Abstract
PurposeThe PTW microDiamond has an enhanced spatial resolution when operated in an edge‐on orientation but is not typically utilized in this orientation due to the specifications of the IAEA TRS‐483 code of practice for small field dosimetry. In this work the suitability of an edge‐on orientation and advantages over the recommended face‐on orientation will be presented.MethodsThe PTW microDiamond in both orientations was compared on a Varian TrueBeam linac for: machine output factor (OF), percentage depth dose (PDD), and beam profile measurements from 10 × 10 cm2 to a 0.5 × 0.5 cm2 field size for 6X and 6FFF beam energies in a water tank. A quantification of the stem effect was performed in edge‐on orientation along with tissue to phantom ratio (TPR) measurements. An extensive angular dependence study for the two orientations was also undertaken within two custom PMMA plastic cylindrical phantoms.ResultsThe OF of the PTW microDiamond in both orientations agrees within 1% down to the 2 × 2 cm2 field size. The edge‐on orientation overresponds in the build‐up region but provides improved penumbra and has a maximum observed stem effect of 1%. In the edge‐on orientation there is an angular independent response with a maximum of 2% variation down to a 2 × 2 cm2 field. The PTW microDiamond in edge‐on orientation for TPR measurements agreed to the CC01 ionization chamber within 1% for all field sizes.ConclusionsThe microDiamond was shown to be suitable for small field dosimetry when operated in edge‐on orientation. When edge‐on, a significantly reduced angular dependence is observed with no significant stem effect, making it a more versatile QA instrument for rotational delivery techniques.
Highlights
Quality assurance (QA) for small field radiotherapy is a challenging task requiring new detectors and QA methodologies
Codes of Practice (CoP) for conventional external photon beam radiotherapy are not suitable for small field dosimetry as they do not account for the lack of lateral charged particle equilibrium (LCPE) or occlusion effects.[1]
The IAEA TRS‐483 CoP for small field dosimetry recommends detectors used for QA should be: small relative to the minimum field size and the range of the secondary charged particles, have a high signal to noise ratio (SNR), high spatial resolution and be energy, dose rate, and angular independent in response
Summary
Quality assurance (QA) for small field radiotherapy is a challenging task requiring new detectors and QA methodologies. Codes of Practice (CoP) for conventional external photon beam radiotherapy are not suitable for small field dosimetry as they do not account for the lack of lateral charged particle equilibrium (LCPE) or occlusion effects.[1]. The IAEA TRS‐483 CoP for small field dosimetry recommends detectors used for QA should be: small relative to the minimum field size and the range of the secondary charged particles, have a high signal to noise ratio (SNR), high spatial resolution and be energy, dose rate, and angular independent in response. Perturbation is created due to the mismatch in stopping power ratios of the detector and its packaging materials relative to water which can lead to large variations in the detector response. Alfonso et al, presented a methodology where the detector response variation with field size can be corrected for by using a detector‐specific sensitivity correction factor, this assumes a certain detector orientation, angular independent response, and isocentric delivery.[9]
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