Evaluation of the applicability of pinpoint ion chambers for SRS dosimetric quality assurance

Abstract

The aim of the present study was to evaluate the applicability of a Pinpoint ion chamber for the measurement of the absolute dose for dosimetric quality assurance (QA) under the same conditions as are used for actual stereotactic radiosurgery (SRS). A PTW 31014 Pinpoint chamber with a active volume of 0.015 cm3 was used to measure the absolute doses of small beams. The PTW 60003 natural diamond detector was used as a reference dosimeter. A custom-made cylindrical acrylic phantom (15 cm diameter, 15 cm long) was produced to obtain measurements, and a noncoplanar arc plan was devised to deliver a prescription dose (15−25 Gy) to 80% of the maximum dose to the target in a single fraction by using the BrainLAB planning system. All irradiations were performed by using a Varian Clinac IX 6 MV equipped with a micro-multileaf-collimators (m3) designed by BrainLAB. The acceptability criterion used was a dose difference of less than 3%. The diameter of the target volume was considered the standard parameter in the present study and was used to divide the cases into two groups, that is, a ≤ 10 mm target diameter group (10 cases) and a > 10 mm target diameter group (13 cases). For the Pinpoint chamber and target diameters of ≤ 10 mm, dosimetric uncertainties of > 3% were seen in 4 of the 10 cases, and differences ranged widely from 0.7% to 4.85%. On the other hand, for the Pinpoint chamber and target diameters of > 10 mm all dose differences were less than 1.6%, and the mean discrepancy was 0.81%. A highly significant, but moderate, correlation between dosimetric uncertainties and all target diameters was observed for the Pinpoint chamber (R2 = 0.483, p 0.001). This result indicates that Pinpoint chambers exhibit a field-size dependency when used for SRS dosimetric QA. Based on the results of the present study, we conclude that the use of a Pinpoint chamber for verification of SRS dosimetric QA is unsuitable for all field sizes, but that it can be used to verify the delivered dose of clinical SRS photon beams given a target diameter of >10 mm. In addition, the use of Pinpoint chambers should be limited to a field size of 12 mm for SRS dosimetric QA, and cross checking with another type of dosimeter should be performed when Pinpoint chambers are used for SRS dosimetric QA.