Dosimetry limitations and a dose correction methodology for step-and-shoot IMRT

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Step-and-shoot intensity-modulated radiation therapy (IMRT) can lead to dose delivery inaccuracies.
• The 'overshoot' effect, caused by dose servo control, results in over- and underdosing in beam segments.
• MLC leaf positional errors can also contribute to dose inaccuracies, particularly at high dose rates and low monitor units.

Purpose of the Study:

• To investigate and quantify the 'overshoot' effect in step-and-shoot IMRT.
• To develop a correction method for segmental dose errors arising from the 'overshoot' effect.
• To establish a quality assurance program for managing the 'overshoot' effect in clinical IMRT delivery.

Main Methods:

• Utilized commercial ionization chambers and radiographic film for dose measurements.
• Employed the Pinnacle(3)-V6.2 treatment planning system and a Varian Clinac 21 EX linear accelerator with a Millennium MLC.
• Compared dose and profiles of reference fields with those from step-and-shoot irradiated fields to assess inaccuracies.

Main Results:

• Quantified the 'overshoot' effect, ranging from 0.1 to 0.6 MU at dose rates of 100-600 MU/min.
• Measured MLC leaf-position errors, generally <1 mm, but exceeding 2 mm at 9.5 cm off-axis distance for specific conditions (600 MU/min, 1 MU/segment).
• Defined a dose ratio for segment dose fluctuation and developed a correction method and QA program.

Conclusions:

• The developed correction method and QA program can improve the agreement between planned and delivered doses in IMRT.
• Understanding and correcting the 'overshoot' effect is crucial for accurate IMRT delivery, especially with high dose rates and small segment monitor units.
• The findings support enhanced quality assurance for IMRT, leading to more precise patient treatments.