LDR treatment Uterine cancer is the most popular application for intracavitary treatment with LDR remote afterloading brachytherapy. These procedures are performed in the patient’s room. The interlock is connected to the patient’s door so that nurses can take vital signs and give medication and family members can visit the patient without risk of radiation exposure.
HDR treatment The most common applications of HDR brachytherapy are for tumors of the vaginal apex, esophagus, lungs, and, most recently, breasts and prostate. Most HDR treatments are performed on an outpatient basis. Allowing the patient to return home the same day after therapy is one advantage of HDR afterloading brachytherapy. Patients with prostate cancer are the exception. They may remain in the hospital for 2 to 3 days during treatment.
Recent advances in planning and treatment
Until recently, CT and simulation were separate pretreatment procedures. Within the past decade, many cancer centers have combined CT and simulation into a single diagnostic-treatment planning unit, known as a CT simulator. The major advantage of this combination is that both procedures can be performed by one unit, and, thus, the patient does not have to make two separate visits to the clinic. Also, CT simulation is bringing the radiation clinic into the digital age, with hospitals reporting an increase in speed, efficiency, and accuracy of treatment planning and delivery.
Procedure In brief, in the first step of this new procedure, the patient is placed on the CT simulator table and undergoes a normal CT study. The physician has the capability of outlining the tumor and any normal structures on each CT slice. A computer performs a three-dimensional (3D) transformation of the CT slices and creates a digitally reconstructed radiograph (DRR).
The DRR resembles a normal diagnostic film, except that it is digital and can be manipulated to achieve better contrast and detail than regular film. The outlines of the tumor and organs are displayed on the DRR for any viewing angle. The physician can then draw blocks on the DRR with a more accurate idea of where the tumor and normal tissues actually are.
The DRR is digitized into the treatment planning computer, and any CT slices and their contours drawn by the physician are transferred as well. This DRR is either sent to the mold room for block construction or is transferred to the treatment planning software for multileaf collimator optimization. Treatment plans are generated as discussed earlier.
At the time of the patient’s first treatment, DRRs and port films are digitized and saved on a local area network (LAN). Physicians can then call up these images on their desktop computers for weekly patient quality assurance.
CONFORMAL RADIATION THERAPY
Conformal radiation therapy is a geometric shaping of the radiation beam that conforms with the beam’s eye view of the tumor. Conformal therapy utilizes the outlining capabilities of the CT simulator. The physician outlines the tumor volume, generates DRRs, and draws an appropriate margin from 1–2 cm around the tumor. These fields conform closely to the shape of the tumor and, thus, shield more critical structures than do normal blocks. The margin allows for setup errors of a few millimeters each day. Appropriate immobilization of the target volume must be achieved in each patient through the use of devices that constrain movement (“casts”), so that the target is accurately localized.
Films are sent to the mold room for block construction. Since the fields are “tighter” around the tumor, the prescribed dose can be increased. By increasing the dose to the tumor, local control will be improved.
INTENSITY-MODULATED RADIATION THERAPY (IMRT)
Intensity-modulated radiation therapy (IMRT), an extension of conformal therapy, allows for shaping of the intensity of the radiation beam. This is an important improvement, especially when the target is not well separated from normal tissues.
A uniform dose distribution can be created around the tumor by either modulating the intensity of the beam during its journey through the linear accelerator or using multileaf collimators. Multileaf collimators consist of 80 or more individual collimators, or “leaves,” located at the head of the linear accelerator, which can be adjusted to the shape of the tumor. (For a technical description, the reader is referred to the text by Khan; see “Suggested Reading.”)
Both of these methods alter the fluence (the number of x-rays per unit area) of radiation exiting the accelerator. The final result is a uniform dose distribution around the tumor and minimal dose to the surrounding normal tissues, often below tolerance levels. This improves the risk-benefit ratio.
The clinical use of IMRT has grown as computer power increases and costs decline. Preliminary clinical data have shown that prostate doses can be increased significantly without increasing the complication rate. IMRT must be administered within a closely monitored program with rigorous quality assurance, since it can potentially cause significant injury if not appropriately applied.
Several types of IMRT delivery are now becoming standard in radiation oncology clinics. Dynamic conformal therapy with multileaf collimators is being used routinely in hospitals around the country. With this approach, collimators conform to the tumor volume with the beam on while the treatment unit is rotating around the patient. This is an example of totally computer-controlled radiation delivery.