Question
Why is it difficult to compare 2D array measurements with water phantom measurements?
Answer
In the context of comparing measurements using a 2D array in a solid phantom with water phantom measurements, there are several key factors that make direct comparison challenging or inaccurate:
Density Differences: Solid phantoms used in radiotherapy are often made of materials like ABS, PMMA or other plastics. Water phantoms, on the other hand, simulate the density and composition of human tissue more accurately. Since water is the standard medium for radiation dosimetry due to its tissue equivalence, measurements in water are considered more representative of human tissue. The density differences between solid phantoms and water can affect the behavior of radiation beams, leading to discrepancies in measurements.
Scattering and Absorption: Radiation beams interact differently with solid materials compared to water. Solid phantoms may cause increased scattering or absorption of radiation compared to water, leading to differences in the dose distribution within the phantom. These differences can affect the accuracy of measurements, especially when comparing dose profiles or dose distributions. Water phantoms also have more backscatter "material", so the phantom scattering is more accurate in water phantoms as in 2D arrays.
Electronic Disequilibrium: Solid materials can exhibit electronic disequilibrium effects, where secondary electrons generated by the primary radiation field are not in equilibrium with the primary electrons. This phenomenon can lead to differences in dose deposition patterns compared to water, particularly at the interfaces between different materials or tissues. Water phantoms are better at mimicking the electronic equilibrium conditions found in human tissue.
Beam Hardening: Depending on the energy spectrum of the radiation beam used in radiotherapy, solid materials may cause beam hardening effects, where lower energy photons are preferentially absorbed, leading to changes in the beam quality and dose distribution. Water phantoms provide a more uniform energy deposition due to their tissue equivalence, resulting in more accurate measurements.
Chamber/Diode Positioning: In 2D array detectors, the chambers (or diodes) are typically arranged in a grid pattern with a fixed distance between them. This spacing can affect the accuracy of dose measurements, especially in regions where the dose gradient is steep, such as in the shoulders of dose profiles. In regions of steep dose gradients, the dose changes rapidly over short distances. The fixed chamber distance in 2D arrays may not capture these variations as effectively as continuous measurement devices or water phantoms, which can result in differences between the measured dose profiles. This effect is particularly noticeable in regions where the dose changes rapidly, such as near the edges of the radiation field or in regions adjacent to critical structures. The distinct chamber distance in 2D arrays can introduce spatial averaging effects, where the measured dose values are averaged over the spacing between the chambers. This averaging effect may lead to differences in the measured dose profiles compared to water phantom measurements, where the dose is measured continuously and with higher spatial resolution.
Beam perturbation caused by the presence of the detector itself: In radiotherapy, the presence of detectors, including 2D arrays, can perturb the radiation beam, leading to changes in the dose distribution downstream of the detector. This perturbation effect can be more pronounced in solid phantoms where the detector is embedded compared to water phantoms where the detector is positioned in the water. The perturbation effect can alter the fluence of the radiation beam and may introduce inaccuracies in the measured dose distribution, especially in regions close to the detector. The magnitude of the perturbation effect depends on various factors, including the size and composition of the detector, as well as the energy and geometry of the radiation beam.