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| ORIGINAL ARTICLE |
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| Year : 2014 | Volume
: 37
| Issue : 1 | Page : 25-29 |
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Radiation dose to radiotherapy technologists due to induced activity in high energy medical electron linear accelerators
Om Prakash Gurjar1, Vikash Kumar Jha2, Sunil Dutt Sharma3
1 Roentgen SAIMS Radiation Oncology Centre, Sri Aurobindo Institute of Medical Sciences, Indore, Madhya Pradesh; Department of Physics, Mewar University, Chittorgarh, Rajasthan, India
2 Department of Physics, Mewar University, Chittorgarh, Rajasthan, India
3 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CTCRS, Mumbai, Maharashtra, India
| Date of Web Publication | 8-Dec-2014 |
Correspondence Address:
Om Prakash Gurjar
Roentgen SAIMS Radiation Oncology Centre, Sri Aurobindo Institute of Medical Sciences, Indore 453 111, Madhya Pradesh
India
 Source of Support: Equipment support of Roentgen Oncologic
Solutions Pvt. Ltd. and Sri Aurobindo Institute of Medical Sciences,
Indore (India)., Conflict of Interest: None  | Check |
DOI: 10.4103/0972-0464.146460
After delivering X-ray beam of 15 MV energy, measurable radiation level near the treatment head of medical electron linear accelerator (LINAC) and in its surrounding is observed due to induced activity generated in target, flattening filter, monitor chamber, cooling tube, collimating jaws, etc., The radiation generated in the head of LINAC has the potential of exposing the radiotherapy staff/personnel (technologists) if he/she comes in close proximity of the machine head immediately after delivering the planned treatment. A systematic study was carried out to quantify the radiation levels near LINAC head, isocenter and 0.5 m lateral to isocenter for planned beam delivery of 50, 100, 200, 300, 400, 500 and 1000 MU (Monitor Units) for field sizes 5 cm × 5 cm, 10 cm × 10 cm, 20 cm × 20 cm, 30 cm × 30 cm and 40 cm × 40 cm. The measured maximum radiation level near LINAC head, isocenter and 0.5 m lateral to isocenter after 1 min of completion of treatment was about 10 μSv/h when the accelerator was operated for < 500 MU. However, the radiation levels at these locations after 1 min of completion of treatment were found to be > 10 μSv/h when the accelerator was operated for 800-1000 MU. It was also observed that the radiation level due to induced activity increases with increasing field size and number of monitor units and shows saturation characteristics when the field size approaches to 30 cm × 30 cm. This study indicates that the radiation dose received by the radiotherapy technologists while standing below the treatment head of the LINAC is significant even after 1 min of completion of the dose delivery when the accelerator is operated for higher number of monitor units (e.g., 1000 MU) in high energy photon mode. Keywords: 15 MV, accelerator, induced activity, occupational safety, radiation level
How to cite this article:
Gurjar OP, Jha VK, Sharma SD. Radiation dose to radiotherapy technologists due to induced activity in high energy medical electron linear accelerators. Radiat Prot Environ 2014;37:25-9 |
How to cite this URL:
Gurjar OP, Jha VK, Sharma SD. Radiation dose to radiotherapy technologists due to induced activity in high energy medical electron linear accelerators. Radiat Prot Environ [serial online] 2014 [cited 2023 Mar 30];37:25-9. Available from: https://www.rpe.org.in/text.asp?2014/37/1/25/146460 |
| Introduction | |  |
Radiotherapy treatment of cancer patients is executed by using photon and electron beams generated by medical electron linear accelerator (LINAC). Radiotherapy technologists are involved in treatment delivery procedure, and they are present at control console while the beam is on and inside the treatment room for removal/set-up of patient while the beam is off. In general, radiotherapy technologists work for about 8 h/day and 5 days/week.
