Investigation of AP/PA Recumbent Technique for Total Body Irradiation

Purpose: Total body irradiation (TBI) is to deplete patient’s bone marrow and suppress the immune system by delivering uniform dose to patient’s whole body with a relatively low dose rate. The widely used total body irradiation (TBI) protocol in many institutions is to extend the source to surface distance (SSD) to over 400 cm in a large treatment room. The TBI techniques currently used at Duke University Medical Center is anteroposterior (AP)/posteroanterior (PA) technique and bilateral technique. Though bilateral technique TBI is executed with simpler treatment planning and setups in a more comfortable position, it could not provide adequate shielding to lungs and kidneys using blocks like AP/PA TBI technique. However, the whole process of block fabricating and verification is labor-consuming and time-costing. This project aims to develop a better AP/PA TBI treatment method in recumbent position, which provides better sparing for lungs and kidneys in any treatment room.

Methods: In this study, we considered different treatment techniques (three-dimensional conformal radiation therapy (3DCRT) and intensity-modulated radiation therapy (IMRT)), different treatment position (on the floor or on the couch), and different setups (gantry rotation and platform movement). TBI treatment plans were simulated in Eclipse treatment planning system by using both water equivalent phantom and patient CT image. Prescription for the treatment plans was 200 cGy per fraction with 4 fractions. The dose homogeneity should within ±10% of the prescription dose. Dose constraints for kidney and lung are 25% of the prescription dose. In 3DCRT TBI, we applied multi-leaf collimators (MLCs) for OARs shielding and used boost field to provide adequate dose to lungs and kidneys. For IMRT TBI, an iterative optimization algorithm was generated for increasing dose uniformity. By using IMRT, dynamic multi-leaf collimators (DMLCs) provided shielding for kidneys and lungs, which were considered in fluence map optimization. Volume dose and dose profiles were used to analyze the dose uniformity. Measurements with solid water phantom in treatment room were performed to verify the simulation results. IMRT QA with portal imager was performed for phantom.

Results: 3DCRT could not ensure the dose homogeneity and dose deliver accuracy at the same time. To ensure the dose homogeneity in 3DCRT TBI, patient/platform position should be changed between field or applying customized wedge to compensate the inverse square law. For IMRT, the optimization algorithm has excellent performance for both phantom and patients. The dose homogeneity in the mid-plane of both phantom and patients were less than ±5% of the prescription dose after a few iterations. Lungs and kidneys could receive around 25% prescription dose. The simulation and measurement results agree with each other. No additional physical compensators or partial transmission blocks were needed. Portal dose and predict dose perfectly agreed with each other. CR film worked well in positioning. Surface dose enhancement under blocked field was observed.

Conclusion: IMRT technique performed much better than 3DCRT in TBI treatment. In this study, we develop an AP/PA recumbent position IMRT TBI technique that could be used in any linac room. This technique can ensure high dose homogeneity, provide better sparing to lungs and kidneys, and reduce the complexity of TBI treatment planning without the need of labor-intensive compensators and partial transmission blocks.