Analysis of dose perturbation factors of a NACP-02 ionization chamber in clinical electron beams

On the wall perturbation correction for a parallel-plate NACP-02 chamber in clinical electron beams

PURPOSE

In recent years, several Monte Carlo studies have been published concerning the perturbation corrections of a parallel-plate chamber in clinical electron beams. In these studies, a strong depth dependence of the relevant correction factors (p(wall) and P(cav)) for depth beyond the reference depth is recognized and it has been shown that the variation with depth is sensitive to the choice of the chamber's effective point of measurement. Recommendations concerning the positioning of parallel-plate ionization chambers in clinical electron beams are not the same for all current dosimetry protocols. The IAEA TRS-398 as well as the IPEM protocol and the German protocol DIN 6800-2 interpret the depth of measurement within the phantom as the water equivalent depth, i.e., the nonwater equivalence of the entrance window has to be accounted for by shifting the chamber by an amount deltaz. This positioning should ensure that the primary electrons traveling from the surface of the water phantom through the entrance window to the chamber's reference point sustain the same energy loss as the primary electrons in the undisturbed phantom. The objective of the present study is the determination of the shift deltaz for a NACP-02 chamber and the calculation of the resulting wall perturbation correction as a function of depth. Moreover, the contributions of the different chamber walls to the wall perturbation correction are identified.

METHODS

The dose and fluence within the NACP-02 chamber and a wall-less air cavity is calculated using the Monte Carlo code EGSnrc in a water phantom at different depths for different clinical electron beams. In order to determine the necessary shift to account for the nonwater equivalence of the entrance window, the chamber is shifted in steps deltaz around the depth of measurement. The optimal shift deltaz is determined from a comparison of the spectral fluence within the chamber and the bare cavity. The wall perturbation correction is calculated as the ratio between doses for the complete chamber and a wall-less air cavity.

RESULTS

The high energy part of the fluence spectra within the chamber strongly varies even with small chamber shifts, allowing the determination of deltaz within micrometers. For the NACP-02 chamber a shift deltaz = -0.058 cm results. This value is independent of the energy of the primary electrons as well as of the depth within the phantom and it is in good agreement with the value recommended in the German dosimetry protocol. Applying this shift, the calculated wall perturbation correction as a function of depth is varying less than 1% from zero up to the half value depth R50 for electron energies in the range of 6-21 MeV. The remaining depth dependence can mainly be attributed to the scatter properties of the entrance window. When neglecting the nonwater equivalence of the entrance window, the variation of p(wall) with depth is up to 10% and more, especially for low electron energies.

CONCLUSIONS

The variation of the wall perturbation correction for the NACP-02 chamber in clinical electron beams strongly depends on the positioning of the chamber. Applying a shift deltaz = -0.058 cm toward the focus ensures that the primary electron spectrum within the chamber bears the largest resemblance to the fluence of a wall-less cavity. Hence, the influence of the chamber walls on the perturbation correction can be separated out and the residual variation of p(wall) with depth is minimized.

Extraction of depth-dependent perturbation factors for parallel-plate chambers in electron beams using a plastic scintillation detector

PURPOSE

This work presents the experimental extraction of the overall perturbation factor PQ in megavoltage electron beams for NACP-02 and Roos parallel-plate ionization chambers using a plastic scintillation detector (PSD).

METHODS

The authors used a single scanning PSD mounted on a high-precision scanning tank to measure depth-dose curves in 6, 12, and 18 MeV clinical electron beams. The authors also measured depth-dose curves using the NACP-02 and PTW Roos chambers.

RESULTS

The authors found that the perturbation factors for the NACP-02 and Roos chambers increased substantially with depth, especially for low-energy electron beams. The experimental results were in good agreement with the results of Monte Carlo simulations reported by other investigators. The authors also found that using an effective point of measurement (EPOM) placed inside the air cavity reduced the variation of perturbation factors with depth and that the optimal EPOM appears to be energy dependent.

CONCLUSIONS

A PSD can be used to experimentally extract perturbation factors for ionization chambers. The dosimetry protocol recommendations indicating that the point of measurement be placed on the inside face of the front window appear to be incorrect for parallel-plate chambers and result in errors in the R50 of approximately 0.4 mm at 6 MeV, 1.0 mm at 12 MeV, and 1.2 mm at 18 MeV.

