Thorax contouring

The breast is a gland: its glandular tissue can produce milk after childbirth, carried to the nipple by a network of ducts, and it is surrounded by fat and supporting connective tissue.

The RTOG breast atlas defines the breast as the glandular breast tissue apparent on CT, together with the clinically marked breast (for example, wire placed at simulation). Its boundary table:

• Top: the clinical reference and the insertion of the second rib.
• Bottom: the clinical reference and where breast tissue is no longer apparent on CT.
• Front: the skin.
• Back: excludes the pectoralis muscles, chest wall muscles and ribs.
• Outside: the clinical reference, typically the mid-axillary line, excluding the latissimus dorsi muscle.
• Inside: the junction of the sternum and ribs.

The atlas notes that the top, outer and inner borders vary a lot with breast size, shape and position, and that the breast contour should not cross the midline. These are the atlas's boundaries for the breast target; the same breast contour is the starting point for both organ-at-risk structures — the uninvolved ipsilateral breast is the treated breast minus the PTV, and the contralateral breast is the opposite side.

The ribs, intercostal muscles and soft tissue of the chest wall protect the heart and lungs and move with each breath.

Kong 2011 builds the ribs/chest wall structure automatically:

• Expand the corrected lung edges by 2 cm to the side, front and back.
• Include the intercostal muscles; exclude other muscles and skin.
• To save effort, the rib contour can be limited to within 3 cm of the PTV.
• Where collapsed lung lies in or near the PTV, edit the contour by hand.

For the upper abdomen, Jabbour 2014 makes the chest wall the same way from the liver and lung (a 2 cm expansion front, back and side), including the ribs and chest wall soft tissue and excluding the sternum and vertebral bodies.

"Chest wall" and "rib" are different structures and sources use different ones: Timmerman 2022 lists a "Rib" row in its 1–8 fraction tables. A limit only applies to the structure it was written for.

The esophagus is the muscular tube that carries food and liquid from the throat to the stomach.

Kong 2011 contours the esophagus on a mediastinal window, including the mucosa, submucosa and all muscle layers out to the fatty outer layer (adventitia):

• Top: the level of the cricoid cartilage.
• Bottom: every slice down through the gastroesophageal junction to the stomach.
• Oral contrast is not routinely recommended unless tumor lies near the esophagus; if it is used, the esophagus can be set to soft-tissue density for the dose calculation.

Timmerman 2022 (Tables 8–11, written for its SBRT dose tables) uses the same wall layers but contours only at least 10 cm above and below the PTV. Jabbour 2014 adds an optional gastroesophageal junction structure — a wedge that overlaps the lowest esophagus and the top of the stomach.

The great vessels — the aorta, the venae cavae and the pulmonary arteries and veins — carry blood between the heart, the lungs and the body.

Timmerman 2022 (Tables 8–11, written for its SBRT dose tables) contours the wall and lumen of the named vessel, at least 10 cm above and below the PTV.

Kong 2011 left the heart and great vessels out of its thoracic atlas because of their complexity. Open published boundaries beyond Timmerman's instruction: verified source required.

The heart pumps blood through the lungs and around the body.

Feng 2011, a cardiac CT atlas built with cardiology and cardiac radiology:

• Top: the heart starts just below the left pulmonary artery. For simplicity, a rounded contour that also takes in the great vessels can be drawn there.
• Bottom: the heart blends with the diaphragm.
• Include the fatty tissue inside the pericardium, because the coronary vessels run in it, even where no heart muscle is visible.
• The superior vena cava can be contoured separately on a contrast scan; on a non-contrast scan it can be included for consistency.
• A window level of 50 and width of 500 shows most structures.

Timmerman 2022 (Tables 8–11, written for its SBRT dose tables) contours the heart with the pericardial sac, starting at the bottom of the aortic arch (the aortopulmonary window) and ending at the heart's apex. The two sources set the top of the heart at different levels, so match the source behind your limit.

The left anterior descending artery supplies blood to the front wall of the heart, including much of the left ventricle.

Feng 2011 describes the LAD as starting from the left coronary artery and running in the groove between the right and left ventricles (the interventricular groove). If it is hard to see, a narrower window helps (level 50, width 150 in the atlas).

Even after training with the atlas, observers in Feng's study overlapped with the reference LAD contour only about 62% of the time on average, so expect variation and take care with this structure.

The lungs bring oxygen into the blood and remove carbon dioxide.

Kong 2011 recommends:

• Contour only the air-inflated lung, leaving out fluid and collapsed lung (atelectasis).
• Auto-contouring is fine with a threshold chosen for each scan, but always review and edit the result.
• Exclude the proximal bronchial tree; include small vessels (under 1 cm, or beyond the hilum).
• Right and left lungs can be one structure or two. Limits are usually based on both lungs minus the GTV — only the part of the GTV inside the lung, so mediastinal nodal GTVs are not subtracted.
• With collapsed lung, IV contrast or PET can help separate tumor from lung.

Timmerman 2022 (Tables 8–11, written for its SBRT dose tables): right and left lung as one structure, including all lung tissue but excluding the GTV and the major airways (trachea and main/lobar bronchi). Sources differ on which target volume is subtracted, so match the source behind your limit.

The skin is the body's protective outer layer; it also helps control temperature and carries the sense of touch.

Skin is built as a shell just inside the body outline, not drawn by hand:

• Timmerman 2022 (Tables 8–11, written for its SBRT dose tables): the outer 0.5 cm of the body surface anywhere within the whole-body contour.
• The EPTN 2018 atlas: the body contour as the outer border and the body contour cropped by 5 mm as the inner border, created automatically and then checked.

Both describe a 5 mm shell; other sources may use a different depth, so check the source behind your limit.

The trachea and bronchi carry air between the throat and the lungs.

Kong 2011 contours the proximal bronchial tree as one structure on a mediastinal window, including the airway wall (mucosa, submucosa and cartilage rings) and the air channel:

• the lowest 2 cm of the trachea, the carina, both mainstem bronchi, both upper lobe bronchi, the bronchus intermedius, the right middle lobe bronchus, the lingular bronchus and both lower lobe bronchi;
• each lobar bronchus ends at its first segmental branch.

For lung SBRT, Kong describes the central zone as a 2 cm expansion of the proximal bronchial tree in every direction except upward.

Timmerman 2022 (Tables 8–11, written for its SBRT dose tables) uses a longer "trachea and large bronchus" structure: the trachea and cartilage rings from 10 cm above the PTV, down into the bronchi, ending at the first branching of the named lobar bronchus.