Technical Note: Influence of entrance window deformation on reference dosimetry measurement in various beam modalities.

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Technical Note: Influence of entrance window deformation on reference dosimetry measurement in various beam modalities.

Med Phys. 2018 Nov 30;:

Authors: Kinoshita N, Kohno R, Oguchi H

Abstract
PURPOSE: Phantoms for horizontal-beam geometry can avoid issues in vertical-beam geometry, such as change in chamber depth due to evaporation, and defining the origin at the water surface. However, their thin entrance-windows would deform when these phantoms are filled, which can change the chamber depth, as pointed out by IAEA TRS-398. Currently, few reports [Arib et al., J. Appl. Clin. Med. Phys. 7, 55-64 (2006), and Kinoshita et al., Rep. Pract. Oncol. Radiother. 23, 199-206 (2018)] are available with practical data on window deformation. Therefore, we investigated the influence of entrance-window deformation on chamber depths in water phantoms and the measurements in various beam modalities.
METHODS: To examine widely used phantoms and phantoms with different characteristics, three phantom types were investigated (the number of phantoms investigated appears in parentheses): PTW-type 41023 (2), Qualita-QWP-04 (2), and IBA-WP34 (2). Prior to the investigation, these phantoms were stored for acclimatization in a room for approximately 10 h under the following two conditions: (1) room temperature: 21 ± 2 °C; (2) room temperature: 27 ± 2 °C. Using a dial indicator, the centers of the windows were monitored every 30 min for 12 h immediately after the phantoms were filled, in a treatment room at the room-temperature of 21 ± 2 °C.
RESULTS: Immediately after the phantoms were filled, the window deformation ranged from -0.07 (inward-deformation) to 0.3 mm (outward-deformation) among the six phantoms, in comparison with empty phantom windows. For 12 h after the phantoms were filled, the change in the deformation was up to 0.23 mm, but typically less than 0.15 mm.
CONCLUSIONS: Reference dosimetry in photon, electron, and proton beams would not be influenced significantly by these window behaviors, whereas the window deformation has a slight impact on those in heavy-ion beams. This article is protected by copyright. All rights reserved.

PMID: 30500984 [PubMed - as supplied by publisher]

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