(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: The total body irradiation (TBI) typically requires a large linac room and patient stand at extended SSD (> 400 cm) with customized compensators and partial transmission blocks. The purpose of this study was to develop a TBI technique using IMRT at extended SSD that can be performed in any size room.
Methods: In this technique, patient lying on a platform is placed directly under gantry close to the floor with cranial-caudal axis parallel to gantry rotation plane and ~200 cm SSD. Two abutting fields of 25×40 cm² at gantry ±21° and collimator 90° are employed, sharing same external isocenter. A Matlab optimization program was developed to generate a uniform dose at the patient mid-plane with adequate shielding to the lungs and kidneys. The optimal fluence was input to TPS for MLC leaf sequencing, final dose calculation, plan evaluation and QA. Treatment planning was practiced in both phantom and patient CT images, and measurement and QA were done in phantom.
Results: The dose distribution in the mid-plane for both phantom and patients were within ±5% of the prescription after a few iterations, and they were confirmed with ion-chamber measurements in phantom. The mean dose to lungs and kidneys can reach as low as 22%. For a prescription of 200 cGy/fraction the total MU was ~ 2500/1800 for superior/inferior fields. The overall treatment time for both AP/PA was therefore ~55 minutes, to meet the maximum absorbed dose rate criteria of 15 cGy/min. IMRT QA with portal dosimetry shows excellent agreement.
Conclusion: We have developed a promising TBI technique using IMRT at extended SSD. Patient is in a comfortable recumbent position with good reproducibility and minimal movement. The technique allows excellent sparing of lungs and kidneys and can be executed in any linac room, without compensators and partial transmission blocks.
Keywords
Taxonomy
TH- External Beam- Photons: IMRT/VMAT dose optimization algorithms
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