ABSTRACT
The tissue- or water-equivalence of dosimetry phantoms used as substitutes for water is essential for absorbed dose measurements in radiotherapy. At our institution, a heterogeneous pelvic phantom that consists of stacked Nylon-12 layers has recently been manufactured for Gafchromic film dosimetry. However, data on the use of Nylon as tissue-mimicking media for dosimetric applications are scarce. This study characterizes the water-equivalence of Nylon-12 for dosimetric measurements in therapeutic photon and electron beams. Employing an Elekta Synergy and SL25 linear accelerator (Linac), photon beam transmission measurements for 6â¯MV and 15â¯MV, acquired in narrow beam geometry with a 0.6â¯cm3 Farmer-type ion chamber showed that the mass attenuation coefficient µm of Nylon-12 agrees with the values of water, water-equivalent RW3 and Perspex phantom materials within 3%. For 6â¯MV, the µm values were 0.0477⯱â¯0.002â¯cm2/g, 0.0490⯱â¯0.003â¯cm2/g, 0.0482⯱â¯0.001â¯cm2/g and 0.0479⯱â¯0.002cm2/g for Nylon-12, water, RW3, and Perspex, respectively. Differences within 2% were attained between depth dose data measured in Nylon-12 slabs with Gafchromic EBT3 films and in water with a Roos ion chamber for 10â¯×â¯10â¯cm2 6, 12 and 20â¯MeV electron beams produced by the Elekta Synergy and SL25 Linacs. Also, a good agreement within 2% was obtained between percent depth doses computed by DOSXYZnrc Monte Carlo simulations in water, Nylon-12 and RW3 materials for photon spectra between 250â¯kV and 15â¯MV. The discrepancies between the ratios of average, restricted stopping powers of Nylon to air and water to air for photon spectra ranging from 2 to 45â¯MV are typically within 1% signifying that Nylon and water have equivalent stopping power characteristics. This study highlights that Nylon-12 can be used as a tissue-mimicking phantom material for dosimetric measurements in clinical megavoltage photon and electron beams as it exhibits good water-equivalence.
Subject(s)
Nylons/chemistry , Phantoms, Imaging , Electrons , Humans , Photons , Water/chemistryABSTRACT
AIM: In this study, the egs_cbct code's ability to replicate an electronic portal imaging device (EPID) is explored. BACKGROUND: We have investigated head and neck (H&N) setup verification on an Elekta Precise linear accelerator. It is equipped with an electronic portal imaging device (EPID) that can capture a set of projection images over different gantry angles. METHODS AND MATERIALS: Cone-beam computed tomography (CBCT) images were reconstructed from projection images of two different setup scenarios. Projections of an Anthropomorphic Rando head phantom were also simulated by using the egs_cbct Monte Carlo code for comparison with the measured projections.Afterwards, CBCT images were reconstructed from this data. Image quality was evaluated against a metric defined as the image acquisition interval (IAI). It determines the number of projection images to be used for CBCT image reconstruction. RESULTS: From this results it was established that phantom shifts could be determined within 2â¯mm and rotations within one degree accuracy using only 20 projection images (IAIâ¯=â¯10 degrees). Similar results were obtained with the simulated data. CONCLUSION: In this study it is demonstrated that a head and neck setup can be verified using substantially fewer projection images. Bony landmarks and air cavities could still be observed in the reconstructed Rando head phantom. The egs_cbct code can be used as a tool to investigate setup errors without tedious measurements with an EPID system.