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ORIGINAL ARTICLE
Year : 2015  |  Volume : 4  |  Issue : 2  |  Page : 83-88

A study on room design and radiation safety around room for Co-60 after loading HDR brachytherapy unit converted from room for Ir-192 after loading HDR brachytherapy unit


1 Department of Physics, Mewar University, Chittorgarh, Rajasthan, Roentgen SAIMS Radiation Oncology Centre, Sri Aurobindo Institute of Medical Sciences, Indore, Madhya Pradesh, India
2 Department of Applied Physics, Guru Jambheshwar University of Science and Technology, Hisar, Haryana; Department of Radiation Oncology, BLK Super Speciality Hospital, New Delhi, India
3 Department of Radiotherapy, Dr. Ram Manohar Lohia Institute of Medical Sciences, Lucknow, Uttar Pradesh, India

Date of Web Publication10-Apr-2015

Correspondence Address:
Om Prakash Gurjar
Roentgen- SAIMS Radiation Oncology Centre, Sri Aurobindo Institute of Medical Sciences, Indore - 453 111, Madhya Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2278-344X.153628

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  Abstract 

Context: Use of Co-60 source in place of Ir-192 in high dose rate brachytherapy unit (HDR unit) has come for discussion in recent publications. Co-60 based system has been advocated for centers which have fewer brachytherapy procedures as it has comparative economically and administrative advantage. This study has direct practical application for such institutions, which are at the cusp of moving from Ir-192 to Co-60 based brachytherapy. Aims: Conversion of Ir-192 HDR room to Co-60 HDR room and to analyze radiation safety around the room. Materials and Methods: Uniform thickness of 15 cm concrete was added to all walls (except one wall adjoining to linear accelerator bunker) to convert existing room forIr-192 HDR unit to suitable room for Co-60 HDR unit. Radiation survey around room was done. Actual and calculated wall thicknesses were compared. Results: Radiation survey data indicates that modified room is suitable for Co-60 HDR unit and all values are in full conformity to annual dose limits mentioned in Safety Code for Radiation Therapy Sources (SCRTS), Atomic Energy Regulatory Body (AERB; the regulatory body in India). Also, modified wall thicknesses are appropriate for annual design dose limits mentioned in Safety Report Series No. 47 of International Atomic Energy Agency (IAEA). However, console wall thickness (0.45 m) is less than the calculated thickness (0.53 m) for instantaneous dose rate (IDR) design dose limit (7.5 ΅Sv/h) as perabove safety report of IAEA. Conclusions: The modified wall thicknesses are appropriate for annual design dose limits. However, console wall thickness is less than the required thickness for IDR design dose limit. It has been suggested to add 2.64 cm steel on console wall. It has been found that design dose limits should be considered while making room layout plan and regulatory body should add these constraints inSCRTS.

Keywords: Co-60 HDR remote after loading brachytherapy, concrete, design dose limits


How to cite this article:
Gurjar OP, Kaushik S, Mishra SP, Punia R. A study on room design and radiation safety around room for Co-60 after loading HDR brachytherapy unit converted from room for Ir-192 after loading HDR brachytherapy unit. Int J Health Allied Sci 2015;4:83-8

How to cite this URL:
Gurjar OP, Kaushik S, Mishra SP, Punia R. A study on room design and radiation safety around room for Co-60 after loading HDR brachytherapy unit converted from room for Ir-192 after loading HDR brachytherapy unit. Int J Health Allied Sci [serial online] 2015 [cited 2019 Sep 20];4:83-8. Available from: http://www.ijhas.in/text.asp?2015/4/2/83/153628


  Introduction Top


Radiotherapy continues to be the main stay of cancer management globally. For the treatment of cancer, more than 50% patients receive radiotherapy and of these 5-15% patients are treated by brachytherapy as a single or combined modality. The advent of precision technology is practices of radiotherapy and better understanding of radiobiology; dosimetry has completely revolutionized the radiotherapy practices. Radiation dose is delivered to a well-defined treatment volume by either external beam radiotherapy using three-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiotherapy (IMRT), and volumetric-modulated arc therapy (VMAT), etc., or by image-guided brachytherapy.

