Dosimetry for ion beam radiotherapy

Abstract

Recently, ion beam radiotherapy (including protons as well as heavier ions) gained considerable interest. Although ion beam radiotherapy requires dose prescription in terms of iso-effective dose (referring to an iso-effective photon dose), absorbed dose is still required as an operative quantity to control beam delivery, to characterize the beam dosimetrically and to verify dose delivery. This paper reviews current methods and standards to determine absorbed dose to water in ion beam radiotherapy, including (i) the detectors used to measure absorbed dose, (ii) dosimetry under reference conditions and (iii) dosimetry under non-reference conditions. Due to the LET dependence of the response of films and solid-state detectors, dosimetric measurements are mostly based on ion chambers. While a primary standard for ion beam radiotherapy still remains to be established, ion chamber dosimetry under reference conditions is based on similar protocols as for photons and electrons although the involved uncertainty is larger than for photon beams. For non-reference conditions, dose measurements in tissue-equivalent materials may also be necessary. Regarding the atomic numbers of the composites of tissue-equivalent phantoms, special requirements have to be fulfilled for ion beams. Methods for calibrating the beam monitor depend on whether passive or active beam delivery techniques are used. QA measurements are comparable to conventional radiotherapy; however, dose verification is usually single field rather than treatment plan based. Dose verification for active beam delivery techniques requires the use of multi-channel dosimetry systems to check the compliance of measured and calculated dose for a representative sample of measurement points. Although methods for ion beam dosimetry have been established, there is still room for developments. This includes improvement of the dosimetric accuracy as well as development of more efficient measurement techniques.

Dosimetric characterization of a synthetic single crystal diamond detector in a clinical 62 MeV ocular therapy proton beam

“…Nonetheless, the superiority of diamond based detectors for absorbed-dose measurement in proton beams has not yet been fully nor reproducibly demonstrated [14,16,17]. Major limitations pointed out for both silicon and state-of-the-art diamond detectors were significant LET, dose rate and energy dependence [9,11]. Such undesired effects were not observed in a novel synthetic diamond dosimeter recently reported in the literature by some of the authors of the present paper [18].…”

“…A comprehensive review on state-of-the-art detectors for measurement of absorbed-dose in clinical proton beams is reported in ICRU Report 78 [9] and Ref. [11]. Low-energy narrow proton beams (60-75 MeV) transported by using passive beam-delivery techniques have been employed, since the beginning of proton therapy, for the treatment of choroidal melanomas.…”

“…Thanks to the high sensitivity per unit volume, small dimensions and high spatial resolution, solid state detectors such as silicon diodes and natural or first prototype synthetic diamond detectors have been reported as promising devices for relative proton dosimetry [9,[11][12][13][14][15][16][17]. The attractive physical and dosimetric properties of diamond make it the material of choice for dosimetric applications.…”

Evaluation of the dosimetric properties of a diode detector for small field proton radiosurgery

“…The average response from each measurement set was plotted versus accumulated dose normalized with respect to the initial baseline value. Finally, data were collected with the diode in two different orientations: the diode was positioned such that the beam was parallel to the detector axis (axial orientation), or perpendicular to the axis (edge‐on orientation) 10 , 19 (see Fig. 1), with the SV centered at 42 mm WED for each orientation for comparison.…”

Water equivalent properties of materials commonly used in proton dosimetry

“…Then WER will be calculated as: water or in modulator wheels, such as PMMA or polystyrene [Karger et al, 2010]. Also 20 in graphite calorimeters the conversion from dose-to-graphite to dose-to-water and 21 WER values are necessary for an accurate dosimetry [Palmans et al, 2004].…”