Attenuation Paper

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Felicia Chu DOS 522 Radiation Dose Calculations 3/1/2019 Enhanced Dynamic Wedge Transmission Factor Calculation

Objective:​ The following analysis of an enhanced dynamic wedge (EDW) will explore the measurement process of the transmission factor, demonstrate the impact of that factor on a monitor unit calculation, and consider clinical implications in a patient case study. Purpose: ​Dosimetrists often use enhanced dynamic wedges to obtain optimal dose distributions in treatment planning. Wedges create a gradual decrease in beam intensity with their sloping geometry, tilting the isodose curve at an angle horizontal to the central axis.¹ Enhanced dynamic wedges are created with the movement of the collimator leaves in the y direction, and eliminate the need for therapists to manually insert a wedge into the head of the machine.² As the beam traverses through the EDW, the moving leaves will attenuate the beam and reduce the dose to Dmax on the central axis. We measure a wedge transmission factor (WF) through the center of the wedge to correct for the presence of the EDW in the beam. This factor must be used in all monitor unit (MU) calculations to deliver the correct dose to patients. The wedge transmission factor is: WF = ​Dose with wedge in the beam path Dose without wedge Methods and materials:​ The linear accelerators at our clinic are calibrated at source-axis distance (SAD), at a depth of 10 cm, 10x10 cm field size, and 1 cGy/MU at Dmax for photon treatments. With Navneeth Hariharan, a physicist at Lahey Hospital, we measured output on a Varian 21EX using 6 and 15 megavoltage (MV) energies, a 10x10 cm field size, and 100 cm source-to-skin distance (SSD). We delivered 100 MU at a dose rate of 300 MU per minute to depth of 10 cm in a solid water build up phantom. Our clinic uses an Exradon A12 standard farmer type ionization chamber, and a Keithley electrometer to measure the accumulated charge in nanocoulombs (nC). We conducted 3 trials for both 6 and 15 MV energies, and used the

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average to calculate the wedge transmission factor. For comparison, we also measured charge with a 30 degree EDW in the beam path.

Results: Table 1 Energy

Readings without EDW (nC)

Readings with EDW (nC)

6 MV

16.090

8.921

16.081

8.922

16.031

8.910

18.253

11.410

18.241

11.390

18.220

11.382

15 MV

Table 2​: We then obtained averages of the charge for 3 trials for each energy, with and without the EDW to account for slight variations in measurements. We summed the results of each trial and divided by 3, the number of trials. Energy

6 MV

15 MV

Average of readings without

Average of readings with

EDW (nC)

EDW (nC)

16.090

8.921

16.081

8.922

+16.031

+8.910

48.202/3=​16.067 nC

26.753/3=​8.918 nC

18.253

11.410

3

18.241

11.390

+18.220

+11.382

54.714/3=​18.238 nC

34.182/3=​11.394 nC

Table 3​: The transmission factor is the ratio of the average measurement with the wedge to that without the wedge. Energy

Reading with EDW/Reading

Wedge Transmission Factor

without EDW 6 MV

8.918 nC / 16.067 nC =

.555

15 MV

11.394 nC / 18.228 nC =

.625

Discussion:​ The wedge transmission factor for the 6 and 15 MV beam was .555 and .625 respectively. The 30 degree EDW attenuates 45.5% of the primary 6 MV beam, and 37.5% of the 15 MV beam. The decrease in fluence occurs because the lead leaves forming the dynamic wedge partially absorb the beam as it shapes the isodose distribution. According to Khan¹, wedge filters impact beam quality by “preferentially attenuating lower energy photons (beam hardening), and, to a lesser extent, by Compton scattering which results in beam degradation (beam softening.)” Table results show that the wedge transmission factor is also dependent on beam energy. Additionally, the wedge transmission factor varies according to wedge angle, field size, and linear accelerator model.³ Ahmad et al² demonstrated that, unlike physical wedges, enhanced dynamic wedges are independent of depth. They measured variation in transmission factor at different depths to be clinically insignificant at under 2%. Clinical Application: ​In an isocentric spine treatment of L1-L5, the impact of the wedge transmission factor is quite apparent. The patient was prescribed 400 cGy per treatment for 5

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fractions to a total dose of 2000 cGy. The plan consists of an equally weighted oblique wedge pair of 15 MV. The following demonstration below shows a monitor unit calculation for each field with the wedge transmission factor. The wedge factor of .625 is the final number in the denominator.

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The image below demonstrates the calculation with identical conditions, minus the wedge factor from the calculation.

The addition of the EDW results in a 37.70% increase in monitor units to deliver the prescribed dose to the target volume. The monitor units are higher because the machine needs more time to deliver the prescription dose through the wedge than time to deliver the prescription

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dose without the EDW in the path. If the patient was treated with the planned 321 monitor units, but by mistake without the EDW in place, the patient would be overdosed. This mistake would be an immense clinical error. Fortunately, there are mechanic safety interlocks in place to prevent this from occurring in practice. The image below is a plan report of the L1-L5 case described. Both fields are the same.

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We typically use Radcalc as a second check that Eclipse, our treatment planning system, is reporting the correct dose. However, because Radcalc incorporates the wedge factor into the scatter rather than listing it independently, it is difficult to use for the purposes of this discussion. I’ve chosen instead to insert a table of a manual monitor unit calculation sheet.

Photon Monitor Unit Calculation Check (Lahey Health-Radiation Oncology) Calculation Site: L1-L5 Prescription Name

A L1-L5

Prescribed Dose

Dose per Treatment

(cGy)

(cGy)

2000

400

Fractions

5

Beam ID

A.A

A.B

Beam Description

RPO

LPO

Accelerator

Varian 21EX

Varian 21EX

Energy

15X

15X

Dose per Treat (cGy)

200

200

Normalization

100%

100%

Gantry

225

135

Collimator Angle

90

90

X1 (cm)

10

10

X2 (cm)

10

10

Y1 (cm)

10

10

8

Y2 (cm)

10

10

Wedge

30 EDW

30 EDW

Wedge Factor

.625

.625

Scp

1.057

1.057

Inverse Square

.879

.879

Cal. Output

1 cGy/MU

1 cGy/MU

Monitor Units (MU)

321

321

Below is a beam’s eye view of the ports with the EDW in.

RPO

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LPO

Conclusion:​ Enhanced dynamic wedges attenuate the beam as they create desirable dose distribution, and must therefore be accounted for in monitor unit calculations. The wedge transmission factor depends on wedge angle, beam energy, field size, and linear accelerator model.³ Overall, the wedge transmission factor is a major consideration in the delivery of correct dose to target volume. If a patient was treated with the planned monitor units incorporating the wedge but for some reason, without the wedge in place for actual beam delivery, he or she would be significantly overdosed. It is therefore critical to understand the impact of an enhanced dynamic wedge on dose delivery.

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References 1. Khan FM. ​Khans the Physics of Radiation Therapy.​ Philadelphia: Lippincott Williams and Wilkins; 2014. 2. Njeh CF. Enhanced dynamic wedge output factors for Varian 2300CD and the case for a reference database. Journal of Applied Clinical Medical Physics. https://aapm.onlinelibrary.wiley.com/doi/full/10.1120/jacmp.v16i5.5498. Published September 8, 2015. Accessed March 18, 2019. 3. Chang SX, Gibbons JP. Clinical Implementation of Non-Physical Wedges. ​AAPM​. 1999.