AbstractID: 9000 Title: Dynamic Intensity Modulated Total Body Irradiation (TBI)
Treatment
Total body irradiation (TBI) aims to deliver a uniform dose to the body while limiting the dose to lung and
kidney. Several techniques have been reported for approaching this goal. At our institution, we use a
computer controlled translating couch combined with transmission blocks for lungs and kidneys. Although
the technique enhances patient comfort and consistency, dose non-uniformity exists due to varying patient
thicknesses, and shielding of critical structures is compromised by the large penumbra produced by beam
divergence. We have developed a technique that combines dynamic multi-leaf collimation in combination
with couch translation to modulate the intensity of the beam during the treatment.
We first deduce one-dimensional intensity profiles for each MLC leaf pair according to the patient’s
geometry and the desired shielding. Using a leaf sequencing routine developed specifically for TBI, we
then calculate the positions of each leaf as a function of the delivered monitor units (MUs). Using
RANDO phantom data we test the prescription, and determine the improvement in dose homogeneity and
the reduction in penumbra around the critical structures. With the blocks, the penumbra region is 3 cm, and
the percent dose fluctuates from 100% - 117% (100% is prescribed to umbilicus level) along the axis. With
dynamic MLC, the penumbra reduces to within 1 cm and the maximum dose stays within 10% of that at the
umbilicus level based on calculation. Combining IMRT and translating couch can thus achieve a dose
distribution unattainable with all previous techniques.