Abstract
PurposeFrequency of conventional kV-image guidance is sometimes sacrificed to reduce concomitant risk, leaving deviations of unguided fractions unknown. MV-imaging and treatment dose can be collectively optimized on Halcyon, where fractional MVCBCT provides complete anatomic records for course-wide dose reconstruction. By retrospective dose accumulation, this work simulated the impact of imaging frequency on patient treatment dose on the platform of Halcyon.MethodsFour hundred and sixteen MVCBCT image sets from 16 patients of various tumor sites treated with radiotherapy on Halcyon were retrospectively selected. After applying the image-guided couch shifts of the clinical records, deformable image registration was performed using Velocity software, to deform the planning CTs to the corresponding MVCBCTs, generating pseudo CTs representing the actual anatomies on the treatment day. Fractional treatment dose was reconstructed on pseudo CTs for accumulation, representing the actual patient dose (Ddaily). To simulate weekly image guidance, fractional dose was reconstructed and accumulated by incorporating 1 CBCT-guided corrections and 4 laser-guided setups of each week (Dweekly). Limited by partially imaged volumes and different organs-at-risk of various sites, only target dose-volume parameters were evaluated across all patients.ResultsGTV_D98%, CTV_D98%, PTV_D90%, PTV_D95%, PGTV_D90%, and PGTV_D95% were evaluated, where Dx% means the minimal dose received by x% volume. Pairwise comparisons were made between plan dose and Ddaily, Ddaily and Dweekly respectively. PGTV_D95% of accumulated Dweekly were significantly lower than those of accumulated Ddaily by up to 32.90% of prescription dose, suggesting that weekly-guidance may result in unacceptable under dose to the target. The broad distribution of fractional differences between Ddaily and Dweekly suggested unreliable patient positioning based on aligning surface markers to laser beams, as a popular approach broadly used on conventional Linac systems. Slight target under-dose was observed on daily reconstructed results compared with planned dose, which provided quantitative data to guide clinical decisions such as the necessity of adaptive radiotherapy.ConclusionFractional image guided radiotherapy on Halcyon provides more reliable treatment accuracy than using sacrificed imaging frequency, which also provides complete anatomic records for deformable dose reconstruction supporting more informed clinical decisions.
Highlights
Image guided radiation therapy (IGRT) using various imaging modalities has been broadly applied to clinics to reduce patient setup errors and associated risk of missing target for tumors at different anatomical sites [1, 2]
Significant differences were observed between plan dose and accumulated Ddaily in terms of CTV_D98%, PTV_D90%, PTV_D95%, PGTV_D90%, and PGTV_D95% (P
Based on faster imaging and incorporable megavoltage cone beam CT (MVCBCT) dose of Halcyon system, the potential benefit of dosimetric accuracy from mandatory fractional image guidance was investigated in this study
Summary
Image guided radiation therapy (IGRT) using various imaging modalities has been broadly applied to clinics to reduce patient setup errors and associated risk of missing target for tumors at different anatomical sites [1, 2]. Guidance is desirable for every fraction to minimize the geometric and dosimetric uncertainties [3,4,5,6], conventional kV imaging frequencies are sometimes sacrificed to balance treatment accuracy and concomitant dose [7,8,9]. Existing studies of imaging frequencies focused more on geometric impact [14,15,16,17], yet analysis based on dose accumulation is missing but clinically desirable. Fractional image guidance does provide more confidence on setup accuracy, and record complete anatomic information for retrospective dose reconstruction, such as using deformable image registration (DIR) method which has been validated quantitatively by our previous study [20]
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