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The Cobalt-60 Machine in the Fight Against Cancer

Introduction

Cancer is a life threatening disease characterized by the development of malignant tumours in various parts of the body. Ravichandran (2009) reveals that this disease is the number one killer in the world. It leads to suffering in the patient and if untreated causes death. Scientists and doctors have therefore sought to come up with effective methods for treating this deadly ailment. The invention of the Cobalt-60 machine in 1951 marked an important milestone in the fight against cancer. This device provided doctors with the means with which to treat deep-seated cancerous tumours without hurting the patient’s skin. The machine was hailed as a major advancement in the radiation treatment of all cancers and it has benefited millions of cancer patients worldwide since invention. This paper will set out to provide an informative discussion on the Cobalt-60 Machine. It will begin by giving its history, concepts, and theories utilized by this invention. The most important advantages and disadvantages of the Cobalt-60 Machine will also be articulated.

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History of the Cobalt-60 Machine

The great revolution in the diagnosis and treatment of cancer can trace its roots to the discovery of X-rays by Roentgen in 1895. The two other important discoveries on the same topic were natural radioactivity by Becquerel and Radium-226 by Marie Curie in 1896 and 1898 respectively (Mould 1993). These three revolutionary discoveries led to the inception of radiotheraphy, which made use of the destructive power of x-rays to destroy cancerous cells. Since the discovery of x-rays and the effect of ionizing radiation on cells, scientists strived to develop technologies that could produce even higher photon energies and intensities. Mould (1993) states that Cobalt-60 sources presented a great improvement to the radium teletherapy that was widely used between 1919 and 1950.

Scientists were able to realize the goal of creating a compact and megavoltage therapy machine with the invention of the cobalt-60 machine. The Canadian physicist Harold E. Johns developed the cobalt-60 machine in 1950 (Podgorsak 2010). Unlike the x-ray tubes and van de Graaff generators, which were unsuitable for isocentric mounting due to their bulkiness and heavy weight, the cobalt-60 machine was a compact unit of moderate weight..The Nuclear and Radiation Studies Board (2008) acknowledges that the cobalt-60 teletherapy machine was the first truly practical megavoltage therapy machine. This machine presented the first method to deliver radiation therapy clinically for the treatment of a wide range of cancers. Cobalt-60 machines had increased penetration and the damage to the skin was minimal making the machines ideal for therapy.

Theory of Operation

The operation of cobalt-60 machines is based on the properties of gamma rays. Gamma rays (symbolically denoted as γ) are high frequency electromagnetic radiations emitted during nuclear fission and radioactive decay (Podgorsak 2010). To produce these rays for radiotherapy, an artificially produced radionuclide is placed within a specially designed source. In this source, the radioactive parent source material undergoes decay and during this process, it produces excited daughter nuclei that stabilize by emitting gamma rays. The operation of cobalt-60 machines is based on some inherent properties of gamma rays.

There exist many radionuclides that produce γ rays during their decay process. However, for these elements to be useful for external beam radiotherapy, they need to fulfil some conditions. To begin with, the radionuclides should have high γ-ray energy, which is greater than 1 MeV. They must also possess specific activity that is great than 100Ci/g. Podgorsak (2010) states that in addition to this, the radionuclides should have a relatively long half-life and a large specific air kerma rate constant. The stringent requirements that radionuclides have to meet exclude most of the available radionuclides from being feasible candidates for external beam radiotherapy. Page (2014) documents that out of the over 3000 radionuclides discovered by scientists over the past century, only three possess the important characteristics necessary for radiotherapy use. Out of these three candidate radionuclines, only cobalt-60 has been exploited for use in radiation therapy units.

Ionizing radiation damages the DNA contained in the cell therefore inhibiting the ability of the cell to divide. With the ability to proliferate destroyed, the cells die off. The focused radiation introduced by the machine is harmful to the cells in the target area and its surroundings. However, scientists theorize that healthy cells are more tolerant of radiation compared to the diseased cells (Podgorsak 2010). While the cancerous cells are damaged and destroyed by the radiation, the healthy cells are damaged but they are not destroyed and over time their health is restored. Ravichandran (2009) notes that 60% of cancer patients require radiotherapy to treat the disease.

