Comprehensive Measurement for Total Body Irradiation (TBI)

Abstract

Purpose: To measure the beam data and dosimetry parameters for Total Body Irradiation (TBI). Materials and Methods: The set of measurements consists of seven items: output ratio, tray and spoiler factors, PDD, TMR, compensator factor, beam profile and dose verification. All measurements were performed on a 6 MV photon beam of Siemens Primus Linac. Experimental setup simulated TBI treatment conditions with SSD at 480 cm, field size at 40×40 cm^2 at SAD 100 cm, and the collimator and gantry angles set to 45° and 270°, respectively. In addition, a 1 cm spoiler screen was placed in front of the phantom to increase the surface dose. The first five items of dosimetry parameters were measured with a Farmer-type ion chamber, an electrometer, a parallel- plate ion chamber and a polystyrene phantom. The beam profile was measured both with films placed on the spoiler screen and with ion chamber in a polystyrene phantom. For dose verification, TLDs were placed at twelve different sites on the patient skin during TBI treatment. Results: The output ratio was 0.0466, very close to that obtained under calibration conditions after applying the inverse square correction and output factor correction for TBI treatment conditions. The tray factor, spoiler factor and the combined tray-and- spoiler factor were 0.949, 0.982 and 0.931, respectively. The discrepancy between PDD and TMR was lower than 2 %. The compensator factor was 0.045 for polystyrene thickness up to 13 cm and the thickness ratio of polystyrene to lead was 0.511. Beam profiles measured with films and ion chamber were almost the same. For dose verification, the discrepancies between TLD and prescribed dose were comparable to the intrinsic measurement uncertainty, ±5 %. Conclusion: A comprehensive set of beam data and dosimetry parameters were measured and all ratio discrepancies seen during the study (except TLD part), consistent with those applied clinically, were within 2 % indicating the study viable.