Air density dependence of the PTW 34013 ionization chamber for soft X-ray

Abstract

Introduction It has been proved that some ionization chambers, when measuring low energy brachytherapy or soft X-ray sources, show a dependence with the air density that it is not entirely corrected with the usual pressure/temperature correction factor, k TP . This is due to the chamber dimensions and materials forming it. Purpose To determine if the response of PTW 34013 chamber, used for dose measurements in superficial radiotherapy, is fully corrected with k TP or a residual air density dependence remains. Materials and methods Measurements for 70 and 200 kVp X-ray beams, generated by a MAXISHOT.200 cabinet, have been performed using a pressurized chamber. Raw data have been corrected with k TP and normalized to its value in standard atmospheric conditions (760 mmHg and 20 °C). Results The corrected measurements for both beams still show a linear dependence with the normalized density ρ / ρ 0 , ρ 0 being the density in standard atmospheric conditions. When the air density is below ρ 0 , k TP undercorrects the measurement, and the contrary occurs when ρ > ρ 0 . In our city, with 700 m AMSL, usual atmospheric conditions will cause 1.2% undercorrection for the 70 kV beam and 2.5% for the 200 kV beam. Conclusion Despite its dimensions, the dependence of the PTW 34013 ionization chamber with the air density is not fully corrected with k TP . The weather or the altitude of the place where measurements are carried out will require an additional correction to reach the accuracy level required in radiotherapy. Disclosure We thank to PTW for material support.