Abstract
Purpose: Because of its small detector volume, the PinPoint ion chamber has been widely used for small field dosimetry measurements. This work investigates the limitations of the PinPoint chamber for cone‐based SRS commissioning and treatment dose verification. Method and Materials:Extensive measurements of PDD were performed in water using three types of ion chambers (0.015 cc PinPoint, 0.125 cc Semiflex and 0.65 cc Exradin), field sizes including one square field (10 × 10 cm2), and five cone‐shaped circular fields (4, 3, 2, 1.5 and 1.0 cm, respectively, in diameter). Two orientations of ion chamber, the chamber axis parallel (vertical setup) or perpendicular (transverse setup) to the beam axis, were used for measurements in the 10×10 cm2 square field while in the circular fields, only the vertical setup is used for the chamber sensitive volume to be covered by the small radiation fields. In addition, Monte Carlo(MC) simulations were also performed to compare with the measurements. Results: Two different chamber orientations (transverse and vertical) gave identical PDDs for each type of ion chambers. Measurements between 0.125 cc and 0.65 cc chambers agree well for all field sizes.MC simulations showed agreement with measurements of 0.125 cc and 0.65 cc chambers for all the field sizes. However, as compared to the other two chamber measurements or MC calculations, the PinPoint chamber presented obvious over‐response beyond certain depths, depending on field sizes (e.g. beyond 4 cm for a 4 cm circular field and beyond 15 cm for a 1 cm circular field). Conclusions: Over‐response of the PinPoint chamber may lead to undesirable measurement uncertainties in small field dosimetry. One should be aware of this limitation when measuring TMR or PDD with the PinPoint chamber beyond certain depths for field sizes typically used in SRS commissioning and treatment dose verification.
Published Version
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