Room: Exhibit Hall
Purpose: Using an arc technique for Total Body Irradiation (TBI) treatment is attractive, as it requires no changes to a linear accelerator, minimal additional equipment and can be done in a standard treatment bunker. This relatively new approach requires careful consideration of shielding the organs at risk, especially because imaging to confirm shielding placement in this geometry is challenging.
Methods: The shielding conditions for an arc TBI were simulated in Excel, assuming ray lines diverging from a point source to a plane representing the midline of a patient. Virtual shielding was positioned in space above the patient and if a ray line was intercepted by the shielding it was considered attenuated. These simulations were compared to measurements made with a MapCheck2 surrounded with 30 cmÂ² blocks of solid water and with a 13 cm by 6 cm by 1.4cm lead shield.
Results: The results showed that as the shields moved away from the central axis at gantry zero (CAG0) or changed height noticeable complications occurred to where the shadow of the shields were cast. It changed more than the shield was moved and the width and the proximal and distal penumbras of the shadow varied too. The divergence simulations captured the patterns and trends correctly but were imperfect, most notably with discrepancies in the penumbra width. This may imply that factors beyond the divergence mentioned and/or represent the challenges of an accurate extended distance set-up.
Conclusion: The results show that with the arc technique it is dosimetrically preferable to place any shields at CAG0 even though locating the patient asymmetrically under the beam will result in longer treatment times. If setting up the shields at CAG0 is not possible, then the shield positioning and size should be determined to ensure dose reduction occurs where desired.