Comparison of 2 transabdominal ultrasound image guidance techniques for prostate and prostatic fossa radiation therapy

Abstract Purpose Our clinic is a long-term user of a first-generation transabdominal (TA) biplanar (2.5-dimensional [2.5D]) ultrasound image guidance (USIG) system for prostate cancer treatments. We are also an early adopter and development partner for a new, second-generation, fully 3D USIG system that allows for volumetric TA localization of the prostate. This new system has been evaluated at our institution by direct comparison with the previously established first-generation TA method for prostate alignment. Methods and materials We compared the 2 TA-USIG methods on the same subjects and same treatment sessions. A total of 1428 fractions delivered to 41 treated patients (16 intact prostate, 25 fossa) were analyzed regarding the agreement of alignments between the 2 US positioning systems. Patients were first aligned to tattoos using treatment room lasers. TA-USIG using the 3D system was then performed to align contours derived during the computed tomography simulation process to their corresponding daily US-visualized structures. The US-3D system image guidance shifts were performed and recorded as the "initial" shifts. A 2.5D system alignment was then immediately performed using the same computed tomography derived reference contours and the indicated shifts, relative to the 3D system, were recorded as the difference between the 2 alignment methods. Results The average difference between the 2 TA-USIG alignments for all patients was 0.4 ± 0.7 mm, 0.7 ± 0.9 mm, and 0.5 ± 0.9 mm in the left-right, anteroposterior, and superoinferior directions, respectively. No significant difference in system agreement between intact prostate versus fossa patients was observed. Conclusion Our comparison of an established 2.5D USIG method with a newer, fully 3D approach for prostate alignment of 41 different patients (1428 fractions) shows excellent agreement with each other, despite the nontrivial difference in imaging approaches. This shows that the 2 specific USIG approaches yield results that are consistent with each other, and that the USIG modality yields consistent results within the modality.