Comparison of microlion liquid filled ionisation chamber with Semiflex and Pinpoint air filled chambers

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Claude Mitchell
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1 Comparison of microlion liquid filled ionisation chamber with Semiflex and Pinpoint air filled chambers Rhys Carrington, Ysbyty Glan Clwyd, Bodelwyddan. MSc Medical Radiation Physics.

2 Small field dosimetry With use of small fields comes requirement for increased dosimetric accuracy. IPEM have recently published report on this specific issue [1]. Acknowledges that improvements in Linac technology encourage use of small field sizes. As a result dosimetric equipment is used that is originally intended for use in broader fields.

3 The volume effect & CAX normalisation Volume effect Volume effect an important consideration when using dosimeters in small fields. Use of detector of finite volume leads to volume averaging effect. CAX normalisation Dose profiles usually evaluated after performing CAX normalisation. CAX normalisation corresponds to multiplying whole curve by 1/0.95. Dose is consequently overestimated. Images from PTW small field dosimetry guide [2]

4 Dosimeters for small fields PTW have a range of dosimeters available. All have respective advantages and disadvantages.

5 The microlion chamber Produced by German manufacturer PTW Key features Waterproof liquid-filled chamber with superior spatial resolution designed for precise beam profile and depth dose (PDD) measurements in water phantoms. Liquid filling of isooctane has density roughly 1000 times greater than air as used in air filled chambers. Very small sensitive volume (0.002cm 3 ). Air filled Could it replace the semiflex as an allround chamber? Liquid filled

Difference of up to 5% dependent of orientation of microlion.

6 Penumbra width Measured at D max, 50mm and 100mm depth. Analysis microlion consistently measures smaller penumbra width at all depths and field sizes. Difference of up to 5% dependent of orientation of microlion. Largest percentage difference recorded of 35% between microlion and Semiflex chamber for 3x3cm field at 14mm depth. Percentage difference decreases with increasing depth. Due to steeper gradient of field at shallower depth. Overestimation of dose due to CAX normalisation observed for Semiflex chamber.

Output factors Calculated for all combinations of field sizes. Increasing difference in calculated output factor with decreasing field size.

7 Output factors Calculated for all combinations of field sizes. Increasing difference in calculated output factor with decreasing field size. Due to apparent widening of field that occurs at small field sizes. Becomes more pronounced with larger volume detectors. Results in drop in central axis dose.

8 2x2 2.5x2.5 3x3 3.5x3.5 4x4 5x5 7x7 10x10 15x15 20x20 25x25 30x30 35x35 Penumbra width (mm) 2x2 2.5x2.5 3x3 3.5x3.5 4x4 5x5 7x7 10x10 15x15 20x20 25x25 30x30 35x35 Penumbra width (mm) 2x2 2.5x2.5 3x3 3.5x3.5 4x4 5x5 7x7 10x10 15x15 20x20 25x25 30x30 35x35 Penumbra width (mm) Penumbra width at 100mm depth Larger field sizes Semiflex Pinpoint 4.00 microlion radial Looked at larger field sizes using Semiflex and microlion. Field size (cm) Penumbra width at 50mm depth See if microlion suitable for use at all field sizes. Would be advantageous not having to change chamber dependent on field size Semiflex Pinpoint microlion radial Field size (cm) microlion measures decreasing penumbra with increasing field size as depth decreases Penumbra width at 14mm 6.00 PTW recommend microlion be used for field sizes up to 20x20cm Semiflex Pinpoint microlion radial Field size (cm)

9 Discussion Similar work published by other groups. Andreoli et al [3] also found microlion recorded shorter penumbra width than Semiflex and Pinpoint 3D. One group [4] found microlion to give superior spatial resolution compared to diamond detector at fields of <3x3cm. Andreoli et al penumbra widths for 1x1cm field [3] Vienna group [5] have done similar investigations and not reporting similar penumbra findings at larger field sizes. They report measurements at 1x1cm, 10x10cm and 35x35cm field sizes.

10 Conclusions Found a difference between measurements taken with microlion and those taken with both the Semiflex and Pinpoint 3D chambers. Combining results with theory, and also published work, evidence suggests that microlion provides more accurate results at small field sizes of dose profiles and output factors. Above field sizes of 20x20cm, a combination of the effects of the flattening filter and microlion response to low energy a possible explanation for observed decrease in penumbra width. Makes microlion ideally suited to dosimetry of small fields. The microlion compliments the semiflex chamber, but is unlikely to replace it.

11 Acknowledgements and References Thank you for the opportunity to present at the All Wales Meeting. Thank you to Jaap for useful discussions and suggestions. Although busy at work in Glan Clwyd, special thanks to Graham Arden, without whom much of the work would not have been possible. [1] Asprakadis M.M et al (2010) Small field MV photon dosimetry, IPEM report number 103 Institue of Physics and Engineering in Medicine. [2] PTW (2011) When small things matter: Small field dosimetry application guide. [3] Andreoli S et al (2011) Dosimetric evaluation of small field radiotherapy photon fields: Micro liquid-filled ionisation chamber compared with conventional detectors [4] [5] - Vienna group

12 Diolch yn fawr! Any questions?

Detector comparison for small field output factor measurements with flattening filter free photon beams

Detector comparison for small field output factor measurements with flattening filter free photon beams 1 Wolfgang Lechner, 1 Lukas Sölkner, 2 Paulina Grochowska, 1 Dietmar Georg 1 Christian Doppler Laboratory