STRENGTHENING BRACHYTHERAPY PROGRAMS WITH ULTRASOUND QUALITY ASSURANCE

Thang Nguyen, Yen Tran and Venkata Seshadri, Epworth Radiation Oncology, Melbourne, Victoria, Australia

BACKGROUND At Epworth Radiation Oncology (ERO), Ultrasound (US) is employed for localisation of target anatomy and Organs At Risk that require delineation in Brachytherapy (BT) treatment planning. US is utilized in both Low Dose Rate and High Dose Rate Brachytherapy for prostate cancer and its application extends to real time guided cervix Brachytherapy and dosimetry. While BT US compliance is not mandatory in Australia, it is fundamental to best practice and has been implemented at ERO. PURPOSE ERO aims to strengthen its BT service through the implementation of a US Quality Assurance (QA) program that follows the recommendations from the American Association of Physics in Medicine Task Group 128: Quality Assurance Tests for Prostate Brachytherapy Ultrasound Systems (AAPM TG 128)

3 using the CIRS 045 Brachytherapy

QA Phantom (CIRS 045) (Fig. 1). The AAPM TG 128 recommendations were also performed in tap water and saline (0.9% w/v sodium chloride) phantoms with different acoustic properties compared to soft tissue, to test which recommendations can be validated and compared to the performance of CIRS 045.

METHOD AAPM TG 128 recommendations were tested using a BK Medical FlexFocus 800 ultrasound scanner, BK Medical Endocavity Biplane 8848 transducer attached to a CIVCO EX3 Stepper, the CIRS 045 and ultrasound gel for coupling. Using 5 x 18 gauge needles inserted through 2 x Template Grids for stabilization (Fig. 2), this configuration was submerged in the tap water and saline phantoms to perform the AAPM TG 128 tests. The lowest gain setting was used to minimise lateral and axial needle blurring

1.

Measurements were taken from the brightest (highest echo amplitude) portion of the first image blur pattern and repeated 5 times to establish baselines.

AAPM TG 128 test CIRS 045 phantom

Saline phantom

Water phantom

Action level from baseline

measures

Grey scale visibility* n/a n/a n/a 10% Depth of penetration Yes No No 10mm

Axial and lateral resolution

Yes Yes Yes 1.0mm

Axial and lateral distance

measurement accuracy

Yes Yes Yes Axial= 2.0mm Lateral= 3.0mm

Area measurement accuracy

Yes No No 5%from nominal

value

Volume measurement accuracy

Yes No No 5%from nominal

value

Needle template /electronic grid

alignment accuracy

Yes Yes Yes 3.0mm

Treatment planning computer

Yes No No Calculated volumes should agree

within 5%

RESULTS AAPM TG 128 tests performed using the BK Medical FlexFocus 800 US unit on the CIRS 045, Saline and Water phantoms.

Resolution Measures

Distance Measures

Needle Template/Electronic Grid Alignment

Grid

Coordinate Axial Resolution

(mm)

Lateral Resolution

(mm) Orientation Grid

Coordinates

Distance between needles*

(mm) Grid

Coordinate

Axial Displacement

(mm)

Lateral Displacement

(mm) Saline B, 5.0 0.6 7.3 axial B1-B5 42.6 B, 1.0 -0.3 -0.4

D, 3.0 0.6 3.2 F1-F5 42.5 B, 5.0 0.1 0

F, 1.0 0.6 1.6 lateral B1-F1 41.7 D, 3.0 0.4 -0.4 B5-F5 41.4 F, 1.0 0.1 -1.1 F, 5.0 0.5 -0.3

Tap Water B, 5.0 0.6 9.6 axial B1-B5 43.3 B, 1.0 0 0

D, 3.0 0.6 3.8 F1-F5 44.8 B, 5.0 1.2 1.2 F, 1.0 0.5 1.5 lateral B1-F1 43.4 D, 3.0 0.6 -1.1 B5-F5 44.3 F, 1.0 1 1.1 F, 5.0 1.2 0.6

CIRS 045 Phantom B, 5.0 0.7 3.76 axial B1-B5 39.8 B, 1.0 -0.1 -0.1

D, 3.0 0.56 2.92 F1-F5 39.8 B, 5.0 -0.1 0 F, 1.0 0.7 1.81 lateral B1-F1 39.6 D, 3.0 -0.2 0 B5-F5 39.7 F, 1.0 -0.2 -0.3 F, 5.0 0 -0.2

DISCUSSION The CIRS 045 made of Zerdine, with an acoustic velocity of 1540m/s matching that of soft tissue, performed all test recommendations with sub millimetre accuracy and consistency (Table 2). Without high contrast volumes with known geometries, the liquid phantoms could not register area, volume and planning system test measurements. The lack of contrast of the US in water meant depth of penetration could not be tested. As a result, the liquid phantoms could only perform 3 of the 7 recommendations (Table 1). The results measured from the 3 recommendations tested in the liquid phantoms could not match the performance of the CIRS 045. However the saline phantom gave more accurate results than the tap water phantom. The different acoustic properties to that of soft tissue resulted in varied focusing of the ultrasound beam

2, giving way to greater variation in test measurements, most notable

in the distance measurement tests (Table 2). With a greater acoustic velocity, the saline had increased performance over the tap water phantom, which did not meet baseline action level standards. This is consistent with Pfeiffer, Sutlieg, Feng, Pierce and Kofler recommendations for phantom solutions

3.

The differing acoustic properties also meant the water and saline phantoms could not reliably assess axial and lateral resolution. Nevertheless, once baseline measures have been recorded, the liquid phantoms can still be used to test for consistency i.e., precision rather than accuracy even in these properties

2.

CONCLUSION US quality control testing in Brachytherapy is not mandatory in Australia. However, to strengthen the quality of our Brachytherapy service, ERO have implemented CIRS 045 Brachytherapy QA Phantom which performs all AAPM TG 128 test recommendations. This QA will be conducted bi-annually to ensure results fall within action levels of the benchmark measurements presented. When a purpose built phantom is not available, one using saline is more reliable option than a phantom of tap water in performing US QA, however neither phantoms meet all AAPM TG 128 recommendations.

Table 1. AAPM TG 128 tests possible on each phantom *Grey scale visibility does not require a phantom, as it is ultrasound unit dependant.

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Excellence, Everywhere, Everyday

a) CIRS 045 Phantom b) Saline c)Tap Water

Table 2. AAPM TG 128 Benchmark Measurements *Needles should measure 40mm, with measurement results outside of baseline action

levels in red.

Figure 2. Ultrasound screen captures. a) CIRS 045 Brachytherapy QA Phantom with monofilaments, b) Saline phantom using lowest gain setting, c) Tap water phantom using lowest gain setting.

References: 1. Goldstein, A, Yudelev, M, Sharma, R, Arterbery E, 2002 ‘Design of quality assurance for sonographic prostate brachytherapy needle guides’, Journal Of Ultrasound in Medicine, vol. 21, pp 947-954

2. Goodsitt, MM, Carson, PL, Witt, S, Hykes, DL, Kofler, JM, 1998, ‘Real-time b-mode ultrasound quality control test procedures - a report of AAPM ultrasound task group no. 1’, Medical Physics, vol. 25, pp 1385-1406

3. Pfeiffer, D, Sutlief, S, Feng, W, Pierce, H, Kofler, J, 2008, ‘AAPM Task Group 128: Quality assurance tests for prostate brachytherapy ultrasound systems’, Medical Physics, vol. 35, December, pp 5471-5489

Figure 1)

Figure 2)

Figure 1. CIRS 045 Brachytherapy QA Phantom Figure 2. Tap water/saline phantom set up