In a lung treatment scenario with AP/PA portals, what is the depth to the tumor in the PA field if the anterior portal depth is 14 cm?

In a lung treatment scenario involving AP (anterior-posterior) and PA (posterior-anterior) portals, the depth of the tumor in the PA field typically accounts for the difference in tissue thickness when shifting the radiation fields from the anterior to the posterior approach.

When the anterior portal depth measures 14 cm, it reflects the measurement from the surface of the skin to the tumor in the anterior position. For the PA portal, the depth is generally greater than the anterior measurement due to the additional distance the radiation beam must travel through the body before reaching the tumor.

Anatomical considerations, such as the position of the lungs, pleura, and potentially the heart and other mediastinal structures, influence the actual depth measurement for the PA view. Given that the question suggests a depth of 16 cm, 18 cm, or 20 cm is considered, with 18 cm taking into account standard adjustments made for the effective path length in the posterior direction.

Thus, the 18 cm depth reflects an appropriate extension beyond the anterior measurement, aligning with clinical considerations for lung treatment. This reasoning supports the choice of 18 cm as the depth to the tumor in the PA field.