Influence of storage temperature and humidity on entrance window deformations of phantoms for a horizontal beam geometry

This study examined the effect of storage temperature and humidity on the entrance window deformations of water phantoms used for reference dosimetry in a horizontal beam geometry. PTW 41023 and Qualita QWP-04 phantoms were investigated. Empty phantoms were first stored for approximately 12 h at (23 ± 1) °C and (50 ± 2)% relative humidity, and then stored for an additional 12 h either at different temperatures (approximately 15, 23, and 30 °C) with the relative humidity maintained at (50 ± 2)%, or at different relative humidities (approximately 20, 50, and 80%) with the temperature maintained at (23 ± 1) °C. Using a dial indicator, window deformations were monitored for 720 min at a room temperature of (23 ± 1) °C and relative humidity of (50 ± 2)%, immediately after the phantoms were filled. Storage humidity had little impact, whereas the maximum deformation difference associated with storage temperature was approximately 0.25 mm. Based on the observed deformation, this 0.25 mm difference was treated as a standard uncertainty component and converted into a relative standard uncertainty component in absorbed dose to water using sensitivity coefficients (0.2%-0.4%/mm) for high-energy photons. The resulting dose uncertainty was at most 0.1%, which is an order of magnitude smaller than the total combined standard uncertainty of reference dosimetry (1.0% according to TRS-398 Rev.1). The influence of storage temperature and humidity within the investigated ranges on the absorbed dose to water was negligible, suggesting that no additional strict control of these storage parameters is required for these phantoms in routine clinical practice.