Uncertainties, Mechanical Uncertainties, Patient-Related Uncertainties
Uncertainties
There are, inevitably, uncertainties in the planning and delivery of a course of radiation therapy. These were well characterized by the esteemed dosimetrist Gunilla Bentel38,39 (1936–2000), to whom we are grateful for the following discussion.
Uncertainties are divided into two general categories. First, there are uncertainties related to the delivery of dose. These include inhomogeneities in the beam, problems related to dose calculations, variables in the output of treatment machines, instability of the beam monitoring technique, and problems related to beam flatness. Spatial uncertainties in the delivery of radiation therapy may be divided into those related to mechanical inaccuracies in the equipment and those related to the patient.
Mechanical Uncertainties
- Field size settings. There can be errors related either to mechanical dials or digital settings in which the field size set on the machine is not precisely the same as that delivered.
- Rotational settings. Mechanical or digital settings that display the degree of angulation of the gantry or the collimator may be in error.
- Cross hairs. Wires in the linear accelerator designed to show the central axis or the field edges may become displaced.
- Isocenter. Deviations in the position of the isocenter may occur as a result of sagging of the gantry head.
- Light-beam congruence. The light beam within the linear accelerator may be in error. These misalignments may be caused by small shifts in the mirror or the light bulb.
- Alignment systems. The laser beam systems used for alignment may be in error. They may not intersect exactly at the isocenter, they may not be perpendicular, and some systems may display relatively thick lines allowing for errors in judgment.
- Couch top. There can be differences in sag between radiation treatment couches. In addition, there may be differences in sag between the simulator couch, the CT couch used for 3D or IMRT planning, and the accelerator couch. Sometimes, in the treatment room, a tennis racket–type insert is used. Over time, couch tops can become tilted from side to side or from end to end.
- Beam-shaping blocks or collimators. If blocks are used rather than a multileaf collimator, there may be errors in constructing the blocks related to user error or because the cutting wire becomes too hot or is moved too fast around a corner when the Styrofoam mold is cut. If multileaf collimators are used, there may be errors in alignment.
Patient-Related Uncertainties
- Target delineation. No matter how sophisticated the computerized treatment-planning system, it will be to no avail if the physician is uncertain about where the tumor is located. The inherent problems related to PET, MRI, CT, and our ability to make correlations between anatomic and functional imaging, and the location of tumor may result in difficulty determining the extent of tumor as well as in transferring anatomic information from imaging studies to the 3D or IMRT treatment-planning system.
- Organ motion. Organ motion can occur from respiration or heartbeat. In addition, it can occur from changes in the size or shape of an organ as a function of digestive or excretory function (i.e., changes in the size and position of the stomach, intestines, bladder, and rectum, and, in the last case, its influence on prostate position) (Fig. 1.8).
- Skin marks. Skin marks can shift relative to deeper tissues. This can change as a result of alterations in patient weight, patient positioning, or the use of steroids during a course of radiation therapy. A particular problem is related to the width of setup lines drawn by therapists on the patient’s skin. Variation in the width of the lines drawn and variation in the position of the light fields in relationship to the lines can cause uncertainty in treatment delivery.
- Repositioning. Day-to-day problems in reproducing the position may occur.
- Patient motion. Some patients simply will not hold still during radiation therapy. Whether this is related to the patient being anxious, in pain, demented, or subject to a neurologic disorder, it can lead to uncertainties in radiation therapy treatment delivery.
Respiratory-Dampened, Respiratory-Gated, and Respiration-Synchronized Radiotherapy
The movement associated with respiration affects the position of multiple organs. If the radiation oncologist wishes to administer highly conformal fractionated or single-fraction treatment(s) to tumors of the liver, lung, pancreas, kidney, retroperitoneum, thoracic wall, mediastinal region, and adjacent structures, it will be necessary to either account for respiration-induced movement by putting a larger margin around the tumor or use an intervention to reduce this movement.
One method for limiting respiratory motion during radiotherapy is the abdominal compression method. This involves placing a plate or some other restrictive device above or around the abdomen and chest, sometimes in association with supplemental oxygen, in an effort to minimize the amount of diaphragmatic motion during radiotherapy.31 This is also referred to as respiratory-dampened radiotherapy.454 Another technique involves general anesthesia and high-frequency jet ventilation to minimize diaphragm motion during liver radiosurgery.169
Respiratory-gated radiotherapy involves turning the beam on only during portions of the respiratory cycle. One such method calls for the patient to hold his or her breath during the irradiation. A device called the Active Breathing Coordinator (ABC, Elekta, Norcross, GA) attempts to standardize breath holding. The patient is coached to hold his or her breath at a certain consistent depth of inspiration by watching a monitor. The ABC device uses a mouth piece, nose plug, bacterial filter, tubing, and balloon valve that, when triggered to inflate by the caregiver, will prevent airflow to and from the patient. The patient controls the switch, which must be enabled to allow the operation of the device. Using the ABC system, the caregiver can initiate a patient’s breath hold at a predetermined title volume. At the time of simulation, the patient practices inhale-exhale breath hold under guidance of the radiation therapist. At the moment of fixed inspiration, a valve device engages to prevent additional inspiration or expiration. The beam-on time is coordinated