Room: AAPM ePoster Library
To examine inter and intrafraction motion and its dosimetric impact for SBRT lung treatments.
4DCBCTs in 393 fractions from 93 SBRT lung patients treated with 4-5 fractions were analyzed retrospectively. The GTV defined at phase-0 of planning CT was propagated to phase images of pre-delivery (with couch correction applied) and post-delivery 4DCBCTs using deformable image registration (Elekta/Admire) to determine if daily GTV moved outside PTV. Target center of mass was used to calculate motion amplitude and target position. Differences in amplitude and position between pre-delivery and post-delivery 4DCBCT was used to assess intrafraction motion. Interfractionally, differences in motion amplitude were evaluated by comparing amplitude between the first fraction and subsequent fractions. Motion in the craniocaudal direction is reported.
Target motion amplitude from all 4DCBCTs was 0.52±0.44 (0.01,1.94)cm. Frequency distributions for inter and intrafraction amplitude variation were similar (Mann-Whitney U-test, p=0.315) with average, std and ranges of 0.03±0.14 (-0.52,0.58) and -0.03±0.11 (-0.60,0.35)cm. Intrafraction target position variation was -0.02±0.23(-1.31,0.88)cm, and was not correlated to motion amplitude or amplitude variation (R2=0.08). Due to intrafraction motion, part of GTV drifted outside PTV in 6 fractions within 4 patients (4.3% of total patients) by up to 0.98cm. Review of bony alignment between pre- and post-delivery 4DCBCTs revealed no difference in patient position suggesting target drift was mostly related to internal anatomy motion. Dosimetric impact of target drift was assessed by reconstructing dose on post-delivery 4DCBCT. 50% dose reduction for daily GTV was seen for one patient, with target cumulative dose slightly below prescription. Other patients had cumulative dose above prescription.
The result of this study indicates that large intrafraction target drift could occur and may result in GTV dose decrease or even target miss. Effective intrafraction target motion monitoring is still an indispensable component in lung SBRT delivery.
Funding Support, Disclosures, and Conflict of Interest: Support in part by Elekta Research Grant