Step and shoot IMRT to mobile targets and techniques to mitigate the interplay effect

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Biology

Background:

• Interplay effect in intensity modulated radiation therapy (IMRT) causes temporal dose variations.
• Accurate dose delivery is crucial for non-small cell lung cancer (NSCLC) treatment.
• Tumor motion significantly impacts radiation therapy efficacy.

Purpose of the Study:

• To evaluate dose rate modulation for mitigating interplay effects in IMRT.
• To investigate the impact of varying motion patterns on dose delivery.
• To analyze the interplay effect in hypofractionation versus traditional fractionation.

Main Methods:

• Utilized MLC settings from 5, 9, and 11-field IMRT plans for NSCLC.
• Simulated tumor motion (1.03-1.95 cm) using MotionSIM XY/4D robotic diode array.
• Investigated dose rate modulation and selective dose rate modulation.
• Employed Pinnacle(3) treatment planning system for dose calculations.

Main Results:

• Computer simulations identified fields with the greatest interplay effects.
• Dose rate modulation and selective modulation significantly reduced temporal dose variations.
• Selective modulation achieved comparable dose variation reduction with shorter delivery times in most cases.
• Interplay effect impact on hypofractionation was not greater than traditional fractionation.
• Variant motion exceeding target volume increased temporal dose variation.

Conclusions:

• Dose rate modulation is an effective strategy to mitigate interplay effects in IMRT for NSCLC.
• Selective dose rate modulation offers an efficient approach to reduce dose variation.
• Tumor motion variability within the target volume has limited impact, but exceeding it increases dose variation.