Concepts

The cobalt-80 machine is referred to as a teletherapy machine since it makes use of gamma ray sources to produce its beam. These machines are external radiation units since the radiation source is kept outside the body and a beam containing a dose of ionising radiation is directed to the cancerous site to damage the genetic material in the cells of target tissue thereby killing the cancerous cells (Podgorsak 2010). An important component of the machine is the source, which consists of high-activity cobalt-60 pellets contained within a specially constructed cylindrical capsule. Ravichandran specifies that “the diameter of the cylindrical teletherapy source is between 10 and 20mm while the height of the cylinder is about 25mm” (p.63). The diameter determines the sharpness of the beam edge during treatment. The cobalt-60 machine typically uses a diameter of 15mm (Page 2014).

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A Cobalt-60 Machine
Fig 1: A Cobalt-60 Machine

A key part of the teletherapy machine is the source head, which is a container encased in protective shielding to contain the radiation emanating from the cobalt-60 radionuclides. The source head contains a mechanism for moving the source in front of the collimator assembly which determines the size and orientation of the radiation beam (Page 2014). The gamma-ray source is mounted on cobalt-60 machine isocentrically, which makes it possible for the source head to be moved around the patient in order to access the target area. The source-axis distance of modern Cobalt-60 machines is 80cm or 100cm.

the Source
Fig 2: Image showing the Source, Source Head and Collimator

Since the source is always emitting radiation, the machine has to have a mechanism for moving it between the ON and OFF positions. The cobalt-60 machine might employ either the sliding drawer or the rotating cylinder techniques for this purpose. A fail-safe mechanism is in-built in the two methods to ensure that the source moves to a default OFF position in the event of a mechanical or technical machine failure. Another important component of the cobalt-60 unit is the ganty which is the unit holding the source head. It is mounted on the machine’s base housing and it can rotate around the patient about a horizontal axis (Page 2014). This feature enables the source to be moved close to the target area therefore increasing the dose rate.

Application

Cobalt-60 machines are primarily used for radiation therapy through the use of gamma rays. Radiation is harmful to all human cells and it would be undesirable to expose the whole body to high doses of radiation (Van Dyk & Battista 1996). The cobalt-60 machine provides a means for delivering desirable amounts of radiation to specific areas of the body while at the same time limiting the exposure to adjacent areas to the minimal extent. The large amounts of energy contained in the gamma rays breaks down the chemical bonds of the cancerous cells effectively killing them (Van Dyk & Battista 1996). The cobalt-60 machine can be used to cure a wide range of cancers including skin, larynx, lung, breast, brain, and prostrate cancers.

Table 1: Cobalt-60 Machine Physical Factors

Buildup Equivalent to 4 MV, buildup 5 mm
Skin dose 40% – 50%
Penetration 54% (10cm)
Penumbra 90% – 10%
Dose Rate 2.5 Gy/min
Energy Lower hence affected by tissue density and air gap

Advantages of Cobalt-60 Machines

A significant advantage of cobalt-60 machines is that they have considerably lower capital, installation, and maintenance costs when compared to more advanced radiation treatment technologies. When compared to the more advanced linear accelerators (LINAC), the cobalt-60 proves to be vastly cost effective. The Nuclear and Radiation Studies Board (2008) reveals that while a single low-energy isocentric LINAC machine has a capital cost of $2,250,000, a single isocentric cobalt-60 machine only costs $750,000. At the same time, the operating expense for the cobalt-60 machine is 50,000 per year compare to $150,000 for the LINAC machine. This cost differentiation is confirmed by Ravichandran (2009) who observes that the operation costs of a cobalt unit are only 33% those of a low energy LINAC and 20% those of a high-energy LINAC machine. These cost effectiveness and reliability of the cobalt-60 has made it the prevalent technology in developing countries. These machines enable radiation therapy institutes to provide effective treatment at a relatively lower cost.

