Appropriate patient instructions can reduce prostate motion.

Area of Science:

• Radiation Oncology
• Medical Physics
• Prostate Cancer Treatment

Background:

• Interfraction prostate motion necessitates larger safety margins in radiotherapy.
• Maintaining consistent rectal and bladder filling can potentially reduce this motion.
• Patient instruction is a key factor in managing interfraction motion variability.

Purpose of the Study:

• To evaluate the effectiveness of patient instruction in reducing interfraction prostate motion.
• To determine the impact of consistent organ filling on prostate displacement during radiotherapy.
• To optimize safety margins for definitive radiotherapy based on motion reduction strategies.

Main Methods:

• Implantation of radio-opaque fiducial markers in 38 patients undergoing definitive radiotherapy.
• Positioning verification using skin marks or infrared body markers.
• Measurement of prostate displacement via ExacTrac (two orthogonal radiographies) across 1252 fractions.
• Determination of systematic and random setup and displacement errors.

Main Results:

• Interfraction prostate displacement was less than 1 mm in the Right-Left (RL) direction and less than 2 mm in the Anterior-Posterior (AP) and Superior-Inferior (SI) directions.
• Systematic errors were slightly lower than random errors, both below 1.5 mm.
• Positioning based on skin marks resulted in greater inaccuracies (±1.5-2 mm RL, ±2-2.5 mm AP/SI).

Conclusions:

• Appropriate patient instructions for constant organ filling enable reduced margins for the clinical target volume to planning target volume (CTV-PTV).
• Bone fusion positioning with patient instruction requires CTV-PTV margins of 2 mm (RL), 4 mm (AP), and 5 mm (SI).
• Studies lacking patient instruction guidance reported significantly larger margins (>1 cm in AP and SI directions).