Detector response in the buildup region of small MV fields

Abstract

Purpose The model used to calculate dose distributions in a radiotherapy treatment plan relies on the data entered during beam commissioning. The quality of these data heavily depends on the detector choice made, especially in small fields and in the buildup region. Therefore, it is necessary to identify suitable detectors for measurements in the buildup region of small fields. To aid the understanding of a detector's limitations, several factors that influence the detector signal are to be analyzed, for example, the volume effect due to the detector size, the response to electron contamination, the signal dependence on the polarity used, and the effective point of measurement chosen. Methods We tested the suitability of different small field detectors for measurements of depth dose curves with a special focus on the surface‐near area of dose buildup for fields sized between 10 × 10 and 0.6 × 0.6 cm2. Depth dose curves were measured with 14 different detectors including plane‐parallel chambers, thimble chambers of different types and sizes, shielded and unshielded diodes as well as a diamond detector. Those curves were compared with depth dose curves acquired on Gafchromic film. Additionally, the magnitude of geometric volume corrections was estimated from film profiles in different depths. Furthermore, a lead foil was inserted into the beam to reduce contaminating electrons and to study the resulting changes of the detector response. The role of the effective point of measurement was investigated by quantifying the changes occurring when shifting depth dose curves. Last, measurements for the small ionization chambers taken at opposing biasing voltages were compared to study polarity effects. Results Depth‐dependent correction factors for relative depth dose curves with different detectors were derived. Film, the Farmer chamber FC23, a 0.13 cm3 scanning chamber CC13 and a plane‐parallel chamber PPC05 agree very well in fields sized 4 × 4 and 10 × 10 cm2. For most detectors and in smaller fields, depth dose curves differ from the film. In general, shielded diodes require larger corrections than unshielded diodes. Neither the geometric volume effect nor the electron contamination can account for the detector differences. The biggest uncertainty arises from the positioning of a detector with respect to the water surface and from the choice of the detector's effective point of measurement. Depth dose curves acquired with small ionization chambers differ by over 15% in the buildup region depending on sign of the biasing voltage used. Conclusions A scanning chamber or a PPC40 chamber is suitable for fields larger than 4 × 4 cm2. Below that field size, the microDiamond or small ionization chambers perform best requiring the smallest corrections at depth as well as in the buildup region. Diode response changes considerably between the different types of detectors. The position of the effective point of measurement has a huge effect on the resulting curves, therefore detector specific rather than general shifts of half th...

Simulation of consequences of using nonideal detectors during beam data commissioning measurements

“…Changes in the depth dose curves between the modifications 1 and 2 and the unmodified beam model as calculated in the treatment planning system (TPS) for a 10 × 10 cm 2 field. Additionally, response differences between two diodes, a PTW‐60012 and a photon field diode (PFD), relative to Gafchromic film are shown 35 …”

“…Changes of the surface dose due to modification 3, relative to the unmodified beam model for a 10 × 10 cm 2 field. Additionally, the response difference between a PTW‐60008 detector and Gafchromic film is shown 35 …”

Diamond Detectors for Radiotherapy X-Ray Small Beam Dosimetry

“…Besides the historical firm [28] it should be underlined that the number of vendors, such as [29][30][31] has increased in the past decade. The next important development is the growing diffusion of a commercial dosimeter, the PTW60019 microDiamond, which has become a reference dosimeter after many comparative studies with other types of dosimeters [32][33][34][35][36].…”

Spectral analysis of Monte Carlo calculated fluence correction and cema conversion factors for high-energy photon beams at different depths

“…A changed detector response for the determination of depth-dose curves or relative distributions for photon beams with ionization chambers is usually not taken into account and the correction factor k NR is assumed to be unity. There are also several studies [36,37] that reported differences in the detector response in the build-up region Frontiers in Physics frontiersin.org depending on the choice of the EPOM. The study of Pena et al [36] assumed that if a measurement of a percentage depth-dose curve is performed without a shift of EPOM, further depth-dependent conversion factors should be used accounting for the variations in the depth-dependent detector response.…”

“…The study of Pena et al [36] assumed that if a measurement of a percentage depth-dose curve is performed without a shift of EPOM, further depth-dependent conversion factors should be used accounting for the variations in the depth-dependent detector response. Wegener et al [37] could reduce the influence of different detector responses on relative measurements by using detector-individually derived effective points of measurement. This study showed that the non-reference correction factor k NR is depth-depending and varies likely by 3% for the Semiflex 3D and 2% for the SNC125c ™ due to different values of the specific fluence correction factors p i from constructive details and due to the depthdepending variations in the Spencer-Attix restricted stopping power ratios in an order of approximately 0.3% (Table 3).…”