Articles
Multi-Modality Markers Enhance the Accuracy of Virtual Simulation – Carole A Digel, RT (R)
New Marking System is Beneficial Both for Patients and Therapists – Carole A Digel, RT (R)
Multimodality Markers Enhance the Accuracy of Virtual Simulation
Author: Carole A. Digel, RT (R) – Johns Hopkins Hospital, Baltimore, Maryland
Computed tomography is a valuable tool not only for diagnosis, but also for treatment. Almost since its development, CT has been used in planning the radiation portals for treatment of malignant tumors. With CT, the radiation oncologist is able to view the relationship of radiation-sensitive organs to the tumor volume. This knowledge allows the radiation therapy department to design treatment portals that deliver a maximum radiation dose to the tumor itself while avoiding surrounding healthy normal tissue. Thanks to CT, tumor extent can be precisely measured and covered with adequate treatment margins.
Recently, CT has provided the radiation therapy department with another advantage: virtual simulation.
In virtual simulation, a computer is used to reconstruct CT patient images three-dimensionally. With this new technology, the radiation oncologist can align the radiation beam by first outlining the tumor volume and critical organs in each CT slice and then by determining the best gantry position and field size for optimum treatment.
Virtual simulation also gives the oncologist the ability to recall any plane and see a "beam's eye view" of the radiation field in relation to the tumor and critical organs. Coupled with advances in patient immobilization, precise beam alignment offers improved localization and reduces the volume of normal tissue that needs to be irradiated.
After all diagnostic studies have been reviewed by the physician, virtual simulation begins by marking the patient's skin over the area of interest. One anterior and two lateral laser points are marked, thus creating the plane from which the treatment isocenter will be calculated.
These skin markings must be visualized on the predetermined CT plane. A number of methods may be used to visualize the markings, but many of these methods have disadvantages. Using wire to delineate clinical markings, fore example, causes a starburst-like artifact on the CT scan that obscures fine detail. Teflon catheters have been used to denote the crosswires of the lasers, but it is necessary to perfectly align the teflon to the perpendicular crosswires.
Our radiation oncology department instead uses gel-filled, circular multimodality markers to visualize skin markings. The markers are placed on the patient, centered over the skin marks. (See Fig. 1.) Being circular, they are easy to place and the small hole in the center accurately visualizes for the treatment planner the premarked level on the CT slice.
Multiple CT cuts are taken superiorly and inferiorly from the premarked plane defined by the multimodality markers. (See Fig. 2.) Using this information, the treatment planes can be chosen from the virtual simulation computer. The patient may remain on the CT couch for the final treatment portal design, but if more complicated treatment planning is required or the patient is uncomfortable on the table, the final treatment portals can be placed just prior to treatment.
Starting with the original premarks, which are outlined and maintained on the patient with IZ Port® tapes the calculated shift in isocenter is performed and final marks are made by changing the IZ Port® tapes and tattooing the center. Treatment blocks are designed and constructed using the digitally reconstructed radiographs from the virtual simulator.
Using the multimodality markers for virtual simulation offers many advantages including:
- Multimodality markers accurately mark the preplanning treatment plane
- It is easy to visualize the central hole in the gel-filled marker, both when placing it on the patient and on the CT plane.
- Multimodality markers are made of Hydrogel, which does not cause artifacts on the CT scan.
- Multimodality markers provide a reference point to any changes that are required in treatment planning.
Multimodality markers are common in diagnostic radiology, where they are used for positron emission tomography (PET) scans and for the superimposition of CT with magnetic resonance imaging. Multimodality markers also offer many benefits for radiation therapy departments that perform virtual simulation.
Back to Top
New Marking System is Beneficial Both for Patients and Therapists
Author: Carole A. Digel, RT (R) – Johns Hopkins Hospital, Baltimore, Maryland
Traditional radiation therapy plans that use simple opposed two-field techniques are being replaced by more complex treatment planning using multiple fields, such as those used in conformal therapy. Maintaining accurate skin markings on the patient to reproduce these complex plans is a never-ending struggle for both the patient and the radiation therapist.