In brachytherapy, radiation dose is delivered to the treatment volume either by implant (permanent or temporary) of radionuclide in tumor volume or by placement of radionuclide after catheters has been put in treatment volume. On the basis of dose rate and its radiobiological consequences brachytherapy has been categorized in three categories viz. low dose rate (LDR; 0.4-2 Gy/h), medium dose rate (MDR; 2-12 Gy/h), and high dose rate (HDR; >12 Gy/h) brachytherapy. [1] Nowadays, HDR afterloading brachytherapy is commonly used amongst the above three, where Ir-192 radionuclide is popularly used isotope with source strength of 10 Ci or less. [2] Interstitial brachytherapy (for the treatment of head and neck tumors, breast, soft tissue sarcoma, and skin tumors), intracavitary brachytherapy (for the treatment of cervical cancer), intraluminal brachytherapy (for the treatment of esophagus, uterus, trachea, bronchus, and rectum), and topical brachytherapy (for the treatment of molds of the body part) are the main applications of brachytherapy. [3]

Very high conformal dose to target volume sparing the organs at risk (OARs) can be delivered by IMRT technique. Recent publications suggest that several brachytherapy procedures have been replaced by these highly precision external beam techniques. However, brachytherapy continues to be the most conformal treatment modality with very high dose delivery to the core of target volume and excellent dose fall to spare the OARs with least integral dose. It continues to enjoy radiobiological superiority and it has the ability to boost the target volume with judicious choice of target volume to achieve better outcome. However, some of the practices have been shifted to external modality because of ease of the procedure. This has resulted in reduction of number of brachytherapy procedures; hence, many radiotherapy centers have very low number of cases. Since half-life (T1/2) of Ir-192 radioisotope is low, that is, 73.8 days, [2] it requires replacement of source every 3-4 months which work outs economical expenses as well as a generation and coordination with statutory authorities for source procurement, loading transporting, and disposal with multiple documentation procedures with regulatory body's rules.

In last few years, Co-60 with enhanced specific activity made possible to design miniaturized sources which are equal to Ir-192 sources. Applicators are same in shape and diameter, application techniques are also same in both, and the irradiation time on average is only 1.7 times longer for Co-60 with initial nominal activities of both the sources (370 GBq for Ir-192 and 74 GBq for Co-60). [4] So, Co-60 radioisotope based HDR remote after loading brachytherapy unit (HDR unit) is the alternate solution of the above problem which provides the optimized service considering clinical benefits, half-life, cost, and repeated processing documentation and administrative workouts. The chief advantage is fairly long half-life of Co-60 (5.26 years), [2] thus source replacement becomes infrequent, although mean energy of this radioisotope is higher (1.25 MeV) than that of Ir-192 (0.38 MeV). This will demand larger thick concrete walls of brachytherapy housing compared to Ir-192 source. [5]

Our institute had prepared room for I-192 HDR unit, but later on decided to install Co-60 HDR remote after loading brachytherapy unit. The existing room had been converted to conform to radiological safety requirements of Co-60 brachytherapy room by doing some necessary alterations. This study presents complete details about design and modifications of Ir-192 housing to converted to suit Co-60 room and evaluation of radiation safety around the converted room with Co-60 HDR unit.


  Materials and methods Top


Ir-192 (max activity 10Ci) HDR unit housing was built at our institution as per room layout plan approved by regulatory body "Atomic Energy Regulatory Body (AERB), Mumbai" as shown in [Figure 1] (i). The wall thicknesses were as follows;

Wall A = 30 cm, wall A' = 40 cm, wall B = 140 cm, maze wall = 30 cm, wall B' = 35 and 23 cm (back to maze wall), and roof C = 47 cm.

After the decision to convert the abovementioned Ir-192 room to house the HDR unit (Bebig GyneSource HDR, Eckert and Zeigler Bebig GmbH, Germany) loaded with Co-60 radioisotope (model Co0. A86, Eckert and Ziegler Bebig GmbH, Germany) with initial nominal activity of 74 GBq and apparent activity of 66.96 GBq, room layout plan as shown in [Figure 1] (ii) was sent to AERB for approval. After getting official approval, all the walls (except wall B) and roof was added with 15 cm uniform thickness of concrete. Since the tenth value layer (TVL) of concrete for Ir-192 is 14 cm, whereas, for Co-60 it is 21 cm; so for the walls (wall A and maze wall) with 30 cm thickness prepared for Ir-192 must be added 15 cm more for housing Co-60. Although the calculated thicknesses for other walls (wall A', wall B', and roof C) came different than 15 cm, but by seeing the surrounding of room and uniformity of wall thicknesses, management decided to add 15 cm additional thickness on all the walls and roof. Also since the old room was with height of 485 cm which was more than required height, a second 15 cm concrete had to be added which was tough just in touch with old ceiling. So, the new ceiling with 15 cm thickness was constructed at 300 cm height. Thus hollow space of 170 cm got created which is totally empty and packed from all sides. The thicknesses of all walls and roof after modification are as follows:

Wall A = 45 cm, wall A' = 55 cm, wall B = 140 cm, maze wall = 45 cm, wall B' = 50 and 23 cm (back to maze wall), roof C = 15 cm + 170 cm air space + 47 cm.

