A cut-away view of such an optical system is shown in Fig. A photograph of a completed hard dock process is shown in Fig.
This photo shows that the docking or alignment process can be a complex and time-consuming task in some situations. The alternative method that avoids the problem of the hard dock procedure is the soft dock procedure. In this method, the treatment applicator is separated from the head of the machine and thus the potential for patient injury is greatly lessened and the treatment eld can be. Figure 2.
دانلود کتاب Encyclopedia of Electronic Circuits [Vol 6 of 6 plus cumul. index] - اینترنشنال لایبرری
Example of electron beam alignment using the hard docking process. The linear accelerator is a conventional machine in the therapy department and the applicator was fabricated inhouse. The disadvantage of this method is that an alternative to the simple mechanical alignment is needed. This need has been answered by a variety of optical systems as explained below.
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A description of two commercial alignment systems is given below. A description of several noncommercial soft-docking systems has been published by several authors The Clinical Rationale for IORT To understand the rationale for intraoperative radiotherapy, it is necessary to understand some of the basic principles of radiotherapy.
Patients are routinely treated for cancer using fractionated radiation. This means that the radiation is delivered in increments on a daily basis, 5 days week1 for up to 8 weeks, depending on the lesion under treatment. The reason for doing this is that if one were to deliver a tumoricidal amount of radiation to the tumor in one or a few fractions, serious long-term side effects to normal tissue and organs would result.
By fractionating the radiation, an equivalent tumoricidal dose can be given to the tumor without serious long-term side effects.
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Fractionation schemes currently in use have developed empirically over the history of radiotherapy going back to the use of early X-ray tubes. The daily fraction dose is limited by acute radiation effects. These effects, such as reddening of the skin in the era of low energy X rays, bowel problems, and so on, appear during the course of treatment, but will generally resolve themselves without long-term consequences after the radiation treatment has been completed. This maximum radiation dose may produce acute effects in a few patients, due to biological variation between individuals and, hence, their response to radiation.
The prescription dose or the maximum overall dose, which is the number of daily fractions times the daily dose, is also limited. This limit is due to normal tissue and organ tolerance.
Some organs, such as the liver, can tolerate varying amounts of radiation, depending on the fraction of liver that is irradiated. The smaller the fraction of organ treated, the larger the dose that can be tolerated. Excess dose to the whole lung can produce brosis, resulting in a nonfunctional lung. One method to improve this ratio is through the use of intraoperative irradiation.
In this technique, the area to be. By irradiating with electrons rather than photons see section below for comparison between therapeutic photon and electron beams , the radiation can be safely and effectively limited to the area of the tumor. Doses delivered through IORT generally range between 7. While IORT can increase the therapeutic ratio by excluding normal tissue and organs from the radiation eld, it suffers from the fact that it is a single fraction procedure, which, as noted above is limited and therefore, by itself may not provide a curative dose of radiation for all tumors.
The reason for this is that such processes as repair of sublethal damage, repopulation, redistribution, and reoxygenation 20 , which contribute to enhancing the therapeutic ratio for fractionated radiation, are limited in single-dose therapy. This means that the patient is treated using the standard fractionation scheme using external radiation and is given the IORT as an additional boost dose. Thus the tumor dose has been increased with only a small increase in dose to a fraction of the surrounding tissue. Table 1 21 shows the radiobiological equivalent fractionated dose of single doses of radiation between 10 and 25 Gy.
Assuming a conventional fractionation scheme for external therapy of 2 Gy per fraction, a single dose of 10 Gy is equivalent to 17 Gy for tumors. Thus normal tissue toxicity determines the maximum dose that can be delivered intraoperatively. He used X rays probably 50 kVp, see below to treat several cases of stomach and colon cancer.
Several years later, Finsterer, in 24 , reported on the treatment of stomach and colon cancer. He used doses between and R using X rays with ltration that varied between none, variable thicknesses of. The R stands for the roentgen, which is a unit of radiation exposure; 1 Gy is roughly equivalent to 87R in air. Note that no external radiation was given to these patients.
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The choice of ltration was dictated by the thickness of the lesion. It was noted by Abe 25 , in his historical review of the topic, that the practice of IORT then was different from that of today; however, it is different only in the sense that kilovoltage radiation is exponentially attenuated and delivers its maximum dose at the surface, whereas electron beams have a maximum dose below the surface and the dose falls sharply beyond a given depth, depending on energy.
Thus, the intent then was the same as it is now, namely, to give additional dose to the tumor and spare normal tissues, even though there were more practical problems in dose delivery and differences in beam quality. However, there are photon beam modalities used in IORT that get around the problems mentioned above with X rays. High dose rate brachytherapy using radioactive Ir sources, for example, is able to deliver a high tumor dose with a low dose to nearby critical structures.
Many of these sites were in the head and neck region, just below the skin; more deep-seated tumors would have been hard to treat with this technique. He reports on 13 patients, all but 1 of whom received IORT from 2 to 12 times, with overall doses ranging from to 30, R. Thus surprisingly, fractionated IORT was practiced then, something that would not be countenanced today.
One of the centers using orthovoltage X rays and another using a dedicated linac in a shielded OR are converting to mobile linear accelerators. The remaining eight responders consisted of six sites with an OR in the therapy department and two that used patient transport. By comparison with the survey, it was found that the greatest decline in the number of centers involved with IORT was the group performing IORT by patient transport, and these were primarily community centers. Thus a higher proportion of those sites still practicing IORT are academic centers.
Of the institutions using nonmobile units, the average date inception of the program was 4 years , whereas all the mobile units were installed after The most commonly used energy was 9 MeV, followed by 12 MeV. This certainly justies the choice of maximum energy of the mobile units. The most commonly used eld size is a 7 cm diameter applicator, followed by a 6 cm diameter applicator. The respective numbers per institution are A similar survey was recently carried 29 out for European institutions and there are some similarities and differences.
Table 3 shows the number in institutions performing each type of IORT. Also, the average number of patients treated is in excess of , compared with in the United States, noted above. Of great importance is the fact that there are a number of clinical trials underway in IORT in Europe, whereas there are no trials in progress in the United States.
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What the two continents have in common is the typical applicator sizes and energies. This result is what one would expect if each were treating the same distribution of disease sites. However, since those distributions are not the same, this commonality is quite remarkable.
Finally, one other similarity is that a large percentage of institutions are performing a few cases and a few institutions. Table 2. Table 3. Schultz 30 describes in great detail the history of the development of X-ray machines of increasing energy. Henschke and Henschke 31 provide considerable detail on the practical application of this technique to the treatment of large elds.
They show that this can be accomplished either by increasing the FSD, primarily to increase the percent depth dose, or by use of multiple, overlapping elds at short FSD. Thus, until X-ray machines operating at higher energies with adequate dose rates at larger distances and covering larger elds became available, the role of IORT was relatively limited. Beginning in the late s through the s, such high energy units became available.
Eloesser 32 described the use of an X-ray machine with ltrations between 0. He notes that the ltration depends on the thickness of the tumor being treated; the FSD was 30 cm.