Heart / Pericardium

The heart lies in the middle mediastinum inside the pericardial sac, with its base superiorly where the great vessels enter and leave and its apex pointing anteriorly, inferiorly and to the left. Its superior boundary on CT is the part most often defined differently between atlases.

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

• SBRT: most relevant for tumors next to the heart or pericardium; toxicity endpoints in SBRT tables include pericarditis.
• Conventional lung RT: cardiac dose has received growing attention in locally advanced lung cancer. Secondary analyses of RTOG 0617 are a good entry point to that literature.
• Esophagus and upper GI: the heart sits directly in front of the lower esophagus and gastroesophageal junction, so mid and distal esophageal plans are often limited by heart dose. Liver SBRT near the dome of the liver can also bring the heart close to high dose.
• Breast: the heart sits directly behind the left breast and chest wall, and Darby 2013 linked mean heart dose to later coronary events — which is why breast protocols ask for heart dose as low as reasonably achievable and suggest considering breath-hold techniques. Internal mammary nodal treatment can increase heart dose further. Breast protocols increasingly limit the LAD as well, because mean heart dose can stay low while the LAD receives much more.

Because the heart is a large organ, both point-dose and volume-based metrics (such as mean dose or a Vx) appear in published sources, depending on the fractionation and the endpoint.

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.