Determination of recombination and polarity correction factors,kS and kP,for small cylindrical ionization chambers PTW 31021 and PTW 31022 in pulsed filtered and unfiltered beams

The aim of this technical communication is to provide correction factors for recombination and polarity effect for two new ionization chambers PTW PinPoint 3D (type 31022) and PTW Semiflex 3D (type 31021). The correction factors provided are for the (based on the) German DIN 6800-2 dosimetry protocol and the AAPM TG51 protocol. The measurements were made in filtered and unfiltered high-energy photon beams in a water equivalent phantom at maximum depth of the PDD and a field size on the surface of 10 cm × 10 cm. The design of the new chamber types leads to an ion collection efficiency and a polarity effect that are well within the specifications requested by pertinent dosimetry protocols including the addendum of TG-51. It was confirmed that the recombination effect of both chambers mainly depends on dose per pulse and is independent of the filtration of the photon beam.     

Monte Carlo calculation of perturbation correction factors for air-filled ionization chambers in clinical proton beams using TOPAS/GEANT

IntroductionCurrent dosimetry protocols for clinical protons using air-filled ionization chambers assume that the perturbation correction factor is equal to unity for all ionization chambers and proton energies. Since previous Monte Carlo based studies suggest that perturbation correction factors might be significantly different from unity this study aims to determine perturbation correction factors for six plane-parallel and four cylindrical ionization chambers in proton beams at clinical energies.Materials and methodsThe dose deposited in the air cavity of the ionization chambers was calculated with the help of the Monte Carlo code TOPAS/Geant4 while specific constructive details of the chambers were removed step by step. By comparing these dose values the individual perturbation correction factors $p_{\text{cel}}$, $p_{\text{stem}}$, $p_{\text{sleeve}}$, $p_{\text{wall}}$, $p_{\text{cav}}\cdot p_{\text{dis}}$ as well as the total perturbation correction factor $p_Q$ were derived for typical clinical proton energies between 80 and 250 MeV.ResultsThe total perturbation correction factor $p_Q$ was smaller than unity for almost every ionization chamber and proton energy and in some cases significantly different from unity (deviation larger than 1%). The maximum deviation from unity was 2.0% for cylindrical and 1.5% for plane-parallel ionization chambers. Especially the factor $p_{\text{wall}}$ was found to differ significantly from unity. It was shown that this is due to the fact that secondary particles, especially alpha particles and fragments, are scattered from the chamber wall into the air cavity resulting in an overresponse of the chamber.ConclusionPerturbation correction factors for ionization chambers in proton beams were calculated using Monte Carlo simulations. In contrast to the assumption of current dosimetry protocols the total perturbation correction factor $p_Q$ can be significantly different from unity. Hence, beam quality correction factors $k_{Q,Q_0}$ that are calculated with the help of perturbation correction factors that are assumed to be unity come with a corresponding additional uncertainty.     

Studies with encapsulated 125I sources. i. apparatus and dosimetry for determination of relative biological effectiveness

Apparatus and dosimetry techniques have been developed which make possible studies of the biological effects of radiation from encapsulated ¹²⁵I sources at clinically relevant dose rates using mammalian cells attached to culture dishes. The variation of dose rate from ¹²⁵I photons as a function of distance from the interface between different materials was investigated. A polystyrene substrate changes the mean dose rate in attached cells by about 21 %, depending on cell thickness. To reduce dosimetry uncertainty caused by this effect, special petri dishes were made from polyvinylidene fluoride, which changes the mean dose in attached cells by only 10 %. Chinese hamster ovary cells attached to these dishes were cultured in incubators which contained ¹²⁵I and ¹³⁷Cs sources, allowing the effects of various dose rates (.005 to 0.80 Gy/hr) of radiation from the two isotopes to be compared. The relative dose rates from these low- and high-energy photons were measured with an accuracy of ± 7 % or better using an air-equivalent ionization chamber designed to resemble one of our special petri dishes. Calculations of dose rates from ¹²⁵I give values within 4% off the measured dose rates used to determine the relative biological effectiveness of ¹²⁵I photons.