Another major advantage of cobalt-60 machines is that they utilize parent source material with a relatively long half-life. While a variety of radioisotopes can be utilized in the machine, the most commonly used is cobalt-60, which is a synthetic radioactive isotope of cobalt (Van Dyk & Battista 1996). This isotope has the considerably extensive half-life of 5years, which removes the need for frequent and costly source replacement when it is used. Page (2014) notes that the source can still be used to treat cancer effectively after the half-life has passed. However, it should be noted that using the use after 5 years leads to decreased efficiency since the patient has to spend more time to achieve the required dose.

Another key advantage of cobalt-60 machines is that they are dependable and robust in nature. The machines are able to generate consistent reliable radiation beams even when the power supply is inadequate or unstable. Ravichandran (2009) declares that cobalt-60 machines present an obvious advantage in that they consume less power and can achieve beam stability even under fluctuating power supply conditions. In addition to this, the electronic systems of a cobalt-60 machine are able to withstand unstable power. Podgorsak (2010) confirms that the high dependability of cobalt-60 units has made them a favourite in developing nations where inadequate or unreliable power supplies are typical.

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The cobalt-60 machine is a simple machine that has fairly basic controls making it very user friendly. Due to the lack of sophistication, staff with basic training are able to effectively use this machine with positive results (Page 2014). Cobalt-60 machines enable easy dose computation leading to increase accuracy in dose administration. These machines make use of cobalt-60 which produces gamma rays. The gamma beam produced is nearly mono-energetic in nature leading to uniformity. Van Dyk and Battista (1996) note that “the primary beam is uniformly attenuated through absorbers such as wedges” (p.11). Hospital personnel with low technical experience are therefore able to properly use these machines.

Disadvantages

A major weakness of cobalt-60 machines is that their radiation beam lacks the desirable sharpness. The ability to target a small area for dose delivery without affecting the surrounding tissue is therefore diminished when these machines are used. Van Dyk and Battista (1996) acknowledge that the probability that healthy cells surrounding the target area will be damaged is increased when cobalt-60 machines are used. Linear accelerators provide a more localized treatment of the tumour through their highlight focused and deeply penetrating x-rays. Due to the high focus, LINAC machines result in fewer side effects in nearby normal tissues. Page (2014) declares that the cobalt-80 machine has fallen out of favour with many health care providers in the developed world due to the superior performance of LINAC machines.

The radioactive nature of the cobalt-60 source used to produce the gamma rays presents a challenge. The Nuclear and Radiation Studies Board (2008) warns that all radioactive sources pose significant risks to the environment both during their use and at the disposal process. Cobalt-60 machines present major radiation safety issues due to the constant radioactive decay taking place at the source. Podgorsak (2010) explains that the source of a cobalt-60 machine is always radiating gamma rays. It is not possible to switch the unit to an “off” state at will and a continuous stream of gamma rays will be emanating from the machine at all times. This increases the risk of leakage radiation from the source head and if not properly insulated, this radiation will be dispersed into the surrounding area. Page (2014) reveals that unlike the LINACs, the cobalt machine is always producing radiation even when it is not in operation. This makes the potential of harm by the source to the staff operating the machine and the people in the surrounding areas very high.

Another demerit of cobalt-60 units is that they require more time for a single treatment to be implemented. This increased treatment time occurs due to the reduced output of the cobalt unit as the source decays. According to Van Dyk and Battista (1996), the dose rate in cobalt units decays at the rate of 1% per month from an initial maximum of 13kCuries. The initial dose rate produced by the cobalt-60 machine is 400cGy/min and this rate decreases to 200cGy/min over time. The treatment time is therefore increased by a factor of two leading to lower patient output. Increased treatment time means that a single cobalt-60 machine cannot serve the optimal number of patients each day. This is a major challenge especially in developing countries where the demand for radiotherapy far outpaces the available megavoltage therapy machine. Many patients are therefore denied treatment due to the reduced patient output caused by the slow cobalt-60 machines (Van Dyk & Battista 1996). The modern LINAC units that have replaced cobalt-60 machines in most developed nations guarantee a constant dose rate to the patient ensuring fast treatment.

