Room: Exhibit Hall
Purpose: Water phantoms for horizontal-beam geometry are used in radiotherapy clinics. These phantoms have the advantage of accurate detector positioning, although users must take care with the outward deformation of their entrance windows due to water pressure on the inner surface. This effect can change the chamber depth in the phantoms. However, few reports are available with practical data on window deformation. In this work, we investigated the influence of phantom window deformation on chamber depth in the phantoms.
Methods: Three phantom types were investigated (number of phantoms investigated appear in parentheses): PTWâ€”type 41023 (2), Qualitaâ€”QWP-04 (2), and IBAâ€”WP34 (2). Prior to the investigation, these phantoms were stored in a room for approximately 10 h under two conditions: (1) room-temperature of approximately 22 Â± 2 Â°C, and (2) room-temperature of approximately 27 Â± 2 Â°C. Using a dial indicator, the center of the phantom window was monitored every 30 min for 12 h right after the phantom was filled, in a treatment room at 22 Â± 2 Â°C.
Results: When the phantoms were just filled, the deformation levels ranged from -0.07 (inward deformation) to 0.3 mm (outward deformation) among the six phantoms. After 12 h, the change in deformation was generally less than 0.15 mm and was always less than 0.23 mm. The window deformation depended on the phantom types, phantom-to-phantom of the same type, time elapsed after phantom was just filled, and the temperature of the storage room.
Conclusion: The change in ion chamber depth in the phantoms was up to 0.4 mm from a desired depth because of their window deformation.
Phantoms, Deformation, Absolute Dosimetry
TH- External beam- photons: Standard field experimental dosimetry