SU-E-T-416: Performance Test Comparing Three Pre-Treatment Isocenter Localization Techniques for Single Fraction...

Area of Science:

• Medical Physics
• Radiation Oncology
• Neurosurgery

Background:

• Accurate patient positioning is critical for effective cranial Stereotactic Radiosurgery (SRS).
• Pre-treatment localization techniques aim to minimize deviations between the radiation isocenter and the target.
• Comparing immobilization methods, such as invasive head frames versus non-invasive masks, is essential for optimizing SRS delivery.

Purpose of the Study:

• To develop and compare the performance of three pre-treatment isocenter localization techniques for cranial SRS.
• To evaluate positioning accuracy using laser alignment versus X-ray verification (ExacTrac, On Board Imaging).
• To assess these techniques with both invasive head frames and immobilization masks.

Main Methods:

• An anthropomorphic head phantom was used with EDR2 film for measurements in two planes.
• CT scans were acquired with either a mask or head frame.
• Positioning was achieved via laser alignment (head frame) or X-ray verification with robotic couch adjustments (mask).
• Gantry star-shot irradiation evaluated isocenter-target differences, with mean and standard deviations calculated.

Main Results:

• ExacTrac for mask-based SRS showed positional accuracy of 1.10±0.86mm (cross plane), 0.67±0.83mm (inline), and 0.59±0.48mm (vertical).
• Frame-based SRS with laser alignment yielded differences of 0.93±0.43mm (cross plane), 0.76±0.18mm (inline), and 0.34±0.12mm (vertical).
• Results for mask-based SRS using On Board Imaging are pending.

Conclusions:

• While frame-based SRS demonstrated smaller standard deviations, the mean differences for both techniques were statistically similar.
• Both head frame and immobilization mask techniques require planning margins that consider target size, especially for small lesions.
• Further results for On Board Imaging are anticipated.