(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose:
Frequency 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 resampling, this work simulated the impact of imaging frequency on patient treatment dose.
Methods:
Using Velocity software, deformable image registration was performed on 416 MVCBCTs from 16 patients of various tumor sites to cast the HU of planning CT onto MVCBCT images representing actual anatomies on the treatment day. Fractional reconstructed dose (1f_fractional) was accumulated representing the actual dose distribution (1f_sum). To simulate weekly guidance, only the first couch shift of every 5 fractions was applied to the reconstructed dose (5f_fractional) and accumulated without remaining 4 setup corrections every week (5f_sum). Limited by partially imaged volumes and different organs-at-risk of various sites, only target dose-volume parameters were compared.
Results:
GTV_D98%, CTV_D98%, PTV_D90%, PTV_D95%, PGTV_D90% and PGTV_D95% were evaluated (Dx%: the minimal dose received by x% volume). Pairwise comparisons were made between 5f_fractional and 1f_fractional, 5f_sum and 1f_sum, 1f_sum and planned dose, respectively. All the difference are (former - latter)/prescription×100%. All parameter mean difference between 5f_sum and 1f_sum were negative, suggesting that weekly-guidance can result in an average under-dose of 1.54% to the target. The maximum difference of PTV_D95% and PGTV_D95% between 1f_sum and 5f_sum reached -10.37% and -32.90% respectively, suggesting larger dose unreliability in the margins of the planned target volumes associated with reduced imaging frequency and increased setup errors. Slight target under-dose was observed on daily reconstructed results compared with planned dose, but is clinically acceptable.
Conclusion: image guidance may introduce dose error up to 32.9% over one course. Fractional image guidance on Halcyon provides more reliable treatment results than IGRT using sacrificed imaging frequency.
Funding Support, Disclosures, and Conflict of Interest: Capital's Funds for Health Improvement and Research[2018-4-1027]; Fundamental Research Funds for the Central Universities/Peking University Clinical Medicine Plus X - Young Scholars Project(PKU2020LCXQ019); National Key R&D Program of China(2019YFF01014405); Ministry of Education Science and Technology Development Center[2018A01019]; National Natural Science Foundation of China[11505012,11905150].Corresponding author: Yibao Zhang, zhangyibao@pku.edu.cn
Keywords
Image-guided Therapy, Megavoltage Imaging, Cone-beam CT
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