Special Simulation Considerations: Thorax

Simulations of the thorax can present with numerous challenges. The most common consideration for cancers of the thorax is tumor motion caused by respiration. Several treatment technologies and devices are available to reduce tumor motion; patient selection for each is important. Some devices utilize abdominal compression that restricts the depth of inspiration for the patient. Other techniques coach the patient how to breathe and include a breath-hold technique, active breathing control (described in the breast unit), and respiratory gating using a 4D CT.

Department protocols will vary based on the devices and technology available, but commonly 4D scans are taken to evaluate target motion. A 4D scan uses “time” as the fourth dimension; they capture multiple images of the patient’s anatomy at different phases of the breathing cycle, creating a dataset representing the range of motion. Patients with tumor motion greater than 5 mm will benefit from motion management treatment, like gating or compression. Both techniques will spare normal tissues by reducing the treatment field size.

During the simulation, patients are educated to establish their eligibility for compression or gating. If a 4D scan is an option, breathing instructions are explained to the patient. It is important to note the patient’s nominal breaths per minute and breathing ratio. These measurements determine the patient’s breathing pattern during the simulation. Respirations are monitored throughout the scan via a belt or tracking device.

Departmental protocols will vary. The University of Iowa acquires an expiration breath-hold scan with contrast for their primary dataset and a 4D scan. The 4D scan is performed using the patient’s established breaths per minute and breathing ratio. After the scan, a physicist or dosimetrist evaluates the CT slices for any artifact. Another option is to obtain a max inspiration and an expiration scan to determine the total amount of motion possible. A free-breathing scan is used for patients that are not good candidates for compression or gating. These scans help determine target volumes and assess tumor motion for treatment planning.

Through analysis of the 4D CT dataset, the radiation oncology team can identify the specific phases of the breathing cycle when the tumor is most stable and accurately target those phases for gated treatment. Most treatments deliver the radiation at the bottom of the patient’s breathing cycle (exhalation). For example, when the beam is on, the gating window may be set at 40% expiration to 40% inspiration. Gated treatments require a system that tracks the patient’s breathing cycle while connected to the treatment delivery system. These treatments take longer because the beam stops when the tumor is outside the treatment volume, relying on the patient to breathe consistently. Established workflows should exist in each facility.

Other challenges for treatments of the thorax are patient position and physical ability. For example, if a patient cannot raise their arms above their head, planning beams to safely reach their tumor may be difficult. Therapists should inform the physician and dosimetry; the best arm position may be akimbo. The akimbo position will move the arms away from the thorax, reducing the chance they will receive dose and open up the thorax to more beam angle options. Patients with COPD or large thorax tumors may have difficulty breathing laying flat; elevating the patient’s head with an incline board or vacloc may be necessary.

Treatment design for thorax malignancies is based on the primary tumor’s size, location, proximity to critical structures, and lymphatic drainage. Commonly affected nodes include the hilar and mediastinal nodes, but lymphatic drainage depends on the tumor’s location. Patients that are not candidates for respiratory monitoring with a free-breathing CT simulation, will have about a 2 cm expansion from the Internal Target Volume (ITV). The ITV encompasses the CTV with a margin for motion. Patients that are 4D candidates, the PTV is an expansion of 5-8 mm from the ITV.

Whole organ TD 5/5 for common OARs (Organs at Risk) for thorax radiotherapy

OAR	TD 5/5 (Whole Organ)	Outcome Associated
Spinal Cord	45 or 47 Gy	Myelitis/Necrosis
Lung	18 Gy	Pneumonitis
Heart	40 Gy	Pericarditis
Esophagus	55 Gy	Stricture/Perforation
Brachial Plexus	55-60 Gy	Nerve Damage

Treatment Techniques Overview: Lung

During the verification simulation appointment, first, position the patient to their 3-point reference (CT) isocenter. Indicated shifts from dosimetry are then applied to move the patient to the planned treatment isocenter. Depending on department preference, new set-up marks may be placed on the patient to indicate the planned isocenter and to eliminate daily shifts. Otherwise, shifts from the reference 3-point marks are applied daily. New marks should only replace existing marks when they can be placed on stable surface anatomy. SSD’s should be documented and table parameters recorded. Patients with lateral isocenters (>8 cm off midline) may require a lateral off-set to reduce clearance issues. For example, if a patient has a right lateral lung lesion greater than 8 cm off midline, placing the patient closer to the left edge allows the right side of the body to be closer to the center of the treatment table. A virtual isocenter may be needed for conebeam (CBCT) clearance if daily imaging is indicated. For safety, always verify gantry clearance before exiting the room.

Most treatments today utilize some form of daily Image Guided Radiation Therapy (IGRT). If the tumor is well defined, align to the planning target volume (PTV) and check bone and carina for verification. The PTV includes margins for uncertainties in beam alignment, tumor motion, and patient positioning. The thoracic spine is the most stable boney anatomy for alignment, especially for posteriorly located lesions. Ensure the spinal cord contour is within the vertebral foramen and verify the carina. For mediastinal tumors, align the carina first and then check the spine.

• Palliation (AP/PA): beam arrangement was historically used for lung treatments but is now commonly reserved for palliation. The initial AP/PA fields were treated to a dose of 45 Gy. If doses above 45 Gy are required, the beam arrangement is modified to obliques termed an “off-cord boost” to avoid overdosing the spinal cord and subsequent transverse myelitis.
• 3D Conformal Radiation Therapy (3DCRT): The size of the PTV and its location to structures dictate the beam arrangement. A three-field conformal plan is used for early-stage disease (Stages 1 & 2). The three fields are an anterior oblique, a posterior oblique, and a lateral. Wedges are used on the oblique beams to compensate for the curvature of the chest wall. Multi-leaf collimator (MLC) shielding conforms each beam shape to the PTV. More beam angles are added with increasing disease stage to improve the overall dose to the tumor and reduce the dose to the normal lung volume and surrounding tissues.
• Volumetric Modulated Arc Therapy (VMAT)/IMRT: Most treatments of the thorax use daily conebeam CT (CBCT) imaging and IMRT or VMAT treatment. These highly conformal plans reduce the dose to normal tissues and maximize the dose to the PTV. Plans are designed by setting dose limits and parameters in the planning system and the computer optimizes a treatment plan to the specifications.