Dosimetric evaluation of the RSCAN PP20 prototype plane-parallel ionization chamber in electron and kilovoltage x-ray beams.

The RSCAN PP20 prototype plane-parallel ionization chamber was evaluated for electron beam dosimetry and medium-energy kilovoltage x-ray beam measurements. This study investigated its response constancy, linearity, depth-ionization characteristics, voltage and polarity dependence, and signal stability in water using a 6 MeV electron beam and selected kilovoltage x-ray beam qualities. Additional time-resolved measurements were performed to assess dark-field charge transients, post-irradiation charge relaxation, in-beam current transients, and cable-related parasitic currents, with a PTW Advanced Markus chamber used as a reference-control detector. In the 6 MeV electron beam, the RSCAN PP20 prototype produced physically plausible depth-ionization curves but showed insufficient quantitative stability. The coefficient of variation reached 1.57%, and the relative range reached 5.10%. The apparent linearity deviation reached 16.8% when background compensation was performed only once per measurement series, but decreased to 0.64% when repeated before each measurement. A polarity correction factor of approximately 0.91 was obtained across the tested voltage range, indicating measurement-chain asymmetry rather than a conventional intrinsic polarity effect. Time-resolved recordings revealed dark-field charge transients, post-beam charge accumulation, and mechanically induced bipolar current spikes, consistent with parasitic currents and possible cable-related triboelectric effects. Fixed-depth measurements in kilovoltage x-ray beams showed better short-term stability, whereas depth-response scans demonstrated cable-dependent distortions. The main limitation of the current RSCAN PP20 prototype is not its basic plane-parallel geometry, which is close to established clinical designs, but instability of the prototype chamber-cable assembly within the tested measurement setup. The prototype is not yet suitable for routine clinical dosimetry in its current configuration and requires hardware improvements followed by comprehensive recharacterization.