TREATMENT PLANNING AND DELIVERY
Determining optimal dose distribution The medical physicist or dosimetrist uses the information from CT and simulation to plan the treatment on a computer. A complete collection of machine data, including depth dose and beam profile information, is stored in the computer. The physics staff aids the radiation oncologist in deciding the number of beams (usually two to four) and angles of entry. The goal is to maximize the dose to the tumor while minimizing the dose to surrounding normal structures.
Several treatment plans are generated, and the radiation oncologist chooses the optimal dose distribution. The beam-modifying devices discussed earlier, such as blocks and wedges, may be used to optimize the dose distribution around the tumor.
Establishing the treatment plan The planning computer will calculate the amount of time each beam should be on during treatment. All pertinent data, such as beam-on time, beam angles, blocks, and wedges, are recorded in the patient’s treatment chart and sent to the treatment machine. The radiation therapist will use this information, as well as any casts, tattoos, and lasers, to set up and treat the patient consistently and accurately each day.
Taking weekly port films As part of departmental quality assurance, weekly port films are taken for each beam. They ensure that the beams and blocks are consistently and correctly placed for each treatment. Port films are images generated by the linear accelerator at energies of 6—20 MeV. Because of the predominance of the Compton effect in this energy range, these images are not as detailed as those at diagnostic film energies (as mentioned earlier), but they still provide important information on treatment accuracy and ensure the quality of setup and treatment.
Brachytherapy is the term used to describe radiation treatment in which the radiation source is in contact with the tumor. This therapy contrasts with external-beam radiotherapy, in which the radiation source is 80—100 cm away from the patient.
In brachytherapy, dose distribution is almost totally dependent on the inverse square law because the source is usually within the tumor volume. Because of this inverse square dependence, the proper placement of radiation sources is crucial.
Isotopes Table 2 lists commonly used isotopes and their properties. In the past, radium was the primary isotope used in brachytherapy. Recently, because of its long half-life and high energy output, radium has been replaced with cesium (Cs), gold (Au), and iridium (Ir). These isotopes have shorter half-lives than radium and can be shielded more easily because of their lower energies.
Types of implants Brachytherapy procedures can be performed with either temporary or permanent implants. Temporary implants usually have long half-lives and higher energies than permanent implants. These sources can be manufactured in several forms, such as needles, seeds, and ribbons.
All temporary sources are inserted into catheters that are placed in the tumor during surgery. A few days after surgery, the patient is brought to the radiation clinic and undergoes pretreatment simulation. Wires with nonradioactive metal seeds are threaded into these catheters. Several films are taken, and the images of the seed placement can be digitized into a brachytherapy treatment planning computer.
Once the treatment plan is complete and the physician has chosen the optimal dose rate (usually 50—60 cGy/h), the sources can be implanted. The actual implantation takes place in the patient’s private room. The duration of treatment is usually 1 to 3 days. The majority of temporary implants are loaded interstitially.
Common uses Interstitial low-dose–rate (LDR) brachytherapy is commonly used for cancer of the oral cavity and oropharynx and sarcoma. Prostate cancer is probably the most common site for which LDR brachytherapy “seeds” are used today. Intracavitary LDR brachytherapy is frequently used in gynecologic applications. High-dose–rate (HDR) brachytherapy is used with remote afterloading techniques, as described below.
Remote afterloading brachytherapy
Because brachytherapy requires numerous safety precautions and entails unnecessary exposure of personnel and family members to radiation, remote afterloading of temporary implants has become popular in recent years. The two types of remote afterloading that can be used for treatment are LDR and HDR sources. The most popular LDR source used today is Cs-137, which has a dose rate of about 1 cGy/min. The most widely used HDR source is Ir-192. This isotope has a dose rate of about 100 cGy/min.
General procedures The pretreatment brachytherapy procedures outlined above are also implemented in remote afterloading brachytherapy. Once the treatment plan has been approved by the physician, the patient is brought into the treatment room. The LDR cesium source or HDR iridium source is connected to the end of a cable inside its respective afterloading unit. This unit is programmed with the data from the planning computer. The cable is sent out from the unit into one of the patient’s catheters. Several catheters can be connected to the unit. Each catheter is irradiated, one at a time, until the prescribed dose has been delivered.
The motor that drives the source out of the treatment unit is connected electronically to the door of the treatment room. If the treatment must be stopped for any reason, simply opening the door triggers an interlock that draws the source back into the unit. Because of this device, oncology personnel will not be exposed to any radiation should they need to see the patient during treatment. This interlock is the main safety advantage of remote afterloading over manual afterloading.