Commissioning of Bilateral (Parallel Opposed) Extended Source-to-surface Distance Total Body Irradiation Technique and Its Long-term Stability.

Aim: This article describes the commissioning of a total body irradiation (TBI) technique using bilateral-parallel-opposed fields at extended source-to-surface distance (SSD). Material: The measurements are based on the actual patient treatment geometry, which requires the patient to be placed inside a Perspex box. The gaps between the patient and the walls of the Perspex box were filled with rice bags to achieve full scattering conditions. The TBI box's lateral separation can be adjusted to 42 cm, 52 cm, or 62 cm by inserting the removable side walls to either of the three pairs of slots. A Farmer chamber, inserted inside a water-equivalent plastic slab phantom placed under full scattering conditions, was used for depth dose and profile measurements. The extended SSD was 333.5 cm, and the available field size was 132 cm × 132 cm. For output measurement, dose-to-water calibration factors for 6 MV and 15 MV energies were derived for the extended SSD. Bilateral-opposed fields were measured at three different separations to calculate lateral tissue effects. Result: For the 6 MV and 15 MV beams at a 42 cm separation, the midline-to-surface dose ratios were 1:1.17 and 1:1.08, respectively. As the separation increased, this ratio increased faster for the 6 MV beam and slower for the 15 MV beam. For the end-to-end quality assurance test (monitor unit to dose verification), the noted deviations were - 2.16% for the 6 MV beam and - 1.27% for the 15 MV beam. Conclusion: This article presents the detailed commissioning and long-term stability of the extended SSD TBI technique.