Radiation Source Use and Replacement Abbreviated Version (2008) / Chapter Skim
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7 RADIOTHERAPY
7 RADIOTHERAPY
Pages 117-134
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From page 117... ...
It is now used mainly to treat brain lesions with the Gamma Knife®, a device with approximately 200 cobalt-60 sources. Radiosurgery can also be practiced with isocentric linacs as well as with a miniature linac mounted on a robotic arm, but many neurosurgeons and some radiation oncologists believe that Gamma Knife® radiosurgery is superior to linac-based radiosurgery.
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From page 118... ...
Modern external beam radiotherapy is delivered with: • x-ray machines producing kilovoltage x-ray beams, typically used for treatment of superficial (skin) lesions; • teletherapy machines producing megavoltage gamma-ray beams, typically used for treatment of deep-seated lesions; • linacs producing megavoltage x-ray beams, typically used for treatment of deep seated lesions and megavoltage electron beams typically used for treatment of superficial (skin)
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From page 119... ...
. STANDARD EXTERNAL BEAM RADIOTHERAPY Since the inception of radiotherapy soon after the discovery of x-rays by Roentgen in 1895, the technology of radiation production has been aimed toward ever higher photon energies and intensities and more recently toward computerization and intensity-modulated radiation therapy (IMRT)
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From page 120... ...
cesium-137 has a relatively low specific activity. The low specific activity implies a relatively low source output and large source diameter; this effectively precludes a teletherapy machine SAD larger than 50 cm, yet in modern radiotherapy a minimum SAD of at least 80 cm is the accepted norm.
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From page 121... ...
To facilitate interchange of sources from one teletherapy machine to another and from one radionuclide production facility to another, standard source capsules have been developed for use around the world. Teletherapy sources are usually replaced within one half-life of installation; however, financial considerations may result in longer source use.
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From page 122... ...
In contrast to a cobalt-60 unit that provides only two gamma rays with energy of 1.17 MeV and 1.33 MeV (average, 1.25 MeV) , a medical linac accelerates electrons to megavoltage kinetic energies in the range from 4 to 25 MeV and can produce two types of radiation beams for use in external beam radiotherapy: electron beams and x-ray beams.
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From page 123... ...
. In comparison with cobalt-60 teletherapy machines, linacs are much more versatile for radiotherapy, and their only disadvantages are that they are significantly more expensive to purchase, operate, calibrate, and maintain (costs are discussed later in this chapter)
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From page 124... ...
Economic Considerations: Megavoltage Linac Versus Cobalt-60 Teletherapy Machine for Standard Radiotherapy Cobalt-60 machine (isocentric, no beam stopper) Capital cost: $750,000 2 Operating expense : $50,000/year including.
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From page 125... ...
The dose in stereotactic irradiation may be delivered through a stereotactic implantation of radioactive sources (stereotactic brachytherapy) or, more commonly, with one or several external radiation sources (stereotactic external beam irradiation)
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From page 126... ...
Radiosurgery based on kilovoltage x-rays was discontinued in the late 1950s but the idea of focal brain irradiation was carried over to other, more suitable radiation beams, first to protons from cyclotrons (Larsson et al., 1958; Lawrence et al., 1962; Kjellberg et al., 1968) then to focused cobalt-60 gamma rays (Leksell, 1968)
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From page 127... ...
This device is the functional equivalent of the isocentric linac dedicated to stereotactic radiosurgery, described below. The Gyro Knife therapy head pivots the 220- to 260-TBq (6,000- to 7000-Ci)
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From page 128... ...
Nonradionuclide Replacements for Cobalt-60 Radiosurgery There are currently three options for nonradionuclide alternatives to cobalt-60 radiosurgery: Stereotactic radiosurgery based on a standard isocentric linac, stereotactic radiosurgery with dedicated isocentric linac, and a miniature linac on a robotic arm (CyberKnife®) .3 Each of these is discussed below.
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From page 129... ...
In the multiple non-coplanar converging arcs technique the patient is stationary on either the treatment couch or chair, while the gantry moves through a given arc. In the dynamic stereotactic radiosurgery technique, both the gantry and the patient rotate simultaneously during the dose delivery (the gantry moves 300 degrees, from 30 to 330 degrees, and the couch moves 150 degrees, from −75 to +75 degrees; see Figure 7-5)
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From page 130... ...
Because the linac is mounted on a robotic arm, it also can perform frameless radiosurgical dose delivery to extracranial targets, such as the spine, lung, and prostate, by using the body skeleton or surgically implanted fiducial markers as frame of reference for targeting purpose. Gamma Knife® Versus Linac-Based Radiosurgery The introduction of linac-based radiosurgery in radiation oncology departments during the late 1980s has very rapidly transformed radiosurgery from an obscure technique practiced in only a few specialized neurosurgery departments around the world into a mainstream radiotherapeutic technique.
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From page 131... ...
Radiation oncologists, on the one hand, embraced the new linac-based radiosurgical techniques with great enthusiasm, but had some reservations about the use of single high-dose irradiation in radiosurgery in contrast to the multifractionated schemes used in conventional radiotherapy. The neurosurgeons, on the other hand, have had previous favorable experience with Gamma Knife® radiosurgery and have expressed concern about the mechanical stability of isocentric linacs when used in radiosurgery.
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From page 132... ...
: $320,000 Total annual operating cost (infrastructure and equipment) : $920,000 Radiosurgery based on modified standard isocentric linac Capital cost (machine)
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From page 133... ...
Because cesium-137–based teletherapy is no longer practiced in the United States and there are only a few cobalt-60 teletherapy machines left in clinical operation in the United States, the committee concludes that teletherapy services, while important, are lesser and declining concerns with respect to radiation source security. It is also possible that the recent rapid growth in Gamma Knife® installations in the United States and the developed world will soon subside given that a viable alternative is offered with linac-based radiosurgery.
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