Simplified low-dose-rate infant total body irradiation

Purpose Low-dose-rate total body irradiation (LDR TBI) in infants possesses unique challenges because of the setup limitations imposed by anesthesia. We present an LDR TBI method with an anteroposterior/posteroanterior arrangement and dose rate < 10 cGy/minute for infants (thickness <14 cm) without the use of a beam attenuator. Materials and methods The delivery used a 6 MV TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA) with adjustable meterset rates below 100 MU/minute. A platform was constructed to support the patient 6 cm above the floor (~2 m from the source). A 40 × 40 cm² field at isocenter along with a 45° collimator rotation was used to achieve a maximum field width of approximately 113 cm at 2 m. The patient was positioned supine with head turned toward the patient's right side for the anteroposterior beam and prone with the head continuing to turn towards the patient's right side for the posteroanterior beam. A scattering plate with custom organ blocks was placed approximately 20 cm from the patient's skin surface. Output, depth-dose, surface-dose, block transmission, and off-axis measurements were taken using solid water slabs, ion chambers, and film. Results The TrueBeam provides several options for this treatment geometry to deliver LDR TBI at less than 10 cGy/min by lowering the linear accelerator meterset rate below 100 MU/minute. This allowed for a compromise between dose rate and treatment time to be selected by the radiation oncology team without the use of a beam attenuator, thus improving patient safety. Depth-dose and off-axis measurements confirmed a dose distribution within ±5% of the central-axis dose. A scattering plate distance of ~20 cm from the patient increases surface dose while allowing easy access to the patient during anesthesia. Conclusions The method described is useful to pediatric radiation oncologists seeking to deliver LDR TBI to an infant patient.