Estimate of the excessive spinal cord dose in AP-PA thorax treatment

By providing quick reference graphs, we have attempted to answer the question "Does this patient need a compensating filter to keep the spinal cord dose within a certain limit?" Such graphs were obtained using several parameters that influence compensating filtration when treating the mediastinum with (anterior-posterior) parallel opposed fields. For typical 5 and 10% excess spinal cord doses above the tumor dose, the reference graphs were designed to provide quick answers for patients with the following measured parameters: (1) patient thickness at the beam axis, (2) difference in source-to-skin distances, (3) air gap between the posterior skin and the couch top, and (4) posterior spinal cord depth. 4, 10, and 18 MV photon beams were used to illustrate the results.