Session Item

Precise patient positioning with image guidance (IGRT) is essential for safe prostate stereotactic radiotherapy (SABR) delivery. We evaluated the potential of a CT-visible hydrogel spacer to serve as a surrogate fiducial marker for prostate IGRT.

Prior to planning prostate SABR, patients underwent placement of three intraprostatic gold markers for alignment and a SpaceOAR VueTM (SOv) to decrease rectal radiation dose, per standard practice. At treatment, table shifts based on cone-beam CT (CBCT) to the planning CT were initially performed by 3-dimensional alignment of the SOv (experimental arm), then again based on the fiducial markers (standard arm). The six directional shifts (three linear and three rotational) were recorded and the differences compared.

140 fractions across 41 consecutive patients were evaluated. Mean/median differences between SOv-based and fiducial-based alignment in linear (vertical, longitudinal, lateral) and rotational (rotation, pitch, roll) shifts were 0.9/0.6mm, 0.8/0.5mm, and 0.6/0.4mm, and 0.38/0, 0.62/0, and 0.35/0 degrees, respectively (table 1). Gland volume did not correlate with shift differences. A learning curve was observed, with significantly less variability between alignment methods in the second half of fractions studied (n=70) compared to the first half in five of the six axes – vertical (p=0.0032), longitudinal (p=0.0464), pitch (p<0.0001), roll (p<0.0001), and rotation (p<0.0001) (Table 1). Further, in the latter 70 fractions, no differences between alignment methods were recorded in 94.3%, 92.9%, and 92.7% of the three rotational shifts.

For precise IGRT in prostate SABR, daily alignment with a readily visible SpaceOAR VueTM was highly comparable to intraprostatic gold fiducial markers, with an observed rapid learning curve. Utilizing a single device for a dual purpose may increase procedural efficiency, decrease risk, and lower cost, warranting further prospective study.