Best Oncologist Specialists Visakhapatnam

20 Jan 2024

WORKFLOW CONSIDERATIONS IMPACTING SI ACCURACY

SI ACCURACY

SELECTION OF REFERENCE SURFACE

Reference surface from CT sim represents the “ gold standard ”:
- captured by SI camera
- created from patient’s external contour (i.e., DICOM)
- Many clinics do not have SI cameras in simulator so internal imaging (e.g., MV ports, CBCT) is used to verify patient’s position before camera is used to c apture a reference surface

SELECTION OF ROI

Requirements	Potential Pitfalls
Accurate surrog ate for target/ task Possess topographic ally salient features to provide unique registration	Too large: Low frame rate for monitoring Insensitive to loc alized changes (e.g., breast swelling) Too small: Cannot chara cterize posture adequately Susceptible to registra tion errors due to la ck of topography

SELECTION OF ROI

ROI that is “just right….”
ROI selection is an “ art” not an “ algorithm ” (Kang et al 2012)
Consid er use of multiple ROIs:
- Large à postural correction
- Small à tra cking

Selection of Threshold

Action threshold to disable beam / alert therapy team
Similarly to setup margins, thresholds are institution-specific
Selected from a) litera ture or b) “ observing ” patients with SI:
- Stanley et al (2017) observed >6000 fractions guided by CBCT in 4 sites (pelvis, abdomen, thorax, chest/ breast) and found discre pancies of 5-6mm
- Lietal (2013) observed volunteers’ range of motion in thermoplastic masks and determined < 2mm threshold
If positioning errors are reduced over time (i.e., improved immobilization), adjust thresholds

Selection of Threshold

Larger	Smaller
When quantifying systematic & random errors Signific ant deformation is expected (e.g., pendulous breast)	For intra-fraction monitoring following acquisition of reference surface (e.g.,frameless SRS)