Our designing consideration envisages erection of maze to bolster the radiation safety, thus there is no need of a heavy lead door to stop the scatter reaching to door. [6] Simple wooden door with door interlock was put at the treatment room entrance.

After requisite furnishing and electric fitting, Bebig GyneSource HDR unit was installed inside the room in such a way that the patient bed is placed in center of the room as shown in [Figure 1]. Gamma area monitor, type no. - GA720H (Nucleonix Systems Pvt Ltd, Hyderabad, India) having range 0-100 mR/h was installed on maze wall towards inner maze entrance and the digital display (connected with gamma area monitor) was installed in console so that the instantaneous dose rate (IDR) inside room can be seen in console.

Co-60 source was loaded with due observance of statutory procedures and all necessary quality assurance (QA) tests were performed as per performa "RP and AD/Remote QA/01"provided by Radiological Physics and Advisory Division, Bhabha Atomic Research Centre. [7] To check the unit leakage, radiation survey around the HDR unit (in source OFF condition) was done at 5 cm from the unit surface and 1 m from center of the source storage in all direction as shown in [Figure 2], and then radiation survey was done all around the room using digital contamination monitor (Micro) Type: CM710P (Pancake) (Nucleonix Systems Pvt Ltd, Hyderabad, India) capable of measuring β and δ rays and having measuring range of 0-200 mR/h. The calibration accuracy of the contamination monitor is ± 15%. All the data were taken in doserate mode. The detector type in this instrument is halogen quenched GM detector. After commissioning approval from AERB, the unit has been commissioned for patient treatment.
Figure 2: Radiation survey points around Bebig GyneSource HDR unit

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Since above alterations were done by in-house expertise of the institution with the help of architect after documented approval from regulatory body, unit installation, and commissioning with the help of service engineer was accomplished. Since this was the case of modified room, so to analyze the above modified room design calculations were done as follows:

The barrier attenuation to give the acceptable external annual dose is given as: [6],[8],[9]



Where

B (The barrier attenuation required to achieve D a ).

Da (The annual dose constraint) =0.3 mSv annually (design limit for public area). [6],[10]

= 6 mSv annually (design limit for occupational exposure). [6],[10]

d [The distance from the source (in the patient) to the far side of the barrier) ≥3.05 m (3.05 is nearest distance for far side of walls facing to public area, see [Figure 1]] and 2.8 m (distance of far side of console wall).
Figure 1: Approved room Layout plan for (i) Ir-192 (walls sketched with dots) and (ii) Co-60 (walls sketched with dots + fully black color) high dose rate (HDR) remote afterloading Brachytherapy

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T w (The average source exposure time per week (hours) covering all procedures) = 1 hour (our center has maximum four patients/week, and the maximum treatment time for one patient is 15 min with initial activity).

Γ (The air kerma rate constant for the radionuclide in the source (μGy/GBq.h@1m)) = 306 μGym 2 h–1 GBq–1 (as mentioned in source certificate).

A (The maximum activity (GBq) of the source (at renewal)) = 66.96 GBq

U (The use factor) = 1

T (The occupancy factor in the area adjacent to the barrier) = 1 [the control area with 100% occupancy and let we consider 100% occupancy in public passage back to wall at position A' as shown in [Figure 1]].

Calculation for wall thickness

Walls facing public area

B = 0.3 mSv × (3.05m) 2 /(1h × 50 × 306 μGym 2 h–1 GBq–1 × 66.96 GBq × 1 × 1).

= 2.72 × 10–3

n (number of tenth value layer (TVL)) = –log 10 B = 2.57 TVL (TVL for concrete is 0.21 m). [5]

Required wall thickness = 0.54 m concrete

Wall facing control console

B = 6 mSv × (2.8 m) 2 /(1h × 50 × 306 μGym 2 h–1 GBq–1 × 66.96 GBq × 1 × 1).

