Optimizing image guidance frequency and implications on margins for gynecologic malignancies

Area of Science:

• Radiation Oncology
• Medical Physics
• Gynecologic Oncology

Background:

• Helical tomotherapy for gynecologic cancers requires precise patient setup.
• Daily megavoltage computed tomography (MVCT) is used for setup verification.
• Reducing MVCT frequency could streamline treatment while maintaining accuracy.

Purpose of the Study:

• To analyze setup deviations in gynecologic cancer patients using daily MVCT.
• To evaluate three protocols for reducing MVCT frequency.
• To determine the optimal protocol for minimizing residual setup errors.

Main Methods:

• Recorded setup errors from 56 gynecologic cancer patients via daily MVCT.
• Calculated systematic and random errors.
• Compared theoretical errors from reduced MVCT protocols (FFF, FTF, 505) with actual errors.

Main Results:

• All three protocols resulted in minor residual deviations, with 95% within 5 mm.
• The 505 protocol (MVCTs first and third weeks) showed the smallest residual deviations.
• The 505 protocol allowed for potentially smaller CTV-PTV margins compared to FFF and FTF.

Conclusions:

• The 505 protocol is effective in minimizing residual deviations for gynecologic cancer patients.
• This protocol may represent an optimal strategy for reducing pre-treatment MVCT frequency.
• Reduced MVCT frequency can be achieved without compromising setup accuracy.