When the photon beam energy is higher than 8-10 MV, components of the LINAC head such as X-ray target, flattening filter, monitor chamber, cooling systems, collimating jaws and multileaf collimator (MLC) gets activated by photo-neutrons. [1] Due to the induced activity measurable radiation is observed near the LINAC head for very short duration after the beam goes off. This radiation level is due to gamma rays coming from photo disintegration and neutron capture reactions. [2] By using gamma spectroscopy methods, variety of radio-isotopes such 24 Na, 28 Al, 54 Mn, 56 Mn, 57 Ni, 53 Fe, 59 Fe, 58 Co, 62 Cu, 64 Cu, 82 Br, 122 Sb and 187 W have been identified by a number of investigators in and around the LINAC head. [1],[3] It has also been reported that two short-lived radionuclides (28Al with T½ = 2.3 min and 62Cu with T½ = 9.7 min) and two long-lived radionuclides (187W with T½ = 23.7 h and 57Ni with T½ = 36 h) are the main contributors of the radiation level at the isocenter of the machine. [1],[2] All the investigators have reported their measurements of radiation level near LINAC head and isocenter for relatively large number of monitor units (1000, 2000, 3000 MU) which does not reflect the realistic condition of clinical operation of the accelerator for the patient treatment. A systematic measurement of radiation level near LINAC head, isocenter and 0.5 m lateral to isocenter was conducted selecting the number of monitor units commonly used for patient treatments in our hospital (50, 100, 200, 300 etc.). The measurement point of 0.5 m lateral to isocenter was selected to quantify the radiation dose to radiotherapy technologists because radiotherapy technologists usually stand at this location during removal/set-up of the patient for treatment and for changing the treatment accessories required for the treatment of the patient.
| Materials and methods | | |
All the measurements were carried out on dual photon energy CLINAC DMX (Varian Medical Systems, Palo Alto, CA, USA) medical electron LINAC. The treatment head of this LINAC contains a tungsten primary collimator, a tungsten X-ray target, copper cooling tubes, monitor chamber mainly containing copper, two pairs of tungsten jaw collimators and a Millenium 80 leaves MLC.
Radiation levels near the head (point - H), isocenter (point - I) and 0.5 m lateral to isocenter (point -LI) [Figure 1] of the LINAC were measured by using digital contamination monitor (Micro, CM710P, Pancake, Nucleonix Systems Pvt. Ltd., Hyderabad, India) capable of measuring the radiation level from beta and gamma rays. This contamination monitor contains halogen quenched Geiger-Muller counter and has the measuring range of 0-2000 μSv/h (0-200 mR/h). The stated calibration accuracy of this contamination monitor is ± 15%. The contamination monitor was operated in dose rate mode for measuring the radiation level at intended points. | Figure 1: Schematic diagram of the medical electron linear accelerator (LINAC) showing the locations of the three points [LINAC head (H), isocenter (I) and 0.5 m lateral to isocenter (LI)] where radiation levels were measured after termination of the irradiation by 15 MV X-rays
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For the experimental measurement of radiation levels, the accelerator was operated in 15 MV photon beam mode for 50 MU at the dose rate of 400 MU/min for a field size of 5 cm × 5 cm and the radiation levels due to induced radio-activity at points H, I and LI [Figure 1] were recorded immediately after irradiation and 1 min after the irradiation goes off. The measurements of radiation levels at these points were repeated by operating the LINAC for 100, 200, 400, 500 and 1000 MU. Similarly, measurements of radiation levels were also repeated for field sizes of 10 cm × 10 cm, 20 cm × 20 cm, 30 cm × 30 cm and 40 cm × 40 cm. A constant time gap of 30 min was maintained between two consecutive exposures. The whole measurements were taken in the period of 15 days, 3 h daily in the morning session from 6 AM to 9 AM. The average room temperature and atmospheric pressure were 19.8°C and 952.3 mbar, respectively.
