Experimental determination of the effect of detector size on profile measurements in narrow photon beams

Abstract

The aim of this work is to investigate experimentally the detector size effect on narrow beam profile measurements. Polymer gel and magnetic resonance imaging dosimetry was used for this purpose. Profile measurements (Pm(s)) of a 5 mm diameter 6 MV stereotactic beam were performed using polymer gels. Eight measurements of the profile of this narrow beam were performed using correspondingly eight different detector sizes. This was achieved using high spatial resolution (0.25 mm) two-dimensional measurements and eight different signal integration volumes A X A X slice thickness, simulating detectors of different size. "A" ranged from 0.25 to 7.5 mm, representing the detector size. The gel-derived profiles exhibited increased penumbra width with increasing detector size, for sizes >0.5 mm. By extrapolating the gel-derived profiles to zero detector size, the true profile (Pt) of the studied beam was derived. The same polymer gel data were also used to simulate a small-volume ion chamber profile measurement of the same beam, in terms of volume averaging. The comparison between these results and actual corresponding small-volume chamber profile measurements performed in this study, reveal that the penumbra broadening caused by both volume averaging and electron transport alterations (present in actual ion chamber profile measurements) is a lot more intense than that resulted by volume averaging effects alone (present in gel-derived profiles simulating ion chamber profile measurements). Therefore, not only the detector size, but also its composition and tissue equivalency is proved to be an important factor for correct narrow beam profile measurements. Additionally, the convolution kernels related to each detector size and to the air ion chamber were calculated using the corresponding profile measurements (Pm(s)), the gel-derived true profile (Pt), and convolution theory. The response kernels of any desired detector can be derived, allowing the elimination of the errors associated with narrow beam profile measurements.

Dosimetric characterization and behaviour in small X-ray fields of a microchamber and a plastic scintillator detector

“…1 However, the required dose accuracy within small photon fields is difficult to achieve, as lateral electronic equilibrium breaks down and traditional detectors have limitations to represent the unperturbed dose distribution in water as accurately as possible. [2][3][4][5][6][7][8] As reported in literature, 9,10 this is mostly due to dose averaging over the finite size of the active volume and the nonwater equivalence of the materials that surround the active volume. With regard to volume-averaging effects, the commercial availability of detectors suitable for measurements of small fields down to the small field sizes mostly used in linear accelerators with multileaf collimator (MLC) reduces the importance of the effect.…”

Assessment of the setup dependence of detector response functions for mega‐voltage linear accelerators

“…Several investigators have proposed measurement techniques and corrections to approximate the profile acquired by an infinitesimally small ionization chamber with good signal to noise ratio, by accounting for the smearing effect introduced by large volume ionization chambers, such as the 0.13 cm 3 CC13 ͑Wellhöfer-Scanditronix, Bartlett, TN͒. These measurement techniques have included, but are not limited to, the use of small volume ionization chambers, 7 solid state detectors, 8 polymer gels, 6 and profile deconvolution. 9 Although all of these methods address the effect of chamber size, each introduces additional issues for data collection and assessment.…”

“…The degree of this effect is heavily dependent on the chamber design and tends to increase as a function of chamber volume. 5,6 Consequently, without consideration of the chamber volume effect such distortions will be included in the TPS beam modeling. Chamber motion, detector noise, setup, and mechanical accuracy can also introduce measurement uncertainty.…”

Analysis Of Small Field Dosimetry For 6 MV Photon Beams

“…Küçük alan foton ışınlarının rölatif doz faktörleri ölçümleri, kullanılan dedektörlerin yeterliliğine bağlıdır 9 . Pek çok çalışmada küçük alanlar için diyot ve diamonddedektörlerinin rölatif doz faktöründe en iyi sonuçları sağladığı ortaya konmuştur 10,12 . Hacim etkisi, farklı voksel boyutları kullanılarak ve bir voksel ile sıfır voksel arasındaki ortalama doz değeri tahmini olarak hesaplanmıştır.…”