Exhibit Hall | Forum 6
Purpose: Optically stimulated luminescent dosimeters (OSLDs) are commonly used in radiotherapy settings to perform coarse in-vivo verification of therapeutic dose delivery, often at sensitive or unusual anatomical locations on the patient skin surface. In MV linac-based image-guided radiotherapy (IGRT), OSLDs are typically placed on the patient at the beginning of a treatment session and retrieved upon its completion. OSLDs are known to overrespond to kV x-rays, however, and if kV cone-beam CTs (CBCTs) are used for IGRT they can contribute spurious excess “dose” which is sufficient to complicate the interpretation of OSLD measurements. We sought to quantify the "pseudo" dose response of our in-vivo OSLDs to standard linac kV CBCT protocols used in our clinic, under a range of common treatment conditions.
Methods: We annealed a large set of used OSLDs and verified they reported negligible dose readout before performing kV CBCT and MV exposures. We performed a variety of measurements, varying such factors as the linac type, CBCT protocol, number of CBCTs delivered, MV beam energy, MV dose delivered, OSLD orientation, phantom type and topography, and applied bolus. In a typical setup we delivered a 2+ Gy dose to one set of OSLDs and then repeated the exposure with another set of OSLDs but adding kV CBCTs; comparison of the two sets of data enabled us to disentangle the average “dose” contribution from the kV CBCT under study.
Results: Our measurements revealed that a linac kV CBCT can contribute 0-10 cGy of apparent OSLD "dose," depending on treatment conditions and the particular imaging protocol used.
Conclusion: We found that commonly used linac kV CBCT techniques can contribute several cGy of apparent “pseudo” dose to OSLD measurements. Our tabulated estimates could be useful to clinics seeking to correct for the effects of kV CBCTs on OSLD dosimetry.
Not Applicable / None Entered.