A Varying Field Size Translational Bed Technique for Total Body Irradiation.
Thesis DisciplineMedical Physics
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Total body irradiation is the irradiation of the entire patient as a conditioning for bone marrow transplants. The conditioning process involves destroying the bone marrow allowing for repopulation of the donor bone marrow cells, suppression of the immune system to allow stop graft rejection, and to eliminate the cancer cell population within the patient. Studies have been done demonstrating the importance of TBI conditioning for BMT5. A range of TBI treatment techniques exist, this department uses a bi-lateral technique which requires bolus packed around the patient to simplify the geometry of the treatment. This investigation will focus on one technique which involves using a translating bed. This technique effectively scans a radiation beam over the patient as the bed moves through the beam. Other investigations on translating beds concentrated on varying the scan speed to achieve a dose uniformity to within ±5%. The recommendations quote a dose uniformity of +5% and -10% as acceptable⁹. The dose uniformity in these investigations was along the midline in the longitudinal direction only. This investigation varied field size to achieve dose uniformity to within ±2.5% along the midline of an anthropomorphic phantom. The goal was to determine if a dynamic multi-leaf collimator could be used to give a uniform in the transverse direction as well as the longitudinal direction. An advantage of utilizing the DMLC for this treatment is the ability to shield organs at risk, i.e. lungs and kidneys, without requiring resources to produce shielding blocks14. Gafchromic-EBT film18 was used as a dosimeter but gave unreliable results due to the lack of film scanning equipment with an appropriate sensitivity for reading the dose to the film. Scans were simulated using Xio treatment planning software. The results from the simulations gave a more reliable indication of the absorbed dose to the midline of the phantom. The disadvantage of this varying field size technique was the time and complexity involved in creating a treatment plan. Within the Xio software exists a limit on the number of beams allowed to be applied in a single plan. There is a maximum of 99 beams allowed which is not enough for complete coverage of a patient. A way around this is to increase the field sizes and decrease the scan speed. This option was not investigated. The advantage of this technique was the increased dose uniformity (±2.5%) in comparison to the varying scan speed techniques (±5%). This technique also allows the patient to be unencumbered during the treatment making the process more comfortable for them.