Attenuation Paper

Fain 1 Maddisen Fain Attenuation Project March 1, 2018 Wedge Transmission Factor Calculation Objective: To determine the transmission factor for a 45 degree wedge in the path of the beam, and to include this wedge transmission factor (WF) in the monitor unit calculation for a patient’s treatment plan. Purpose: Physical and dynamic wedges are used to compensate for uneven patient surfaces and to improve the dose distribution of a treatment plan. Physical wedges, made of lead or steel, are placed in the path of the beam to attenuate it. Depending on the slope of the wedge used, the intensity across the beam gradually decreases and the isodose lines are tilted away from their normal position.1 Using a physical wedge, the amount of radiation under the thick part of the wedge (the heel) is decreased and the radiation under the thin part of the wedge (the toe) is increased,2 which causes the shift in the isodose lines. When a wedge is used, the output of the machine is decreased. This decreased output must be accounted for using the wedge transmission factor when calculating the monitor units needed for treatment. The wedge transmission factor (WF) is found by the ratio of the dose with the wedge and dose without the wedge:1 𝐃𝐨𝐬𝐞 𝐰𝐢𝐭𝐡 𝐰𝐞𝐝𝐠𝐞

WF= 𝐃𝐨𝐬𝐞 𝐰𝐢𝐭𝐡𝐨𝐮𝐭 𝐰𝐞𝐝𝐠𝐞 Methods and Materials: Measurements of the transmission factor for a 45 degree wedge were performed on the Varian Clinac 6EX Linear Accelerator at my facility. Using a solid water phantom with a pre-cut hole at 10 centimeters depth, a PTW micro chamber was inserted and connected into a Keithley 614 electrometer which measured the charges in nanocoulombs (nC). The measurements were taken with the collimator set to 90 degrees at a 100 cm source to axis distance (SAD) and 100 MU. Using this setup, two measurements without wedges were measured on the electrometer. Two readings were then performed with the addition of a 45 degree physical wedge placed in the “in-out” position, followed by two readings of the 45 degree wedge in the “left-right” position with the collimator moved to 180 degrees. The electrometer readings for each measurement were then used to determine the wedge transmission factor.

Fain 2 Results: Table 1: Electrometer readings with and without a 45° wedge, in nC No Wedge

45° In-Out

45° Left-Right

1st Exposure

346

174

173

2nd Exposure

346

172

174

Average

346

173

173.5 (= 174)

Table 2: Average of 45° Wedge readings, in nC 45° Wedge Average

(174 + 173) / 2

= 173.5

Table 3: 45° Wedge Transmission Factor Calculation Wedge Transmission Factor

173.5 / 346

= 0. 501

Discussion: From the data obtained, the wedge factor for a 45 degree physical wedge using a 6 MV photon beam is 0.501. Thus, when the 45 degree wedge is placed in the path of the beam, the wedge attenuates 49.9% of the beam and 50.1% of the beam is transmitted through the wedge. Without the wedge factor included in the MU calculations, the patient would be underdosed by about 50% for each field that uses the 45 degree wedge. Clinical Application: The use of a 45 degree wedge is seen in the treatment of a left breast. The treatment plan implemented two tangential fields and a field in field on the medial field, and a 45 degree wedge was used on the lateral field with the collimator at 180 degrees. The medial field was weighted 47.9%, the medial field in field was weighted 4.4%, and the lateral field with the 45 degree wedge was weighted 47.7%. The patient received 16 fractions of 266 cGy per day, for a total of 4256 cGy. Images 1 and 2 are included in order to show the comparison of the plan with and without the wedge. Image 1 shows the plan without the 45 degree wedge on the lateral field. Without the wedge, the hot spot is 121.1% of the maximum dose, and viewing a sagittal image of the patient shows the isodose lines shifted inferiorly. Image 2 shows the plan with the 45 degree wedge, and it has a decreased hot spot of 108.6% of the maximum dose. While this hot

Fain 3 spot is still located inferiorly, it has significantly decreased and the 100% isodose line now covers the tumor volume better than the plan without the wedge did. Image 1: Left breast treatment plan without 45° Wedge

Image 2: Left breast treatment plan with 45° Wedge

Fain 4 The measurements of the wedge factor were tested using hand calculations to determine the monitor units needed for the plan with the 45 degree wedge, which were then compared to the monitor units calculated by the treatment planning system. A hand calculation was also performed without the wedge factor in order to determine the change in monitor units that would occur if the wedge factor was not included in the treatment calculation. The monitor unit formula, along with the hand calculations with and without the wedge factor, are shown below in Image 3a and 3b. Image 3a: Monitor Units Hand Calculation with Wedge Factor

Image 3b: Monitor Units Hand Calculation without Wedge Factor

Fain 5 Comparing the monitor unit hand calculation using the wedge factor obtained in our measurements to the monitor units calculated by the treatment planning system showed a 1% difference, which is an acceptable discrepancy. This shows the wedge factor calculation found in this study correctly matches the wedge factor programmed in the treatment planning system. Comparing the monitor unit hand calculation with the 45 degree wedge factor to the monitor unit calculation without the wedge factor shows the patient would receive only 50% of the prescribed dose of the wedge field if the wedge factor was accidentally omitted from the monitor unit calculation. This absence of the wedge factor would result in the patient being under-dosed, highlighting the importance of including the wedge factor in monitor unit calculations. Most treatment planning systems have the wedge factor for every available wedge programmed into the calculation algorithms, but it is important to ensure that a wedge factor is included if you have to perform a hand calculation. Below, Image 4 shows the monitor units calculated by the treatment planning system. Image 4: Monitor Unit Calculation from Treatment Planning System

Fain 6 Conclusion: Using a 45 degree wedge in a treatment plan on the Varian Clinac 6EX Linear Accelerator causes approximately 50% of the radiation to be absorbed and not delivered to the patient. Due to this, it is important that the use of a wedge is compensated for in the monitor unit calculations using the measured wedge factor. It is also crucial that the wedge factor is programmed into the treatment planning system so it can be used in calculations. If a wedge factor is forgotten or incorrect, this could result in the patient being under- or over-dosed for the duration of their treatment.

Fain 7 References 1. Khan FM, Gibbons JP. Treatment Planning I: Isodose Distributions. In: Pine JW, Moyer E, eds. The Physics of Radiation Therapy. 5th ed. Philadelphia, PA: Lippincott Williams &Wilkins; 2014:170-194. 2. Bentel GC. Dose Determination for External Beams. In: Wonsiewicz MJ, Navrozov M, eds. Radiation Therapy Planning. 2nd ed. New York, NY: McGraw-Hill; 1996: 33-58.