Extra Credit: A patient is being treated for a mediastinal m…

Extra Credit: A patient is being treated for a mediastinal mass on a linear accelerator with a calculated isocenter of 100 SSD.  The patient’s separation is 24 cm.  The tumor is located 4 cm posterior to the anterior surface of the patient. The doctor wants to treat the patient using a non-isocentric source to skin distance set-up.  What is the patient’s AP SSD?