Practical time considerations for optically stimulated luminescent dosimetry (OSLD) in total body irradiation.

Abstract

Total body irradiation (TBI) treatments are used to treat the whole body in preparation for hematopoietic stem cell (or bone marrow) transplantation. Our standard clinical regimen is a 12Gy in 6 fraction, bi-daily technique using 6 MV X-rays at an extended Source-to-Surface distance (SSD) of 300cm. Utilizing these characteristics, the beam dose rate is reduced below 7cGy/min as is standard for TBI treatment. Dose received by the patient is monitored using optically stimulated luminescent dosimetry (OSLD). This work presents some practical calibration corrections based on time-dependant factors for OSLD calibration related to TBI procedure. Results have shown that a negligible difference is seen in OSL sensitivity for 6MV X-rays irradiated in standard SSD (100cm) and high dose rate (600cGy/min) conditions compared to extended SSD (300cm) and low TBI dose rate (6cGy/min) conditions. Results have also shown that whilst short term signal fading occurs in the OSL after irradiation at a high dose rate (37% reduction in signal in the first 15min), thereafter, negligible differences are seen in the OSL signal between 600 and 7cGy/min irradiations. Thus a direct comparison can be made between calibration OSLs and clinical TBI OSLs between 15min and 2h. Finally a table is presented to provide corrections between calibration OSL readout and clinical TBI dose readout for a period up to 7 days. Combining these three results allows users to pre-irradiate their calibration OSLs at standard dose rate and SSD, up to 1 week prior to clinical treatment, and still provide accurate in-vivo dosimetry. This can help with time saving and work efficiency in the clinic.