Simulation of large x-ray fields using independently measured source and geometry details

PURPOSE

Obtain an accurate simulation of the dose from the 6 and 18 MV x-ray beams from a Siemens Oncor linear accelerator by comparing simulation to measurement. Constrain the simulation by independently determining parameters of the treatment head and incident beam, in particular, the energy and spot size.

METHODS

Measurements were done with the treatment head in three different configurations: (1) The clinical configuration, (2) the flattening filter removed, and (3) the target and flattening filter removed. Parameters of the incident beam and treatment head were measured directly. Incident beam energy and spectral width were determined from the percent-depth ionization of the raw beam (as described previously), spot size was determined using a spot camera, and the densities of the flattening filters were determined by weighing them. Simulations were done with EGSnrc/BEAMnrc code. An asymmetric simulation was used, including offsets of the spot, primary collimator, and flattening filter from the collimator rotation axis.

RESULTS

Agreement between measurement and simulation was obtained to the least restrictive of 1% or 1 mm at 6 MV, both with and without the flattening filter in place, except for the buildup region. At 18 MV, the agreement was 1.5%/1.5 mm with the flattening filter in place and 1%/1 mm with it removed, except for in the buildup region. In the buildup region, the discrepancy was 2%/2 mm at 18 MV and 1.5%/1.5 mm at 6 MV with the flattening filter either removed or in place. The methodology for measuring the source and geometry parameters for the treatment head simulation is described. Except to determine the density of the flattening filter, no physical modification of the treatment head is necessary to obtain those parameters. In particular, the flattening filter does not need to be removed as was done in this work.

CONCLUSIONS

Good agreement between measured and simulated dose distributions was obtained, even in the buildup region. The simulation was tightly constrained by independent measurements of parameters of the incident beam and treatment head. The method of obtaining the input parameters is described, and can be carried out on a clinical linear accelerator.

Treatment head disassembly to improve the accuracy of large electron field simulation

PURPOSE

The purposes of this study are to improve the accuracy of source and geometry parameters used in the simulation of large electron fields from a clinical linear accelerator and to evaluate improvement in the accuracy of the calculated dose distributions.

METHODS

The monitor chamber and scattering foils of a clinical machine not in clinical service were removed for direct measurement of component geometry. Dose distributions were measured at various stages of reassembly, reducing the number of geometry variables in the simulation. The measured spot position and beam angle were found to vary with the beam energy. A magnetic field from the bending magnet was found between the exit window and the secondary collimators of sufficient strength to deflect electrons 1 cm off the beam axis at 100 cm from the exit window. The exit window was 0.05 cm thicker than manufacturer's specification, with over half of the increased thickness due to water pressure in the channel used to cool the window. Dose distributions were calculated with Monte Carlo simulation of the treatment head and water phantom using EGSnrc, a code benchmarked at radiotherapy energies for electron scatter and bremsstrahlung production, both critical to the simulation. The secondary scattering foil and monitor chamber offset from the collimator rotation axis were allowed to vary with the beam energy in the simulation to accommodate the deflection of the beam by the magnetic field, which was not simulated.

RESULTS

The energy varied linearly with bending magnet current to within 1.4% from 6.7 to 19.6 MeV, the bending magnet beginning to saturate at the highest beam energy. The range in secondary foil offset used to account for the magnetic field was 0.09 cm crossplane and 0.15 cm inplane, the range in monitor chamber offset was 0.14 cm crossplane and 0.07 cm inplane. A 1.5%/0.09 cm match or better was obtained to measured depth dose curves. Profiles measured at the depth of maximum dose matched the simulated profiles to 2.6% or better at doses of 80% or more of the dose on the central axis. The profiles along the direction of MLC motion agreed to within 0.16 cm at the edge of the field. There remained a mismatch for the lower beam energies at the edge of the profile that ran parallel to the direction of jaw motion of up to 1.4 cm for the 6 MeV beam, attributed to the MLC support block at the periphery of the field left out of the simulation and to beam deflection by the magnetic field. The possibility of using these results to perform accurate simulation without disassembly is discussed. Phase-space files were made available for benchmarking beam models and other purposes.

CONCLUSIONS

The match to measured large field dose distributions from clinical electron beams with Monte Carlo simulation was improved with more accurate source details and geometry details closer to manufacturer's specification than previously achieved.