Program Information

Inter-Fractional Variations in Internal Tumor Volumes Using Multiple 4DCTs and CBCTs

M Xue*, S Feigenberg , S Tan , A Reese , B Yi , W D'Souza , W Lu , University of Maryland School of Medicine, Baltimore, MD

Presentations

SU-E-J-77 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To quantitatively evaluate the inter-fractional variations in ITVs over the course of treatment using multiple 4DCTs and CBCTs for patients treated with SBRT for lung targets. To compare using ITV generated from all phases of a 4D or average intensity projection (ITV-AIP) as reference contours for image guidance.

Methods: An IRB approved study included five patients who received SBRT for a lung target using daily image guidance with a CBCT. Three 4DCTs were acquired before (simulation), during and at the end of treatment. ITV-4Ds (on all 10 phases) and ITV-CBCTs were contoured manually. All CT images and contours were registered to the reference image - AIP1 from 4D1. The registration consists of intensity-based image registration followed by manual soft-tissue alignment, where physicians manually make shift to ensure that the tumor seen on CBCT is encompassed in ITV-4D1 or ITV-AIP1. The relationship between two ITVs was quantified by centroid distance, volume inclusion, and volume overlap (Dice Coefficient).

Results: The average volume for ITV-4D, ITV-AIP, and ITV-CBCT were 23.1, 17.2, and 14.1 cm3, respectively. When using ITV-4D1 as a reference contour in the manual soft-tissue alignment, an average of 93% of ITV-CBCTs was included in ITV-4D1, with a centroid distance of 2.0 mm. Only 81% of ITV-4Ds were included in ITV-4D1. The Dice Coefficient between ITV-4D1 and ITV-4Ds was only 0.68, suggesting that the two ITVs were not highly overlapping. For all comparisons, the results were similar using either ITV-AIP1 or ITV-4D1 as a reference contour in the manual soft-tissue alignment.

Conclusions: There were noteworthy inter-fractional variations in ITV-4Ds. Compared to ITV-4D, ITV-AIP was much similar to ITV-CBCT. However, there was no significant difference between using ITV-4D1 and using ITV-AIP1 as a reference contour for image guidance. Further dosimetric studies are required to evaluate actual tumor coverage.

Funding Support, Disclosures, and Conflict of Interest: This study was supported in part by Philips Healthcare.

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