The measurement of photoneutron dose in the vicinity of clinical linear accelerators

Evaluation of photoneutron dose equivalent in 10 MV and 15 MV beams for wedge and open fields in the Elekta Versa HD linac

In a high-energy medical linear accelerator (linac), if the interaction of photon energy is higher than the neutron binding energy of high atomic material, it emits a neutron field through a photonuclear reaction. The objective of this current study is to measure the photoneutron dose equivalent produces in a motorized wedge field and open field of 10 MV and 15 MV photon beams in Elekta Versa HD™ linac. The PNDE values were recorded at various positions along the patient plane using the Bubble Detector-Personal Neutron Dosimeter (BD-PND). The results revealed that the PNDE values are higher in 20 × 20 cm² than 10 × 10 cm² field sizes for both the 60° wedge and open fields of 10 MV and 15 MV beams. In addition, the 60° wedge fields generate higher photoneutron contamination when compared with the 45°, 30° wedge fields and open field sizes. Hence, on average PNDE values produced by the 15 MV beam were higher by a factor of 1.98 and 2.11 times for open and 60° wedge fields than the 10 MV beam, respectively.

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

PURPOSE

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 pile-up 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 pile-up in the spectra, the Poisson distribution was used, employing the average number of photons per pulse (μ) interacting with the detector. The quantity, μ, 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 pile-up 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 pile-up 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 pile-up 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.

Novel 6MV X-ray photoneutron detection and dosimetry of medical accelerators

PURPOSE

Dosimetry of fast, epithermal and thermal photoneutrons in 6MV X-ray beams of two medical accelerators were studied by novel dosimetry methods.

METHODS

A Siemens ONCOR and an Elekta COMPACT medical accelerators were used. Fast, epithermal and thermal photoneutron dose equivalents in 10cm×10cm 6MV X-rays fields were determined in air and on surface of a polyethylene phantom in X and Y directions. Polycarbonate dosimeters as bare or with enriched ¹⁰B convertors (with or without cadmium covers) were used applying a 50Hz-HV electrochemical etching method.

RESULTS

Fast, epithermal and thermal photoneutron dose equivalents were efficiently determined respectively as ∼1145.8, ∼45.3 and ∼170.6μSv in air and ∼1888.5, ∼96.1 and ∼640.6μSv on phantom per 100Gy X-rays at the isocenter of Siemens ONCOR accelerator in air. The dose equivalent is maximum at the isocenter which decreases as distance from it increases reaching a constant level. Tissue-to-air ratios are constants up to 15cm from the isocenter. No photoneutrons was detected in the Elekta COMPACT accelerator.

CONCLUSIONS

Fast, epithermal and thermal photoneutron dosimetry of 6MV X-rays were made by novel dosimetry methods in a Siemens ONCOR accelerator with sum dose equivalent per Gy of ∼0.0014% μSv with ∼0.21MeV mean energy at the isocenter; i.e. ∼150 times smaller than that of 18MV X-rays. This observation assures clinical safety of 6MV X-rays in particular in single-mode machines like Elekta COMPACT producing no photoneutrons due to no "beryllium exit window" in the head structure.

Evaluation of Photoneutron Dose Measured by Bubble Detectors in Conventional Linacs and Cyberknife Unit: Effective Dose and Secondary Malignancy Risk Estimation

This study aims to reduce the uncertainty about the photoneutron dose produced over a course of radiotherapy with high-energy photon beams and evaluate photoneutron contamination-based secondary malignancy risk for different treatment modalities. Dosimetric measurements were taken in Philips SL25/75, Elekta Synergy Platform (Elekta AB, Stockholm, Sweden), Varian Clinac DHX High Performance systems (Varian Medical Systems, Palo Alto, CA), and Cyberknife Robotic Radiosurgery Unit (Accuray Inc., Sunnyvale, CA) using bubble detector for neutron dosimetry. The measurement data were used to determine in-field and out-of-field neutron equivalent dose in 6-MV 3D conformal radiotherapy, sliding window-intensity-modulated radiotherapy, and stereotactic body radiotherapy and to calculate the effective dose in 18-MV 3D conformal radiotherapy and sliding window-intensity-modulated radiotherapy techniques for patients with prostate cancer undergoing a standard treatment. For the 18-MV treatment techniques, the secondary malignancy risk due to the neutron contamination was estimated using the risk factors published by The International Commission on Radiological Protection. The neutron contamination-based secondary malignancy risk for the 18-MV 3D conformal radiotherapy and sliding window-intensity-modulated radiotherapy modalities was found to be 0.44% and 1.45% for Elekta Synergy Platform and 0.92% and 3.0% for the Varian Clinac DHX High Performance, respectively. For 6-MV 3D conformal radiotherapy, sliding window-intensity-modulated radiotherapy, and stereotactic body radiotherapy treatment techniques, neutron equivalent doses inside the treatment field were found to be lower than 40 mSv. Our measurements reveal that equivalent dose and effective dose due to the neutron contamination are at a considerable level for 18-MV sliding window-intensity-modulated radiotherapy treatments, while 6-MV photon beams used in different modalities still induce only negligible photoneutrons. The secondary malignancy risk based on photoneutron should be therefore taken into consideration in case of selecting 18-MV photons in a sliding window-intensity-modulated radiotherapy treatment instead of 6 MV.