| Results and discussions | | |
[Table 1], [Table 2], [Table 3] present the measured radiation levels due to induced activity after termination of irradiation by 15 MV X-ray beam at points H, I and LI respectively for different field size and monitor units. Moreover, [Figure 2]a-g represents the bar diagram of the measured radiation levels from induced activity at points H, I and LI for different field sizes after delivering different number of MU. A close observation of the measured data for these three points gives the following information. | Figure 2: Bar diagram of the measured radiation levels from induced activity at points H, I and LI for different field sizes after delivering (a) 50 MU (b) 100 MU (c) 200 MU (d) 300 MU (e) 400 MU (f) 500 MU and (g) 1000 MU
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 | Table 1: Measured radiation level at LINAC head (point H) after termination of irradiation
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 | Table 2: Measured radiation level at the isocenter (point I) after termination of irradiation
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 | Table 3: Measured radiation level at 0.5 m lateral to the isocenter (point LI) after termination of irradiation
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Point H: The radiation level even after 1 min of termination of the beam was (is) higher than 1 mR/h (10 μSv/h) except for field sizes 5 cm × 5 cm and 10 cm × 10 cm for all monitor units and 20 cm × 20 cm for 50 MU.
Point I: The measured radiation levels for ≤ 300 MU for all the (entire) field sizes were less than 1 mR/h (10 μSv/h). This measurement has relevance to the clinical treatment of three-dimensional conformal radiotherapy where total number of monitor units used for the majority of such cases is about 300 MU. However, for bulky patients with the cancer of the pelvic region, intensity modulated radiotherapy (IMRT) treatment is done with 15 MV X-rays [4],[5] and more than 800-1000 MU are delivered by single IMRT plan. Hence, in the case of IMRT treatment for bulky patients of the pelvic region, the dose due to the induced activity at the isocenter is significant even after 1 min of termination of irradiation.
Point LI: The radiation level at 0.5 m lateral to the isocenter was <1 mR/h (10 μSv/h) when the beam was (is) put on for up to 500 MU. However, for 1000 MU irradiation (generally delivered by IMRT) and for larger field sizes, the radiation level due to induced activity is about 1 mR/h after 1 min of termination of irradiation.
As per radiation safety code of Atomic Energy Regulatory Board, [6] radiation dose limits to radiation workers are:
- An effective dose of 20 mSv/y averaged over 5 consecutive years (calculated on a sliding scale of 5 years)
- The equivalent dose to the extremities (hands and feet) of 500 mSv/y
- The equivalent dose to the skin of 500 mSv/y.
Moreover, as per the International Commission on Radiological Protection (ICRP 2011), [7] the new radiation dose limit for the lens of the eye is 20 mSv/y. [8]
On the basis of above dose constraints instant radiation level of <1 mR/h (10 μSv/h) can be considered within the prescribed dose limit assuming that the radiotherapy technologist is receiving the continuous exposure for the 8 h of the working duration per day and 40 h in a week. In general, after completion of treatment of a patient the radiotherapy technologist enters the treatment room for removing the treated (old) patient and setting up the treatment of the next patient by standing at position shown in [Figure 3]a for brain and head- and-neck cases and at position shown in [Figure 3]b for chest and pelvis cases. Average time taken by the technologist to arrive near the treatment couch from the console is about 15 s. So after delivery of treatment by 15 MV X-ray beams, the standing at position shown in [Figure 3]a should be avoided. | Figure 3: The standing positions of radiotherapy technologists during patient set-up in case of (a) brain and head and neck treatments and (b) chest and pelvic treatments
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Almιn et al. [1] reported that the contribution of gamma-rays from 28 Al (T½ = 2.3 min) and 62 Cu (T½ = 9.7 min) is considerable (dominant) in dose rate immediately after termination of irradiation. Contribution of gamma-rays from 187 W (T½ = 24 h) and 57 Ni (T½ = 36 h) is also important and so absorbed dose rate at isocenter is about twice that of beside the treatment couch. Considering these radionuclides and their half-lives, the time interval of 30 min was kept between two measurements so that activation of Al and Cu due to first exposure may not contribute significantly to the measurement after next exposure. However, contribution from 187 W and 57 Ni can't be avoided due to their relatively long half-lives. Because in routine radiotherapy schedule, the LINAC is operated from the morning to the evening and the time interval between LINAC switch-off (evening) to switch-on (morning) is less than 24 h (T½ of 187 W and 57 Ni), if a radiotherapy technologist stands at position shown in [Figure 3]a, the radiation dose contribution of these radionuclide cannot be avoided.
