3.2 Immobilization Devices & Radiopaque Marking Protocols

Purpose and Hierarchy of Patient Immobilization

\nThe goal of radiation therapy is to deliver high therapeutic doses to target volumes while minimizing dose to adjacent normal tissues. This precision demands that the patient's position during each treatment fraction is identical to their posture during CT simulation. Immobilization devices restrict physical movement, minimize inter-fraction variations (day-to-day setup shifts), and mitigate intra-fraction motion (movement during beam delivery). \nImmobilization systems are categorized into non-rigid positioners (e.g., head cups, pillows, knee sponges) and rigid immobilization devices (e.g., thermoplastic masks, vacuum cushions, stereotactic frames). Devices must be rigid, comfortable, non-toxic, and constructed from low-attenuation radiolucent materials such as carbon fiber, low-density foam, or thin thermoplastics.

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Thermoplastic Mask Systems and Indexing Protocols

Thermoplastic Masks

\nThermoplastic sheets are composed of low-melting-point polymers (polcaprolactone) that become pliable when heated in a water bath at 65\u00b0C to 70\u00b0C. Once softened, the mask is draped and molded over the patient's anatomical contours and fastened to a rigid baseplate. As it cools to room temperature (over 3 to 5 minutes), the plastic hardens, forming a rigid shell.

• 3-Point Masks: Secure the head and forehead to the baseplate. Used primarily for brain tumors and cranial irradiation.
• 5-Point (Head, Neck, & Shoulder) Masks: Extend down over the clavicles and shoulders, fastening at the head, neck, and supraclavicular baseplate notches. Mandated for head-and-neck malignancies (e.g., nasopharynx, larynx, oral cavity) to suppress shoulder movement and cervical spine rotation.

      [ Head Baseplate Notches ]
               /    \
         (   Patient Head   )
               \    /
      [ Neck Baseplate Notches ]
               /    \
        [ Shoulder Lock Pins ]

Mechanical Indexing

\nTo ensure the immobilization baseplate does not slip or rotate on the flat couch, all modern positioning boards utilize mechanical indexing. Carbon fiber lockbars clip into pre-drilled notches located at fixed intervals (e.g., notches A, B, C, D) along the edges of the CT and linac couch tops. Indexing locks the immobilization hardware into exact, reproducible $X, Y, Z$ spatial coordinates relative to the treatment couch origin.

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Vacuum Cushion Systems (Vac-Lok) and Body Positioning Accessories

Vac-Lok Cushion Mechanics

\nA Vac-Lok cushion consists of a airtight vinyl shell filled with small polystyrene (styrofoam) spheres. During initial patient positioning, the therapist drapes the soft bag around the patient's body contours (e.g., pelvic cradle, arms-up thoracic support). A vacuum pump evacuates the air from the bag. Atmospheric pressure compresses the rigid polystyrene beads together, locking the cushion into a hard, custom-molded rigid negative impression of the patient's body.

Specialty Positioning Accessories

• Belly Board: A specialized foam platform with a central cutout designed for patients lying prone during pelvic irradiation (e.g., rectal cancer). Gravity forces the small bowel anteriorly into the cutout, displacing mobile small bowel loops out of the high-dose posterior pelvic fields.
• Breast Board: An inclined baseplate that elevates the patient's torso (10\u00b0 to 25\u00b0). Elevation causes the ipsilateral breast tissue to fall inferiorly and away from the neck, while keeping the arm abducted superiorly on a dedicated handgrip to clear lateral treatment fields.
• Abdominal Compression Plate: A mechanical screw-press or inflated bladder applied to the upper abdomen during lung or liver SBRT. Compression restricts diaphragmatic excursion, reducing respiratory tumor motion from > 2 cm down to < 5 mm.

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Radiopaque Marking Protocols and Isocenter Triangulation

\nDuring CT simulation, external reference marks must be placed on the patient's skin or thermoplastic mask to establish a spatial coordinate system.

Skin Marker Types

1. Lead / Dense BB Markers: Small radiopaque spheres (1.0 mm to 2.0 mm diameter) placed directly over skin tattoo sites to mark reference points.
2. Radiopaque Wires: Flexible metallic or solder wires taped along surgical scars (for breast or sarcoma fields) or field borders.
3. Anatomical Markers: High-density markers inserted into body cavities, such as vaginal cylinders, anal markers, or nipple BBs, to delineate target margins on CT axial slices.

Triangulation Alignment Protocol

\nTherapists place three radiopaque BBs on the patient's skin to form a triangulation coordinate origin:

• One anterior midline marker ($X=0, Z=0$).
• Two lateral markers positioned symmetrically on the left and right sides ($Y_{\text{left}}, Y_{\text{right}}$).

                    [ Anterior Laser BB ]
                              |
                              |
   [ Right Lateral BB ] ------+------ [ Left Lateral BB ]
                              |
                     ( CT Isocenter Origin )

\nAfter the CT scan is completed, the radiation oncologist contours the target volume, and the dosimetrist selects the optimal treatment isocenter. The treatment planning system calculates the exact directional shift vectors from the initial reference BB triangulation origin to the final treatment isocenter:

$\Delta X = X_{\text{isocenter}} - X_{\text{BB}}$ $\Delta Y = Y_{\text{isocenter}} - Y_{\text{BB}}$ $\Delta Z = Z_{\text{isocenter}} - Z_{\text{BB}}$ \nTherapists execute these calculated shifts at the linear accelerator using motorized couch translation or alignment lasers prior to treatment delivery.