Evaluation of Optimum Room Entry Times for Radiation Therapists after High Energy Whole Pelvic Photon Treatments

Abstract

High energy photon beams (greater than 10 MV) are routinely employed in clinical use for treatment of deep-seated tumors 1,2) . However, they induce undesirable photonuclear and electronuclear reactions 3) that produce neutrons and radioisotopes. Neutron production increases with photon energy, and the induced radioactivity depends on the neutron radiation level 3,4) . This phenomenon induces potential exposure for radiation therapists due to neutron, gamma and beta radiations emitted from decay of activation products 1, 3) . As occupational doses are of concern, undesirable photoneutrons should be minimized and actions taken to reduce unnecessary exposure. Neutron productionThe minimum energy required to remove a neutron from a nucleus decreases with increase in target atomic number 5) and lies between 6 to 16 MeV for nuclei heavier than carbon 6) . Linear accelerators generate high energy photon beams by accelerating electrons to 6-25 MV and then converting them to therapeutic X-rays. For photon energies below 10 MV, minimal photonuclear reaction occurs, and neutron production is negligible 5) . Neutrons are also produced by photodisintegration processes. However, since these have fewer interactions with the nuclei of the accelerator head components 7) , photonuclear reactions are considered to be the main cause of neutron contamination in external beam radiotherapy.Neutron production takes place inside machinery components of the linear accelerator head and the bodies of patients, as well as in the walls of the treatment room [1][2][3][4][8][9][10][11] . The target, beam collimation systems and multi-leaf collimators (MLCs) are the major sources of neutron production where the photon This study estimated gamma dose contributions to radiation therapists during high energy, whole pelvic, photon beam treatments and determined the optimum room entry times, in terms of safety of radiation therapists. Methods: Two types of technique (anteriorposterior opposing and 3-field technique) were studied. An Elekta Precise treatment system, operating up to 18 MV, was investigated. Measurements with an area monitoring device (a Mini 900R radiation monitor) were performed, to calculate gamma dose rates around the radiotherapy facility. Measurements inside the treatment room were performed when the linear accelerator was in use. The doses received by radiation therapists were estimated, and optimum room entry times were determined. Results: The highest gamma dose rates were approximately 7 μSv/h inside the treatment room, while the doses in the control room were close to background (~0 μSv/h) for all techniques. The highest personal dose received by radiation therapists was estimated at 5 mSv/yr. To optimize protection, radiation therapists should wait for up to11 min after beam-off prior to room entry. Conclusions: The potential risks to radiation therapists with standard safety procedures were well below internationally recommended values, but risks could be further decreased by delaying room entry times. Dependent on the te...

Radiation therapy in patients with cardiac implantable electronic devices

“…The minimum delay after the previous session should be considered if beam energy is higher than 6MV. The energy exceeding 6MV may result in longer persistence of ionizing radiation in the room [17]. Neutrons produced during the LINAC treatment might generate the CIED dysfunctions only during the radiation, even when no direct exposure to the radiation beam was present [18].…”

Risks posed by neutron contamination in high energy radiotherapy

“…Israngkul-Na-Ayuthaya et al (2015) measured a dose of 4.14 μSv/hour at 100 cm source to surface distance (SSD) following a 1000 MU 15 MV photon beam delivered at 400 MU per minute. Annual estimates of dose to staff resulting from activation following high energy conventional 3D conformal radiotherapy range from 0.7 mSv (Almen et al 1991) and 0.9 mSv (Donadille et al 2008) up to 2.5 mSv (Perrin et al 2003) and 5 mSv (Ho et al 2012). Data from Rawlinson et al (2002) shows that for IMRT which requires longer beam times activation may result in an annual dose of up to 17 mSv compared to 3 mSv for conventional radiotherapy.…”

“…Reported staff annual dose estimates from activation caused by 3D conformal radiotherapy techniques range from 0.7 mSv (Almen et al 1991) and 0.9 mSv (Donadille et al 2008) for a variety of linear accelerator models, up to 2.5 mSv (Perrin et al 2003) and 5 mSv (Ho et al 2012) both estimated from measurements performed on Elekta Precise linear accelerators. Rawlinson et al (2002) report the estimated dose to staff from activation as a result of IMRT treatments (up to 17mSv/year) and conventional treatments (3 mSv/year) from measurements made on a Varian 21EX.…”

Technical Note: Induced radioactivity in stereotactic body radiation therapy with a flattening‐filter‐free 10 MV beam model

“…Using γ spectroscopy and dose decay curve measurements on various linacs, many induced radioisotopes have been identified, including 24 Na, 28 Al, 38 Cl, 56 Mn, 57 Co, 57 Ni, 62 Cu, 122,124 Sb, 187 W, and 196 Au. 14,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Based on the properties of the radioisotopes, several models were established to describe the induced activity and discuss the occupational dose [20][21][22] that ranged from 0.1 mSv/y to 5.9 mSv/y [21][22][23][24][25][26][27][36][37][38][39] and 17 mSv/y under extreme conditions. 21 However, establishing a universal model to predict the dose is difficult because it depends sensitively on many factors, such as the different treatment installations (linac, bunker, console room, etc.…”

“…Many studies have been dedicated to the investigation of induced activity in various aspects 14–40 . In general, as beam energy increases, the induced activity tends to increase greatly 17–20 .…”