TH-CD-304-02: Clinical Uncertainty of in Vivo Dosimetry for Intensity-Modulated Radiation Therapy Using Optically-Stimulated Luminescent Dosimeters

Authors


Abstract

Purpose:

Several studies have reported the physical properties of optically-stimulated luminescent dosimeters (OSLDs) and suggest their efficacy for clinical in vivo dosimetry, but few publications have assessed the clinical uncertainties associated with OSLD-based in vivo dosimetry for conformal and intensity-modulated treatment. The purpose of the current work is to identify and characterize clinical uncertainties for OSLD-based in vivo dosimetry.

Methods:

OSLDs are placed in dosimetrically-appropriate locations on a weekly basis, covered with small 5 mm bolus squares, exposed during patient treatment, read, and compared with doses predicted from the treatment planning system. Six (6) parameters were identified as significant contributors to uncertainty in the process: Inherent physical OSLD uncertainty (σ_ OSLD), OSLD reader uncertainty (σ_reader), dose calculation uncertainty in the build-up region (σ _calc), uncertainty of depth for planned dose (σ_d), dosimetric uncertainty due to daily image-guided shifts (σ_ s), and placement uncertainty (σ_p). σ_d, σ_ s, and σ_p were estimated by analyzing clinical OSLD dosimetry for inverse-planned intensity-modulated treatment plans (prostate, lung, and head-and-neck) and field-in-field intensity-modulated treatment plans (breast). Total uncertainty was estimated by summing the 6 components in quadrature.

Results:

σ_OSLD was defined by the manufacturer (±3.0%). σ_calc was assumed to be approximately ±5.0% at 5 mm depth from other works in the literature. σ_reader and σ_s were measured to be ±1.0 and ±5.8% at 5 mm depth respectively. σ_p was found to be ±3.6% for breast, ±4.2% for prostate, ±4.4% for lung, and ±10.6% for head-and-neck. Total uncertainty was ±9.0% for breast, ±11.5% for prostate, ±13.2% for lung, and ±16.1% for head-and-neck.

Conclusion:

Site-specific clinical uncertainty for a limited selection of sites ranged from ±9.0% to ±16.1%. The largest components were image-guided shifts and estimated placement uncertainty. A wider selection of anatomical sites, site-specific correction factors, and clinical tolerance/action level guidelines will be presented.

Ancillary