Cumulative sum method in nonzero fixed action level setup correction strategy: an application of statistical process...

Area of Science:

• Radiation Oncology
• Medical Physics
• Statistical Process Control

Background:

• Accurate patient positioning is critical in image-guided radiotherapy (IGRT) to ensure precise radiation delivery.
• Systematic shifts in prostate position can lead to underdosage or overdosing of the target and surrounding organs.
• Existing methods for detecting and correcting setup errors may not be sufficiently sensitive or timely.

Purpose of the Study:

• To integrate the cumulative sum (CUSUM) method into a nonzero fixed action level setup correction strategy.
• To detect and correct systematic shifts in prostate position during image-guided radiotherapy.
• To evaluate the efficacy of this strategy in improving treatment accuracy.

Main Methods:

• A retrospective analysis was performed on 34 patients undergoing prostate radiotherapy with fiducial markers.
• A daily nonzero action level setup correction strategy utilizing CUSUM charts was implemented.
• A cumulative error index monitored localization offsets; exceeding a decision interval triggered correction of prescribed offsets.

Main Results:

• The CUSUM-based strategy demonstrated high detection rates for systematic shifts, with 42% detection for a 3 mm shift.
• The mean magnitude of detected systematic shifts were 3.1, 4.4, and 5.2 mm for shifts of 3, 5, and 7 mm, respectively.
• Implementing the protocol reduced the frequency of localization offsets exceeding action levels by approximately half.

Conclusions:

• The CUSUM chart incorporation provides an objective method for detecting systematic shifts in patient localization offsets.
• The strategy effectively quantifies the magnitude of systematic changes based on cumulative observations.
• This approach enables correction of prescribed offsets, accounting for systematic shifts in patient setup position.