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

Abstract

In Brazil, the replacement of rather old cobalt and cesium teletherapy machines with high-energy (E > 10 MV) medical linear accelerators (linacs) started in the year 2000, as part of an effort by the Ministry of Health to update radiotherapy installations. Since then, the contamination of undesirable neutrons in the therapeutic beam generated by these high-energy photons has become an issue of concern when considering patient and occupational doses. The walls of the treatment room are shielded to attenuate the primary and secondary X-ray fluence, and this shielding is generally considered adequate also to attenuate neutrons. However, these neutrons are scattered through the treatment room maze and might result in a radiological problem at the door entrance, an area of high occupancy by the workers of a radiotherapy facility. This paper presents and discusses the results of ambient dose equivalent measurements of neutron using bubble detectors. The measurements were made at different points inside the treatment rooms, including the isocentre and the maze. Several radiation oncology centres, which are users of Varian Clinac or Siemens machines, have agreed to allow measurements to be taken at their facilities. The measured values were compared with the results obtained through the semi-empirical Kersey method of neutron dose equivalent calculation at maze entrances, with reported values provided by the manufacturers as well as values published in the literature. It was found that the measured values were below the dose limits adopted by the Brazilian Regulatory Agency (CNEN), requiring no additional shielding in any of the points measured.

Measurement of the secondary neutron dose distribution from the LET spectrum of recoils using the CR-39 plastic nuclear track detector in 10 MV X-ray medical radiation fields

“…The absorbed dose and dose equivalent due to the secondary neutron evaluated from the LET spectrum of recoiled tracks produced in CR-39 are compared with the previous results carried out in the other works [6,[51][52][53][54] as summarized in Table 2. The quite large differences in the dose value, ranging from 0.05 to 1.4 mSv/Gy, are found in the similar experiments.…”

Evaluation of Photoneutron Dose Measured by Bubble Detectors in Conventional Linacs and Cyberknife Unit

“…In the literature, there are several studies about the photoneutron dose measurement, with different dosimetric systems such as bubble detector, thermoluminescance dosimeter, activation detector, and polycarbonate film. 6,7,9,[11][12][13][14][15][16][17] However, there is sparse information about the secondary malignancy risk due to the neutron contamination in 18-MV 3D conformal radiotherapy (3D-CRT) and sliding window (SW)-IMRT treatment techniques. In this study, we therefore investigate the photoneutron-based secondary malignancy risk created using 18-MV photon beams in 3D-CRT and SW-IMRT treatments.…”

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

“…The neutron dose as well as the accompanying photon dose at the maze entrance has been studied for many years by Monte Carlo simulations, [4][5][6] semiempirical analysis, [6][7][8][9][10] and measurements. 4,8,[10][11][12][13] Monte Carlo simulations are capable of providing a variety of dose information in advance. However, an accurate neutron spectrum simulation is required, with detailed information on the geometry and the materials of the linac head and the treatment room.…”

“…The relationship between neutron dose equivalent and the absorbed dose at isocenter has been intensely studied by several research groups. 6,9,[12][13][14][15][16][17] The parameters of neutron source strength ͑Q n ͒, neutron dose equivalent ͑H 0 ͒ near the isocenter, and tenth value distance ͑TVD͒ on commonly used medical accelerators under different vault and maze shielding conditions were provided. The studies include linacs manufactured by Elekta, 12 General Electric, 6,[12][13][14]17 Mitsubishi, 14 Philips, 9,12,14,16 Siemens, 6,[12][13][14] Toshiba, 14 and Varian.…”

“…6,9,[12][13][14][15][16][17] The parameters of neutron source strength ͑Q n ͒, neutron dose equivalent ͑H 0 ͒ near the isocenter, and tenth value distance ͑TVD͒ on commonly used medical accelerators under different vault and maze shielding conditions were provided. The studies include linacs manufactured by Elekta, 12 General Electric, 6,[12][13][14]17 Mitsubishi, 14 Philips, 9,12,14,16 Siemens, 6,[12][13][14] Toshiba, 14 and Varian. 6,9,[12][13][14][15] However, the relationship between the neutron dose equivalent at the maze door and the absorbed dose from the photon beam for new linac models, including the Varian Clinac 23EX ͑Varian Medical Systems, Inc., Palo Alto, CA͒, has not been thoroughly evaluated.…”