Determination of the photon spectrum of a therapeutic linear accelerator near the maze entrance: Comparison of Monte Carlo modeling and measurements using scintillation detectors corrected for pulse pile‐up

Abstract

The determination of x-ray spectra near the maze entrance of linear accelerator (LINAC) rooms is challenging due to the pulsed nature of the LINAC source. Mathematical methods to account for pulse pileup have been examined. These methods utilize the highly periodic pulsing structure of the LINAC, differing from the effects of high-intensity radioactive sources. Methods: Sodium iodide (NaI) and plastic scintillation detectors were used to determine the energy spectra at different points near the maze entrance of a medical LINAC. Monte Carlo calculations of the energy distribution of scattered photons were used to simulate the energy spectrum at the maze entrance. The proposed algorithm uses the Monte Carlo code, FLUKA, to calculate a response function for both detectors. To determine the effects of the pileup in the spectra, the Poisson distribution was used, employing the average number of photons per pulse (l) interacting with the detector. The quantity, l, was obtained from the ratio of the number of events detected to the number of pulses delivered. The energy spectra at various distances from the maze entrance were measured using NaI and plastic scintillation detectors. From these measurements, the values of µ were calculated, and the pileup probability was determined. The FLUKA Monte Carlo code was used to calculate the spectrum at the maze entrance and the response matrices of the NaI and plastic scintillation detectors. The algorithm based on the Poisson distribution was applied to calculate the spectrum. Results: The agreement between the calculated and measured spectra was within the first standard deviation of the variance expected in µ. This agreement confirms that photons at the maze entrance have energies between 30 and 240 keV for a maze with three turns, with an average energy of around 85 keV. After pileup correction, the range of the pulse height distribution with the plastic scintillation detector, which has a low atomic number, was decreased (0 to 140 keV). In contrast, the range of the pulse height distribution with the NaI scintillation detector was closer to the photon spectrum (0 to 240 keV). Conclusions: The corrected spectrum demonstrates that using a FLUKA Monte Carlo code and an algorithm based on the Poisson distribution are effective methods in removing the distortion due to the pileup in LINAC spectra when measuring with NaI and plastic scintillation detectors. The agreement between the corrected and measured spectra indicates that Monte Carlo modeling can accurately determine the spectrum of a LINAC machine at the maze entrance.

Estudio de la radiación de fuga generada en el cabezal de un acelerador lineal

“…La información contenida en los espacios de fase generados en la simulación permitió su empleo en el estudio de la radiación de fuga producida en los cabezales de los aceleradores lineales. Los resultados confirman que dicha técnica es adecuada para evaluar la radiación de fuga en dichos dispositivos [1, 3,6].…”

“…La exposición a sobredosis debidas a las radiaciones secundarias es un problema conocido desde hace tiempo, a finales de la década de 1980 se produjo un accidente importante relacionado con la exposición de un paciente a la radiación de un acelerador, el problema se debió al malfuncionamiento del sistema de control de la radiación aplicada en el paciente [5]. La determinación de los niveles de radiación de fuga en entornos asociados a los aceleradores lineales es un proceso complejo, tal actividad es posible simulando los espectros de radiación empleando simulación MC [6]. La caracterización de los espectros de radiación de fuga en entornos cercanos al acelerador lineal es posible al emplear simulación MC para determinar los niveles de contaminación en puntos específicos cercanos al acelerador [7].…”

Monte Carlo simulation of photons backscattering from various thicknesses of lead layered over concrete for energies 0.25–20 MeV using FLUKA code

“…This technique is based on using a few millimetres of lead to cover the concrete walls of the maze of a radiotherapy room. Dose rates of backscattering photons from radiotherapy rooms walls, floors and passageways can be equal or even exceed the dose rate of transmission photons through the room walls 3 – 6 .…”