Advances in the determination of absorbed dose to water in clinical high-energy photon and electron beams using ionization chambers

Abstract

During the last two decades, absorbed dose to water in clinical photon and electron beams was determined using dosimetry protocols and codes of practice based on radiation metrology standards of air kerma. It is now recommended that clinical reference dosimetry be based on standards of absorbed dose to water. Newer protocols for the dosimetry of radiotherapy beams, based on the use of an ionization chamber calibrated in terms of absorbed dose to water, N(D,w), in a standards laboratory's reference quality beam, have been published by several national or regional scientific societies and international organizations. Since the publication of these protocols multiple theoretical and experimental dosimetry comparisons between the various N(D,w) based recommendations, and between the N(D,w) and the former air kerma (NK) based protocols, have been published. This paper provides a comprehensive review of the dosimetry protocols based on these standards and of the intercomparisons of the different protocols published in the literature, discussing the reasons for the observed discrepancies between them. A summary of the various types of standards of absorbed dose to water, together with an analysis of the uncertainties along the various steps of the dosimetry chain for the two types of formalism, is also included. It is emphasized that the NK-N(D,air) and N(D,w) formalisms have very similar uncertainty when the same criteria are used for both procedures. Arguments are provided in support of the recommendation for a change in reference dosimetry based on standards of absorbed dose to water.

Impact of Beam Quality Q for 137Cs on NK and ND,W for Different Types of Ionization Chambers

“…This fact has also been noted by other authors (Bjerke H. and Hult A., 2004) and (Huq M. and Andreo P., 2004).…”

Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration

“…A priori , one of these improvements is a reduction in the uncertainty during determination of the absorbed dose to water. However, a recent publication, (7) which is also analyzed in the present work, has questioned this argument.…”

“…The uncertainty value provided by Ciemat in $N_K$ for the NE 2571 chamber is 0.45%. The remaining parameters that enter into the determination of $N_{D,\text{air}}$ and of the components associated with the perturbation factors in the user beam $p_Q$, and to the water–air stopping power ratio $s_{w,\text{air}}$, were taken from Huq and Andreo (7) . Thus the uncertainty in $N_{D,\text{air}}$ can be set for a cylindrical chamber at 0.8%.…”

“…Recent estimates have yielded a typical uncertainty value in this step of the process of between 1.4% and 2.1% ( 6 – 8 ) —the largest contribution to the uncertainty being associated with the factor $k_{Q,Q_0}$. ( 7 , 8 ) …”

Estudo e desenvolvimento de dosímetros luminescentes opticamente estimulados de a-Al2O3 com nanoestruturas de carbono e óxidos terras-raras para aplicação em radioterapia

“…Em Radioterapia, a dose é verificada através de métodos protocolados de controle da qualidade e do uso de dispositivos, os chamados dosímetros, com características adequadas, entre elas sensibilidade, reprodutibilidade e repetitividade e baixo desvanecimento do sinal dosimétrico (HUQ et al, 2004). Alguns dos dosímetros mais utilizados na prática clínica são as câmaras de ionização, utilizada como dosímetro de referência por protocolos internacionais (HUQ et al, 2004;HSU et al, 2010), os filmes e os dosímetros termoluminescentes (TL).…”