| Conclusion | | |
Radiation levels due to induced activity were measured near LINAC head, isocenter and 0.5 m lateral to the isocenter for 15 MV X-ray photon beam for 50, 100, 200, 300, 400, 500 and 1000 MU at field sizes 5 cm × 5 cm, 10 cm × 10 cm, 20 cm × 20 cm, 30 cm × 30 cm and 40 cm × 40 cm. A survey of the measured data indicates that the significant radiation dose is received by the radiotherapy technologist while standing with below the LINAC head even after 1 min of termination of irradiation when the accelerator is operated for a higher number of monitor units (1000). When the accelerator is operated for < 500 MU, the radiation level at isocenter and 0.5 m lateral to isocenter is <10 μSv/h after 1 min of termination of irradiation for all the field sizes. However, when the accelerator is operated for delivering larger monitor units (e.g., in case of IMRT), the measured radiation levels at these positions due to induced activity after 1 min of termination of irradiation is significant. Sufficient time gap should be maintained in such situations to avoid the unnecessary exposures. It is also observed that the radiation level at the three measurement points increases with increasing field size and the number of monitor units.
| Acknowledgments | | |
We thank to Dr. Virendra Bhandari, HOD, Roentgen-SAIMS Radiation Oncology Center, SAIMS, Indore for his interest in conducting this study. We are thankful to ROS and SAIMS, Indore for providing the necessary equipments and permitting us to carry out this study. We also thank Mr. Ramesh Kumar, Radiotherapy Technologist for his support during the measurements.
| References | | |
| 1. | Almén A, Ahlgren L, Mattsson S. Absorbed dose to technicians due to induced activity in linear accelerators for radiation therapy. Phys Med Biol 1991;36:815-22.  |
| 2. | Wang YZ, Evans MD, Podgorsak EB. Characteristics of induced activity from medical linear accelerators. Med Phys 2005;32:2899-910. |
| 3. | Rawlinson JA, Islam MK, Galbraith DM. Dose to radiation therapists from activation at high-energy accelerators used for conventional and intensity-modulated radiation therapy. Med Phys 2002;29:598-608. |
| 4. | Hussein M, Aldridge S, Guerrero Urbano T, Nisbet A. The effect of 6 and 15 MV on intensity-modulated radiation therapy prostate cancer treatment: Plan evaluation, tumour control probability and normal tissue complication probability analysis, and the theoretical risk of secondary induced malignancies. Br J Radiol 2012;85:423-32. |
| 5. | Pathak P, Vashisht S. A quantitative analysis of intensity-modulated radiation therapy plans and comparison of homogeneity indices for the treatment of gynecological cancers. J Med Phys 2013;38:67-73. [ PUBMED]  |
| 6. | Atomic Energy Regulatory Board (AERB). Safety Code for Radiation Therapy Sources, Equipment and Installations. Available from: http://www.aerb.gov.in. [Last accessed on 9 th September, 2014]. |
| 7. | International Commission on Radiological Protection (ICRP). Statement on Tissue Reactions. Ottawa, ON: International Commission on Radiological Protection ICRP; 2011. |
| 8. | O'Connor U, Gallagher A, Malone L, O'Reilly G. Occupational radiation dose to eyes from endoscopic retrograde cholangiopancreatography procedures in light of the revised eye lens dose limit from the International Commission on Radiological Protection. Br J Radiol 2013;86:20120289. |
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3]
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