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A Study of the Correlation of 3D Surface Matching and KV Imaging for Chestwall IMRT

H Al-Hallaq*, E Gerry, The University of Chicago, Chicago, IL

SU-E-J-70 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: We investigated the accuracy of surface matching compared to kV positioning of 10 post-mastectomy chestwall patients receiving inverse-planned, non-gated IMRT.

Methods: During 130 treatment sessions, non-gated 3D surfaces were captured using AlignRT (v4.5) before and after table translations as indicated by kV imaging. Surfaces were compared to a reference generated from CT data. Pearson's correlations between 'indicated' AlignRT and kV shifts were studied for surface registration of user-defined regions-of-interest (ROIs): the entire surface ('all') and the chestwall ('cw'). In 21% of sessions, two consecutive surfaces of patients in identical positions were used to estimate variability. Finally, the 'implemented' shifts detected by AlignRT were calculated and compared to the absolute table translations.

Results: Correlations between 'indicated' AlignRT and kV shifts were higher for 'cw' than for 'all': r=0.65 (Anterior-Posterior), r=0.65 (Superior-Inferier), r=0.44 (Left-Right). Correlations exhibited large inter-patient variability; 60% had r<0.6 in at least 1 direction thereby resulting in low correlation between 3D Euclidian shift distances (r=0.48). Correlations between 'cw' and kV increased (r>0.8) for 'implemented' table shifts; only 2 patients demonstrated r<0.6 in any single direction. Comparison of consecutively-acquired 'cw' surfaces controlled for patient movement demonstrated standard deviations (STDs) of: 1.2mm (A-P), 2.0mm (S-I), 1.6mm (L-R). While STDs between 'implemented' and kV shifts were on the same order, STDs between 'indicated' and kV shifts were twice as large: 2.9mm (A-P), 3.5mm (S-I), 4.1mm (L-R). Rotational differences >1degree were calculated more often when registering 'cw' (22%) than 'all' (5%). At the treatment position, residual STDs remained high (3.0-3.6mm).

Conclusions: On average, 'cw' shifts correlated with kV shifts but exhibited significant inter-patient variability and larger rotations than 'all'. Differences between AlignRT and kV were ~3mm for initial patient positioning. The lack of a one-to-one correspondence between surface and kV shifts in any single session must be further investigated before clinical implementation.

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