SAD, I did not have to calculate the inverse square factor (ISF) and it remained at a value of 1. The calculation point was also at the isocenter, so there was also no need for an off axis ratio (OAR) correction. Furthermore, there was no tray factor (TF) so this was also omitted from the calculation. Finally, our wedge factor (WF) could be incorporated at a value of 0.686. I have inserted a photo of the plan report with the Field ID, equivalent square size, and Sc, Sp, and TMR values highlighted. This plan report also provided its own wedge factor, calculated to be 0.677, which was approximately 1.3% different than the value I had calculated. The photo of the beams eye view of the LLAT field may also be seen below. Following these documents is a photo of the calculation using the above variables to find the necessary MU value. This hand calculation yielded a value of 94.35 MUs while the treatment planning system yielded a value of 93.2 monitor units. This is only a 1.23% difference from the system’s value, well within the Task Group 71 standards of 5%.4 This discrepancy may be due to the difference of normalization style used in the treatment plan versus the IDL definition used in the calculation. It is also in part due to a dose of 43.9 cGy used in the TPS calculation, as greater than 100% of the dose was delivered at that point.

3

Figure 1. Photo demonstrating field data.

4

Figure 2. Photo showing beams eye views of treatment fields.

Figure 3. Photo of hand calculation performed and percent difference between calculated and planning system MU values.

5 For this patient, the use of this wedge allowed dose to be pushed anteriorly towards the center of the patient’s anatomy where coverage was difficult to attain due to the patient’s larger size. The thick heel of this wedge also worked to attenuate the beam and prevent excess dose delivery in areas where multiple beams intersected. While watching the treatment monitor during the QA session measuring the wedged field, we were able to note the change in dose rate while the machine simulated the wedge as well as the increase in time needed to complete the delivery in comparison to the open field. The following photos depict the dose rate change during the Wedged Field delivery.

6

Figure 4. Demonstration of dose rate variance during wedged field delivery.

Figure 5. Additional demonstration of dose rate variance during wedged field delivery.

For these reasons above, it is absolutely necessary to take into account the wedge factor in the calculation of monitor units. Had we not, the MU value would have decreased, specifically to 89.6. If delivery were dependent on monitor units alone, this decrease would have resulted in a

7 lesser amount of time in which the beam would be delivering radiation, and ultimately, the underdosing of the treated volume. Though we do not know exactly the consequences of an underdose for this particular patient, one may surmise the efficacy of the treatment would certainly be diminished solely as a result of a simple mistake.

8 References 1. Abling, C. Week Five: Beam Modifying Devices in Calculations. [SoftChalk]. La Crosse, WI: UW-L Medical Dosimetry Program; 2018. 2. McConnell, K. Attenuation Project. March 2019. 3. Saminathan S, Manickam R, Supe SS. Comparison of dosimetric characteristics of physical and enhanced dynamic wedges. Rep Pract Oncol Radiother. 2012;17(1):4–12. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3863232/. Accessed March 13, 2019. 4. Gibbons JP, Antolak JA, Followill DS, et al. Monitor unit calculations for external photon and electron beams: Report of the AAPM Therapy Physics Committee Task Group No. 71. https://www.aapm.org/pubs/reports/rpt_258.pdf. Published 2014. Accessed March 12, 2019. 5. Abling, C. Week Four: SAD Calculation Techniques. [SoftChalk]. La Crosse, WI: UW-L Medical Dosimetry Program; 2018.