A Simple 3D AP-PA Solution for Multiple-Isocenter Total Body Irradiation
Abstract
Purpose
Recently, a modern total body Irradiation method using volumetric-modulated arc technique (VMAT-TBI) has gained popularity. Although many advantages are associated with VMAT-TBI but the disadvantages such as time-consuming planning process and longer treatment time. For the full-dose TBI, the effort to create the complicated VMAT plans can be justified, but for the low-dose TBI, it seems unnecessary to use such a sophisticated solution. We have simplified this treatment method by replacing all VMAT beams with AP/PA 3D fields. This simplified technique will offer much shorter planning time, faster treatment time, no PSQA required, and higher tolerance for setup uncertainties.
Methods
This 3D AP-PA solution utilizes multiple isocenters and overlapping AP/PA beams to deliver a desired dose distribution. Compared to VMAT fields, these big open AP/PA fields have the advantage of larger setup margins. An in-house python script for our treatment planning system (RayStation) was created to generate and optimize AP/PA fields with feathering overlaps. The script will create 5 feathering sub-fields for the junctions and reduce hot/cold spots by adding 2 more sub-fields for each beam.
Results
Two plans were generated with an anonymized patient’s images. The first plan was prescribed to 2 Gy and the lungs was not spared. The second plan was prescribed to 12 Gy and the lung was spared. Both plans were compared to the VMAT-TBI plans generated with the same dose criteria. For the low-dose prescription without sparing the lungs, the dosimetric results from the VMAT-TBI and 3D AP-PA plans are very similar. For the high-dose prescription sparing the lungs, the dosimetric results from the VMAT-TBI plan is superior to the 3D AP-PA plan due to the chest walls above and below the lungs received insufficient dose.
Conclusion
The 3D AP-PA technique is a great solution for low-dose TBI patients.