DOS 523 - Week 1 Discussion
Initial Post: Devices and Practices for Simulation and Verification
Simulation and setup verification are tightly related processes that must be planned in conjunction with one another according to the needs of the plan, treatment site, and treatment room. At Loyola University Health System, we have a in-house CT simulator, and four linear accelerators with varying capabilities.
At the time of CT scanning, treatment parameters such as patient positioning, immobilization devices, and setup tolerances will have already been decided by the patient's physician, in consultation with therapists, dosimetrists, and physicists as necessary.
Patient immobilization devices for head and neck can involve thermoplastic masks of varying designs, custom-molded or pre-formed head rests, screw-in head frames for stereotactic radiosurgery (SRS), bite blocks to maintain jaw or tongue position, and mouth guards to protect oral mucosa from secondary scatter off of dental work.
Body immobilization can include inclined breast board, full-body vacloc bags, alpha cradle molds, knee wedges, shoulder retractors, adjustable arm supports, vacuum-fitted plastic sheets to control motion, abdominal compression blocks and straps, or just something as simple as a rolled-up towel, depending on the positioning tolerance level for the patient's specific treatment. An en-face electron field with clinical setup does not need to be as exacting as breath-gated SBRT setup.
Some devices are used only in simulation and not during treatment. These include wires that mark scars, incision sites, intended field edges, and borders of tissue types such as breast tissue. These wires are visible on the CT, which allows dosimetrists to use their location information during planning, but their densities are eventually overridden to air because they are not in place during treatment. Other removable markers include vaginal markers, anal markers, urethral contrast agent to localize prostates, and various kinds of injected or ingested contrast agents. These are all used in similar fashion to the wires.
In most cases, marks are made either on the patient's skin (tiny tattoos) or on their thermoplastic mask in the case of head & neck treatments. These marks may indicate either a setup baseline to establish a CT coordinate system, or they may indicate a physician's decision about the location of the plan's isocenter, based on what they have seen on the CT's console immediately after scanning while the patient is still on the table. In either case, the marks are placed, and these marks are used during daily setup to establish initial positioning by lining up the treatment room laser system to the same marks. The usefulness of these marks varies based on the ability of the marked tissue's ability to move around, but in many cases, the marks are only intended to get the patient approximately into position before finalizing setup through other techniques. In some cases, the only point of the marks is to establish a roll angle and they are not actually used for tumor positioning at all.
Digestive preparation for scanning and treatment is generally kept as consistent as possible for each patient, although the instructions may differ quite a bit from patient to patient based on their anatomy, disease site, and physical condition. For abdominal or pelvic treatments, the patient may be instructed to drink a specified amount of water ahead of treatment and to maintain a full bladder. Other patients may be instructed to empty their bladder immediately prior to treatment. The same is true of rectal filling. In some cases, it is advantageous to have a full rectum because it moves part of the rectal wall away from the treatment field, but in other cases such as prostate treatments, a full rectum may end up pushing the target volume around by an unpredictable amount. Patients are often directed to take Gas-X prior to treatment in order to reduce abdominal gas production, which can produce bubbles in unpredictable places. These bubbles can displace anatomy, or they can reduce local scatter dose contribution, so they are not desirable. With the exception of contrast agents, each patient's instructions for digestive preparation for scanning and for treatment are the same. Unfortunately, many patients can not comply fully or at all with their instructions due to incontinence, pain, frequent urgency, difficulty in voiding completely, or complications from their disease. In extreme cases, it may be necessary to perform an enema or insert a Foley catheter to force the emptying or filling, but these are invasive procedures that are avoided when not absolutely necessary. Inflatable rectal balloons are sometimes used at other sites with the goal of making prostate setup more predictable, but rectal balloons are not typically used at Loyola.
Treatment Room 1 - Novalis
The Novalis linac uses a specialized treatment head with micro-sized multileaf collimators (MLCs) that can only treat up to a 9.8x9.8 cm2 field size. As such, the linac is only able to treat small lesions, and the primary purpose of the machine is stereotactic radiosurgery (SRS). The treatment room is equipped with an ExacTrac patient positioning system that includes orthogonal non-coplanar kilovolt imaging panels, and a robotic couch with six degrees of freedom to move the patient to the exact location desired. The table is equipped for frame-based SRS which uses a halo bolted to the patient's skull, which is then in turn bolted to the table for precise positioning and motion mitigation. For less exacting needs, a frameless system can be used, which consists of a carbon fiber cage that is positioned around the patient's head. It has a series of infrared-reflective spheres that are used by an automated tracking camera to move the couch into the correct position to establish a baseline. In either scenario, kilovolt imaging is used for final verification and adjustments if necessary. These systems allow positioning with 1 mm or less tolerance.
Treatment Room 2 - TrueBeam
The Varian TrueBeam linac can also be used for SRS and for stereotactic body radiotherapy (SBRT), plus a variety of other more common techniques such as VMAT, IMRT, and conformal therapy. For SBRT treatments, a patient may be placed in a full body vacloc bag with a plastic sheet draped over them and sealed at the edges with adhesive. The space between the vaclog bag and plastic sheet is then evacuated with a vacuum pump, effectively sealing the patient into a cocoon that allows minimal motion other than breathing. If the target site is in the lung or near the diaphragm, a compression block may be placed near the solar plexus and then strapped down to minimize the possible movement range. A tracking beacon can then be placed on the patient's abdomen to monitor their breathing cycle, only turning the beam on at specified points such as full exhalation. The TrueBeam is equipped with a cone beam CT (CBCT) scanner which allows 3D imaging for setup verification. It can also perform kV orthogonal imaging and megavolt imaging through the treatment port.
Treatment Room 3 - Upgraded Clinac 21 EX
This room houses one of two Clinac 21EX machines, and this one has been upgraded to include kilovolt imaging panels and a respiratory tracking system for breath-hold treatment of left breasts. This unit is capable of delivering both VMAT and IMRT plans as well. Patients assigned to this room do not usually have exotic immobilization devices, although the arsenal of mainstream devices such as thermoplastic masks, alpha cradle molds, and others is quite diverse.
Treatment Room 4 - Clinac 21 EX
This room is used for cases that are not particularly exotic. The machine is only equipped with megavoltage portal imaging capabilities, so the cases that are sent here are usually ones that are relatively easy to set up, such as breasts, electron boosts, whole brains, and other plans with generous margins. Conformal and 2D photon plans can be delivered on this machine. This room also houses our total body irradiation (TBI) equipment.
Whichever room is used, patients usually have an initial scan on their first day of treatment to make sure that their anatomy is positioned as expected. This is a process we call pre-porting, although I have heard other names used at other institutions. A physician must be present during pre-porting, and they must approve the alignment of the treatment imaging with the plan imaging. After the first treatment, physicians have the discretion to order daily, weekly, or no followup imaging during treatment, depending on the setup tolerances needed and the capabilities of the machine the patient is being treated on. In some treatments, permanent markers are used to trace the edges of field lights projected onto the patient's skin, and the patient is instructed not to scrub the marks off when showering. These marks, which can be refreshed as needed, are used to help line up the patient in addition to other indicators such as tattoos.