An important disadvantage of cobalt 60 machines is that they require high costs to replace the cobalt 60 source once it is depleted. While the source has a long half-life, the decay eventually decreases necessitating replacement. The facility has to obtain the source material from the machine manufacturer. Podgorsak (2010) reveals that the cost of cobalt-60 radionuclides has been on the rise due to reduced supply. In addition to this, most manufacturers are moving to LINAC machine production. Disposal of spent sources and acquisition of new ones is therefore becoming a major expense.

Table 2: Key issues between Cobalt-60 Machine and LINAC

Cobalt-60 Machine LINAC
Maintenance Source to be changed after 5 years Constant Quality Assurance check
Safety Issue Radioactive source has to be handled carefully No Radioactive source
Staffing Easy to operate with limited technical knowledge Requires thorough staff education on machine operation
Cost Capital cost of costs $750,000 Capital cost of $2,250,000,
Clinical Acceptability Approved for use Approved for use

Limitations

A major limitation of cobalt-60 machines is that the beam edge produce is not sharp due to the large penumbra in the machine. Van Dyk and Battista (1996) explain that this limitation occurs due to the geometry of the source and the distance between source head and the patient. LINAC machines do not suffer from this limitation since the sharpness of the beam edge is largely independent of the source geometry and distance between source and patient.

Another significant limitation is in the maximum beam energy that can be produced by cobalt units. Cobalt-60 radioisotopes emit high-energy gamma rays within the range of 1.17 and 1.33 MeV. Podgorsak (2010) confirms that the energy of emitted photons cannot be increased beyond this range. This is in spite of the fact that higher energy beams provide more advantages to the patient. Van Dyk and Battista (1996) acknowledge that an increase in beam energy provides numerous benefits to the treatment efforts. Due to the lower energy output compared to LINACs, cobalt-60 machines have lower efficiency in treating deep-seated cancerous tumours since they cannot deliver the maximum radiation dose at increased depths.

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Finally, cobalt-60 machines suffer from a relatively lower focus to skin distance. For adequate doses to be delivered to the target area, the distance is usually 80cm (Mould 1993). Doctors are sometimes forced to reduce this distance to even 60cm in order to increase dose penetration. This increases the risk of damage to the skin of the patient. In contrast to this, LINAC machines have a focus to skin distance of over 100cm making them safer for the patient.

Conclusion

Curing cancer is a major goal of many hospitals and currently, radiotherapy is the most cost-effective method of achieving this. This paper set out to discuss cobalt-60 machines, which are one of the radiation therapy technologies used to treat cancer. It described the history of the machine and provided a theoretical basis for the operation of the teletheraphy machine. The paper then highlighted the inherent merits and demerits of the cobalt-60 machine. The key advantages include low capital costs, long half-life of the source material, robustness, and ease of operation. The paper has acknowledged that cobalt-60 machines suffer from limitations of lower energies, decreased dose rates and larger penumbras. Due to these setbacks, this machine is not widely used in the developed world where LINACs are the recommended machines for radiation therapy. In spite of the clear technological and practical advantages that linear accelerators hold over cobalt-60 machine, these units continue to play a crucial role in the provision of radiation therapy services. It can therefore be concluded that the significant advantages offered by cobalt-60 machines will ensure that this technology remains active especially in developing nations where resource constraints are pervasive.

References

Mould, RF 1993, A Century of X-Rays and Radioactivity in Medicine: With Emphasis on Photographic Records of the Early Years, CRC Press, NY.

Nuclear and Radiation Studies Board 2008, Radiation Source Use and Replacement: Abbreviated Version, National Academies Press, NY.

Page, BR 2014, ‘Cobalt, Linac, or Other: What Is the Best Solution for Radiation Therapy in Developing Countries’, Int J Radiation Oncol Biol Phys, vol. 89, no. 3, pp. 476 – 480.

Podgorsak, E 2010, Radiation Physics for Medical Physicists, Springer Science & Business Media, Quebec.

Ravichandran, R 2009, ‘Has the time come for doing away with Cobalt-60 teletherapy for cancer treatments’, J Med Phys, vol. 34, no. 2, pp.63-65.

Van Dyk, J & Battista, J 1996, ‘Cobalt-60: An Old Modality A Renewed Challenge’, Current Oncology, vol. 3, no. 1, pp. 8-17.

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