Photoneutron contamination from an 18 MV Saturne medical linear accelerator in the treatment room

Dose escalation with high-energy X rays of medical linear accelerators (linacs) in radiotherapy offers several distinct advantages over the lower energy photons. However, owing to photoneutron reactions, interaction of high-energy photons (>8 MV) with various high-Z nuclei of the materials in the linac head components produces unavoidable neutrons. The aim of this study was to evaluate the photoneutron dose equivalent per unit therapeutic X-ray dose of 18 MV, GE Saturne 20 linac in the treatment room using Monte Carlo (MC) MCNP linac head full simulation as well as thermoluminescence dosemeter measurements. This machine is one of the old linac models manufactured by General Electric Company; however, it is widely used in the developing countries because of low cost and simple maintenance for radiotherapy applications. The results showed a significant photoneutron dose from Saturne 20 linac head components especially at distances near the linac head (<150 cm). Results of this work could be used in several applications, especially designing bunker and entrance door shielding against neutrons produced by photoneutron reactions in GE Saturne 20. However, a detailed cost optimisation for a specific room would require a dedicated calculation.

The neutron dose equivalent evaluation and shielding at the maze entrance of a Varian Clinac 23EX treatment room

PURPOSE

To evaluate the neutron and photon dose equivalent rate (H(n,D) and H(G)) at the outer maze entrance and the adjacent treatment console area after the installation of a Varian Clinac 23EX accelerator with a higher beam energy than its predecessor. The evaluation was based on measurements and comparison with several empirical calculations. The effectiveness of borated polyethylene (BPE) boards, as a maze wall lining material, on neutron dose and photon dose reduction is also reported.

METHODS

A single energy Varian 6 MV photon linear accelerator (linac) was replaced with a Varian Clinac 23EX accelerator capable of producing 18 MV photons in a vault originally designed for the former accelerator. In order to evaluate and redesign the shielding of the vault, the neutron dose equivalent H(n,D) was measured using an Andersson-Braun neutron Rem meter and the photon dose equivalent HG was measured using a Geiger Müller and an ion chamber gamma-ray survey meter at the outer maze entrance. The measurement data were compared to semiempirical calculations such as the Kersey method, the modified Kersey method, and a newly proposed method by Falcão et al. Additional measurements were taken after BPE boards were installed on the maze walls as a neutron absorption lining material.

RESULTS

With the gantry head tilted close to the inner maze entrance and with the jaws closed, both neutron dose equivalent and photon dose equivalent reached their maximum. Compared to the measurement results, the Kersey method overestimates the neutron dose equivalent H(n,D) by about two to four times (calculation/measurement ratio approximately 2.4-3.8). Falcão's method largely overestimates the H(n,D) (calculation/measurement ratio approximately 3.9-5.5). The modified Kersey method has a calculation to measurement ratio about 0.6-0.9. The photon dose equivalent calculation including McGinley's capture gamma dose equivalent equation estimates about 77%-98% of the measurement. After applying BPE boards as a lining material on the inner corner of the maze wall, the H(n,D) and the H(G) at maze entrance were decreased by 41% and 59%, respectively.

CONCLUSIONS

This work indicates that the Kersey method overestimates the neutron dose equivalent H(n,D) for a Varian Clinac 23EX accelerator. The Falcão method overestimates the H(n,D) partially due to the discrepancy in the International Commission on Radiological Protection (ICRP) conversion factors caused by the uncertainties of the estimated average neutron energy. The modified Kersey method gives the closest estimation of a Varian Clinac 23EX accelerator operated at 18 MV photon mode in a maze with a similar design as in the authors' study. However, it should be used with caution because of its tendency to underestimate the H(n,D). A borated polyethylene lining can provide a cost effective method to reduce neutron and photon dose equivalent at the maze door for an existing linac vault, following the installation of a higher energy linac.

Photoneutron and capture gamma dose equivalent for different room and maze layouts in radiation therapy

In this paper the effect of treatment room and maze layout on the photoneutron and capture gamma dose equivalent in the maze was studied. MCNPX Monte Carlo (MC) code was used to simulate the Varian 2100 C/D Clinac 18 MV and four different room layouts. Two analytical methods, Wu-McGinley and McGinley, were used for dose calculations. The analytical methods overestimated the photoneutron dose (13-43 %) and gamma capture dose (16-95 %) comparing with the MC method at the maze entrance door. The results of MC method revealed that additional bend can cause a great reduction in photoneutron (5000 times) and capture gamma dose (50 times) in the maze entrance door.

A neutron track etch detector for electron linear accelerators in radiotherapy

Background

Electron linear accelerators in medical radiotherapy have replaced cobalt and caesium sources of radiation. However, medical accelerators with photon energies over 10 MeV generate undesired fast neutron contamination in a therapeutic X-ray photon beam. Photons with energies above 10 MeV can interact with the atomic nucleus of a high-Z material, of which the target and the head of an accelerator consist, and lead to the neutron ejection.

Results and conclusions.

Our neutron dosimeter, composed of the LR-115 track etch detector and boron foil BN-1 converter, was calibrated on thermal neutrons generated in the nuclear reactor of the Josef Stefan Institute (Slovenia), and applied to dosimetry of undesirable neutrons in photon radiotherapy by the linear accelerator 15 MV Siemens Mevatron. Having considered a high dependence of a cross-section between neutron and boron on neutron energy, and broad neutron spectrum in a photon beam, as well as outside the entrance door to maze of the Mevatron, we developed a method for determining the effective neutron detector response. A neutron dose rate in the photon beam was measured to be 1.96 Sv/h. Outside the Mevatron room the neutron dose rate was 0.62 μSv/h. PACS: 87.52. Ga; 87.53.St; 29.40.Wk.