Comparing Setup Errors Using Portal Imaging in Patients With Gynecologic Cancers by Two Methods of Alignment.

Area of Science:

• Radiation Oncology
• Medical Imaging
• Gynecologic Oncology

Background:

• Abdomino-pelvic radiotherapy (RT) setup accuracy is crucial for effective cancer treatment.
• Alignment tattoos on lax abdominal regions can lead to patient misalignment during RT.
• Improving setup reproducibility is essential to minimize radiation dose to healthy tissues.

Purpose of the Study:

• To compare the setup reproducibility of two patient alignment methods in gynecologic cancer patients undergoing radiotherapy.
• To evaluate the impact of alignment technique on systematic and random errors.
• To determine the required planning target volume (PTV) margins based on setup accuracy.

Main Methods:

• A prospective study involving 35 gynecologic cancer patients undergoing radical RT.
• Patients were tattooed with reference points for alignment.
• Two alignment methods were compared: conventional field center (Arm I) and a new setup isocenter with cranial offset (Arm II).
• Setup deviations were analyzed using portal images (PI) over 3 days and twice weekly.
• Systematic and random errors were calculated, and PTV margins estimated using van Herk's formula.

Main Results:

• Arm II demonstrated a lesser systematic error in the mediolateral (ML) direction compared to Arm I.
• Patients with large pelvic girth (>95 cm) showed greater ML shifts in Arm I (37%) versus Arm II (22%).
• Arm II required significantly smaller PTV expansion (0.9 cm) compared to Arm I (1.6 cm).

Conclusions:

• Alignment using a setup isocenter with cranial offset (Arm II) is superior to conventional field center alignment for gynecologic cancer patients.
• This improved alignment technique, utilizing tattoos on the lower abdomen, enhances setup reproducibility and reduces PTV margins.
• The findings suggest a more precise and potentially safer approach to radiotherapy for this patient population.