= 4.59 × 10–2

n = –log 10 B = 1.34 TVL (TVL for concrete is 0.21 m) [5]

Required wall thickness = 0.28 m concrete

Calculation for IDR

The IDRis calculated as follows: [6],[8],[9]



For public area

IDR = 306 μGym 2 h–1 GBq–1 × 66.96 GBq × 2.72 × 10–3 /(3.05 m) 2

= 5.99 μSv/h

For control console

IDR = 306 μGym 2 h–1 GBq–1 × 66.96 GBq × 4.59 × 10–2 /(2.8 m) 2

= 119.96 μSv/h


  Results and discussions Top


The radiation survey data are as shown in [Table 1] and [Table 2].
Table 1: Radiation survey around Co - 60 HDRafterloading brachytherapy unit (with initial maximum activity 66.96 GBq)

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Table 2: Radiation survey around Co - 60 HDR afterloading brachytherapy room (with initial maximum activity 66.96 GBq)

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As per "RP and AD/Remote QA/01", [7] the dose limits at 5 cm from surface and 1 m from the center of the source storage are ≤100 and ≤10 μGy/h, respectively. From the table we can see that the maximum dose rate at 5 cm distance from surface is 8.49 mR/h (≈84.9 μGy/h) and at 1 m distance is 0.21 mR/h (≈2.1 μGy/h), hence the radiation level around unit is well within the tolerance limit. Also the maximum dose rate around room is 1.12 mR/h (≈11.2 μGy/h) at room door. However no one is allowed to stay there while the source is ON. And the maximum dose measured in control console is 2.37 mR/h (≈23.7 μGy/h). If we calculate the annual dose based on work load (maximum 1 h/week, so 52 h in a year), then it comes 58.24 mR/year (≈0.58 mGy/year) and 123.24 mR/year (≈1.23 mGy/year); while the annual dose constraint for general public is 1 mSv/year, [6],[8],[9],[11] and for radiation workers an effective dose of 20 mSv/y averaged over 5 consecutive years (calculated on a sliding scale of 5 years). [11] Hence the radiation survey data indicates that there is no radiation hazard around the room with abovementioned maximum work load and room is suitable for Co-60 HDR unit as per norms of regulatory body.

It is imperative to discuss the design dose limits; although there is no design dose rate constraint mentioned in safety code of regulatory body in India, [11] but as per Safety Report Series No. 47 of International Atomic Energy Agency (IAEA) [6] giving the reference of National Council of Radiation Protection and Measurements (NCRP), USA and Health and Safety Executive (HSE), UK; the annual design dose limit for public area is 1 [12] and 0.3 mSv/year, [10] respectively, and the design dose limits for occupational exposure is 10 [13] and 6 mSv/year, [10] respectively. Using formula 1 and design dose limits, 0.3 (for public) and 6 mSv/year (for radiation workers), the calculated wall thickness facing public area is 0.55 m and wall thickness facing control console is 0.29 m. So, adding the uniform thickness of 15 cm concrete to roof and all walls (except wall B) as shown in [Figure 1] (ii) was safe for the radiation safety point of view based on annual design dose limits.

As per Safety Report Series No. 47 of IAEA [6] giving the reference of Nuclear Regulatory Commission (NRC), USA and Institute of Physics and Engineering in Medicine (IPEM), UK; the IDR design dose limit for public area is 20 [14] and 7.5 μSv/h, [15] respectively, and the IDR design dose limit for occupational exposure is 7.5 μSv/h. [15] Using formula 2, the calculated IDR in public area is 5.99 μSv/h and in control console is 119.96 μSv/h. The calculated IDR for public area is well safe, but the calculated IDR in console is much higher than the abovementioned dose limits. Also the measured IDR in console is 0.55 mR/h (≈5.5 μSv/h). When position of HDR unit in the room is between the source ON position and console wall, and 2.37 mR/h (≈23.7 μSv/h) when direct facing the console wall to source like other walls. Although, we maintain the position of HDR unit in the room in such a way that its position remain between the source ON position and console wall while doing actual patient treatment, so that dose rate in console remain low. The measure IDR at room door is 1.12 mR/h (≈11.2 μSv/h), which is less than the constraint value of NRC, USA, while higher than that of IPEM, UK. The area having IDR > 7.5 μSv/h is considered as supervised. [6],[15] Therefore, this place has been declared as supervised area and no one is allowed to stay there while source is ON. Measured IDR values at all other positions in public area are well within tolerance limit. To bring the IDR in console less than 7.5 μSv/h, the required wall thickness of console can be calculated using formula 2 and IDR design dose limit 7.5 μSv/h as follows:

7.5 μSv/h = 306 μGym 2 h–1 GBq–1 × 66.96 GBq × B/(2.8 m) 2

B = 2.87 × 10–3

n = –log 10 B = 2.54 TVL (TVL for concrete is 0.21 m) [5]

Required wall thickness = 0.53 m concrete.

