Applications

We have explored three applications of the rapid prototyped immobilization devices.

Streamlined SABR planning

The RP cradle can be used as part of a streamlined stereotactic ablative radiation therapy (SABR) process for out-of-town patients who live more than a few hours’ drive from the treatment center. The process allows the patient to travel to the treatment center for simulation, stay overnight, be treated the next day, and travel home. Previously, patients had to travel for simulation, travel home, wait for 1-2 weeks, and return for treatment. With the new process, one week before the patient arrives for treatment, the physician delineates target and normal tissue structures on a prior diagnostic CT. A SABR treatment plan is created and reviewed by the physician. This pre-plan serves as a starting point for the final simulation CT plan. In parallel, a foam cradle is fabricated to match the diagnostic CT. A limitation to planning on a previous CT is lack of control over parameters like slice thickness. Also, for tumors with respiratory motion, 4DCT is preferable. Therefore, the patient arrives the day before first treatment. Simulation CT is performed using the RP cradle. The diagnostic CT contours are deformed onto the simulation CT using MIM, manually edited, and approved. This step is rapid, since patient positioning exactly matches the prior CT, ensuring accurate contour transfer.

A treatment plan is created, starting with the same planning parameters (beam arrangement, optimization objectives) from the pre-plan. The physician approves the plan and patient-specific quality assurance measurements are performed. The patient starts treatment the next day on the linear accelerator using the RP cradle.

For targets in the thorax with anticipated respiratory motion, a slightly modified workflow is used. When doing initial contouring on the diagnostic CT, an extra superior/inferior GTV to PTV expansion is used in addition to our standard 5mm margin. For upper lobe tumors, an extra 2mm superior/inferior expansion is used; for lower lobe tumors, which move more with respiration, a larger expansion is used.1 The intent is to create a better correspondence between the pre-plan PTV based on a free breathing CT, and final PTV based on a 4DCT with motion-inclusive contour.

We implemented the streamlined SABR process for two out-of-town patients with early stage lung cancer. One had a cT1bN0M0 cancer of the left upper lobe (Fig. 3, patient #2); the other had two cT1aN0M0 cancers, both in the right upper lobe (Fig. 3, patient #3). All three targets were planned to receive a dose of 25 Gy in one fraction. For both patients, the RP cradle was fabricated based on the CT portion of a recent PET/CT. Contouring was performed on this CT and a SABR pre-plan was generated. Each patient arrived and underwent 4DCT simulation in the RP cradle. Body position corresponded well to the prior diagnostic CT (Figure e1). Contours were deformed from the diagnostic CT to the simulation 4DCT and edited to create a motion-inclusive ITV, then expanded by 5mm to create a PTV. For one patient, the esophagus contour was slightly modified; all other normal structures for both patients required no editing. The day after simulation, each patient underwent smooth administration of single-fraction SABR. The pre-plan and final PTVs generally corresponded well. For the first patient the pre-plan PTV

volume was 12.4cc and the final PTV volume was 15.2cc. For the second patient the pre-plan PTV volumes were 5.2 and 13.3cc, and the final PTV volumes were 6.0 and 14.1cc.

We acknowledge that there could be other, simpler ways to streamline treatment planning for out-of-town patients, such as use of plan templates, knowledge-based planning, or dedicating experienced personnel to planning. The advantages of these various approaches would depend on the specific department’s procedures, equipment, and personnel.

Replace leaking vacuum bag

The vacuum bags commonly used for immobilization sometimes suffer a puncture and lose their form mid-way through a treatment course. In this situation, treatment is typically delayed by several days while a new CT simulation is performed and a new treatment plan is made. Rapid prototyping technology enables a second option, in which a foam cradle based on the patient’s position from the initial simulation CT is fabricated on short notice and used for the remainder of the treatments.

We used a RP cradle to replace a vacuum bag that had punctured in between simulation and first treatment of a 6-week treatment course for a right thigh sarcoma (Figure e2). This avoided the need to perform a new simulation and treatment plan, and prevented delay of care. The attenuation of the 6MV beam from the RP cradle was calculated and found to be negligible because of its low density, making it safe to treat with the initial plan. Daily positioning was verified using orthogonal kV films and seen to match well to digitally reconstructed radiographs from simulation.

Pre-formed head and neck immobilization

Young children or claustrophobic patients unable to tolerate a long simulation session could undergo awake simulation and treatment in a pre-made cradle or head positioning system. By using pre-made immobilization based on a prior diagnostic CT, the simulation session becomes shorter and more tolerable, sparing the patient the use of general anesthesia. This could be valuable, as there is emerging evidence that childhood exposure to general anesthesia can have long-term effects on neurocognitive development.2-3

We created a prototype head and neck immobilization system that could be used for this purpose. (Figure e3). The prototype was tested on an adult head and neck cancer patient, but was not used for treatment. A RP cradle was fabricated to support the posterior head and neck. A mask to immobilize the anterior head and neck was fabricated in two steps. First, a high density urethane mold (10 lb/ft3) was fabricated using the CNC machine. Then, a sheet of high-impact polystyrene was warmed and shaped over the urethane mold using a vacuum forming machine. The finished polystyrene mask was attached to the cradle using plastic fasteners. The patient found the devices comfortable.

References

1. Liu HH, Balter P, Tutt T, et al. Assessing respiration-induced tumor motion and internal target volume using four-dimensional computed tomography for radiotherapy of lung cancer. Int J Radiat Oncol Biol Phys. 2007; 68(2):531-40.

2. Raper J, Alvarado MC, Murphy KL, Baxter MG. Multiple anesthetic exposure in infant monkeys alters emotional reactivity to an acute stressor. Anesthesiology 2015;123:1084-1092.

3. Ing C, DiMaggio C, Whitehouse A, et al. Long-term differences in language and cognitive function after childhood exposure to anesthesia. Pediatrics 2012;130:e476-485.

Supplemental figures

Figure e1. Rapid prototyped (RP) cradle accurately reproduces position from prior CT. Simulation CT in RP cradle (blue) overlaid on prior CT (red). Bony alignment is generally within 1-2mm. Patient 1 underwent mid-treatment simulation in the RP cradle. Patients 2-3 were treated in RP cradle with the streamlined SABR process.

Figure e2. A foam cradle was used to replace a vacuum bag that had lost its shape mid-way through a treatment course for a tumor in the right thigh.

Figure e3. Prototype immobilization device for head and neck/brain treatment. Foam cradle immobilizes posterior head/neck. A urethane mold is used to vacuum form a polystyrene mask to immobilize anterior head/neck. Mask is fastened to cradle at four corners. By pre-forming immobilization devices before simulation, simulation can be performed much more quickly, possibly avoiding anesthesia for pediatric patients.

Video captionComputer numeric control router fabricates topmost slice of rapid prototyped foam cradle. In the first pass the rough outline of the cradle is produced (stair-step pattern seen farther from camera). In the second pass, the fine details are milled.