You can tell just from the frontal chest X-ray that this mediastinal mass is posteriorly located by using the cervicothoracic sign. The anterior mediastinum ends superiorly at the level of the clavicles, while the posterior mediastinum extends superiorly above the level of the clavicles. (See for yourself that this is true by scrolling through the sagittal images). This means that a mediastinal mass that extends above the clavicles has to be in the posterior mediastinum. The CT confirms that the mass is posterior.

Most posterior mediastinal masses are neurogenic tumors and this patient had a known schwannoma.

The International Thymic Malignancy Interest Group (ITMIG) recommends replacing the terms anterior, middle and posterior mediastinum with prevascular, visceral, and paravertebral compartments respectively. The ITMIG classification is detailed in the extensively illustrated reference below ¹.