Hence, 8 cm uniform thickness of concrete (with density ≥2.35 g/cc) or 2.64 cm steel (equivalent to 8 cm concrete) is additionally needed to add on console wall to bring IDR within tolerance limit.


  Conclusions Top


It is possible to modify the existing room for Ir-192 HDR brachytherapy unit to room for Co-60 HDR brachytherapy unit with minimal alteration if judicious planning is done. To modify the existing room for Ir-192 (370 GBq) HDR unit to suitable room for Co-60 (66.96 GBq) HDR unit, additional thickness of 15 cm of concrete was added on all existing walls (except wall B) and roof. Radiation survey data shows that the modified room is completely congruent to annual dose limits mentioned in Radiation Safety Code provided by regulatory body (AERB).

The modified wall thicknesses are appropriate for annual design dose limits for public area as well as for occupational exposure. However, the thickness of console wall (0.45 m) is less than the required thickness (0.53 m) for IDR design dose limit. It has been suggested to management to add 2.64 cm steel on console wall.

Design dose limits should also be considered while planning room layout for radiation facility and regulatory body may incorporate these constraints in radiation safety code. This study has direct practical application for such institutions, which are at the cusp of moving from Ir-192 to Co-60 based brachytherapy in the light of new developments.


  Acknowledgement Top


We thank to Dr. Virendra Bhandari, HOD, Roentgen-SAIMS Radiation Oncology Centre, SAIMS, Indore for his positive support in conducting this study.

 
  References Top

1.
International Commission on Radiation Units and Measurements (ICRU) Report 38: Dose and volume specification for reporting intracavitary brachytherapy in gynecology, 1985.  Back to cited text no. 1
    
2.
Khan FM. The physics of radiation therapy. 4 th ed. Philadelphia: Lippincott Williams and Wilkins; 2010.  Back to cited text no. 2
    
3.
Washington CM, Leaver DT. Principles and practices of radiation therapy: Physics, Simulation, and Treatment planning. 1 st ed. Maryland Heights: Mosby; 2003.  Back to cited text no. 3
    
4.
Andrassy M, Niatsetsky Y, Perez-Calatayud J. Co-60 versus Ir-192 in HDR brachytherapy: Scientific and technological comparison. Rev Fis Med 2012;13:125-30.  Back to cited text no. 4
    
5.
McGinley PH. Shielding techniques for radiation oncology facilities. 2 nd ed. Madison: Medical Physics Publishing; 2002.  Back to cited text no. 5
    
6.
Radiation Protection in the Design of Radiotherapy Facilities, Safety Reports Series No: 47, IAEA, Vienna, Austria, 2006.  Back to cited text no. 6
    
7.
Performa for acceptance/quality assurance tests or remote after loading brachytherapy unit, RP and AD/Remote QA/01. Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai, India, 2001.  Back to cited text no. 7
    
8.
The Design of Radiotherapy Treatment Room Facilities, Report No: 75, IPEM, York, UK, 1998.  Back to cited text no. 8
    
9.
Structural shielding design and evaluation for megavoltage X- and gamma-ray radiotherapy facilities, NCRP Report 151, NCRP, Bethesda, USA, 2005.  Back to cited text no. 9
    
10.
Health and Safety Executive, Ionising Radiations Regulations, S. I. No. 3232, HMSO, London, 1999.  Back to cited text no. 10
    
11.
AERB (Atomic Energy Regulatory Board), 2011, Safety Code for Radiation Therapy Sources, Equipment and Installations, No. AERB/RF-MED/SC-1 (Rev. 1) published by AERB, Mumbai, India.  Back to cited text no. 11
    
12.
National Council on Radiation Protection and Measurements, Recent Applications of the NCRP Public Dose Limit Recommendation for Ionizing Radiation, Statement No. 10, NCRP, Washington, DC, 2004.  Back to cited text no. 12
    
13.
National Council on Radiation Protection and Measurements, Limitation of Exposure to Ionizing Radiation, Rep. 116, NCRP, Bethesda, MD, 1993.  Back to cited text no. 13
    
14.
Nuclear Regulatory Commission, Standards of Protection against Radiation, 10CFR20, US Office of the Federal Register, Washington, DC, 1991.  Back to cited text no. 14
    
15.
Institute of Physics and Engineering in Medicine, Medical and Dental Guidance Notes, IPEM, York, 2002.  Back to cited text no. 15
    


    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2]



 

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