In the past, radiation portals were marked on the skin with silver nitrate sticks and gentian violet. When the silver nitrate stick was placed against the patient's skin, the marking was not visible immediately. When the patient returned the following day for treatment, however, a brown stain appeared where the stick had been applied. Because of this delayed reaction, some overzealous physicians who wanted to see the marking immediately would keep applying the silver nitrate. This often would result in burn marks on the patient.
The marking method used most frequently today is carbol fuchsin. These temporary skin markers are applied during simulation and intended for use throughout the treatment. Although a few patients are allergic to carbol fuchsin, it is safer than silver nitrate and more indelible than gentian violet. But carbol fuchsin has disadvantages too. The markings fade and may wash away, requiring resimulation. If the carbol fuchsin reaches the patient's clothing or the therapist's, the dye becomes permanent, and patients are advised to avoid getting these skin markings wet. They should not swim, although they may shower, they cannot scrub their skin.
A new skin marking system, however, eliminates these drawbacks. Called (I.Z.) Port®, the markers are manufactured by IZI Medical.
I.Z. Port® comes as a set of markers – two isocenters, two sets of points, four corners and eight field edge markers. They are made of woven nylon with pressure-sensitive skin contact adhesives and appear to be tolerated even by patients who are allergic to standard tape. They are premarked, easy to place on the skin and remain in place up to two weeks on most patients and for as long as five or six weeks on some patients.
Our radiation therapy department's experience with this new skin marking system has been very positive. Patients report that they love the freedom of not worrying about their marks. When worm in areas not covered by clothing, the tapes look more like bandages than embarrassing purple lines. When worn under clothing, the ink does not rub off onto the patient's clothing like carbol fuchsin lines, even if the patient perspires heavily.
Another advantage of the I.Z. Port® system is that it is easily visible on darkly pigmented skin. Carbol fuchsin lines do not show up well on darkly pigmented skin, whether the skin was pigmented before treatment or has darkened because of changes caused by reactions to radiation or side effects.
We have found that using the I.Z. Port® tape is more accurate than other methods because the tapes provide a fine line for matching. With carbol fuchsin, it often is necessary to retouch the marks daily, resulting in broader marks. Carbol fuchsin lines also bleed when damp, resulting in lower accuracy of patient positioning.
Maintaining fine, accurate lines is particularly important for the treatment of craniospinal patients when a feathered or moving gap technique is used. A 0.5 cm gap, cycled every three days, is the technique our department uses for treating these patients. Bleeding skin marks on the patient's neck at the junction of the brain and spinal portals could cause mismatches or overlaps over the spinal cord.
After verification films are approved to check the simulation portals and treatment is initiated, department policy regarding tattooing prevails. Two of the crosswire or central beam tapes are designed with a hole in the center so tattooing may be performed without removing the tape. But the I.Z. Port® tapes are more convenient to use than tattoos through daily set-ups. If changes are made after verification films are checked, the tapes can be changed without leaving residual marks, while the original carbol fuchsin marks may not be totally erased on some patients.
While not all therapist have problems with patients loosing or deliberately washing marks off the skin, we have identified three groups of patients who seem to have problems with skin markers – those who feel too self-conscious to wear marks, those who remove the marks because of denial of their disease, and those who are mentally ill, alcoholic, or homeless and have little incentive to assist in their treatment. These three groups of patients repeatedly come into radiation therapy without their marks, which can disrupt patient appointments and daily scheduling. A marking system that uses tape is ideal for these patient populations.
There is at least one other method of positioning that does not use skin markers. The Aquaplast face mask is used for positioning head and neck patients without skin markings, but we have experienced erythema due to the bolus effect of the plastic on a few patients. In addition, some patients who began treatment in the face mask switched to skin markers because they could not tolerate the confinement of the mask.
In our department, we do not tattoo above the shoulders, no matter how small the mark. And despite the rule about universal precautions for all patients, we have decided not to tattoo any patient who is HIV positive or has AIDS. A high number of our patients fall into this category, making the I.Z. Port® tapes especially useful for these patients.
In our experience, the only disadvantage to I.Z. Port® tapes occurs when using them on skin covered with thick hair. Their major advantages are:
|
