Extending the Eclipse^TM AcurosXB output factor table for small field radiosurgery

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Treatment Planning

Background:

• Accurate dose calculation is critical for stereotactic radiosurgery (SRS) and stereotactic body radiosurgery (SBRT).
• Small fields used in SRS/SBRT pose challenges for traditional output factor (OF) tables in treatment planning systems.
• The Varian Eclipse system's default OF table may lack sufficient data for these small fields.

Purpose of the Study:

• To evaluate the necessity of extending the output factor (OF) table in the Varian Eclipse Treatment Planning System.
• To assess the impact of an extended OF table on dose calculation accuracy for small field SRS and SBRT treatments.

Main Methods:

• A new AcurosXB beam model with an extended OF table (1x1 cm to 40x40 cm) was created in Varian Eclipse.
• Output factors were measured using a scintillator detector in a water tank for various small field sizes.
• Dose calculations from the extended and default OF tables were compared for SRS/SBRT treatment plans.

Main Results:

• The extended OF table required higher monitor units (MUs) for 6FFF and 6MV beams, indicating improved accuracy.
• Differences of up to 7.5% were observed for small fields (1x1 cm) between the models.
• Eclipse calculations significantly overestimated dose for fields smaller than 0.5x0.5 cm, even with the extended table.

Conclusions:

• The Varian Eclipse system provides satisfactory dose calculations for primary jaw sizes down to 2 cm.
• Extending the OF table in Eclipse is necessary for improving dose calculation accuracy for fields smaller than 1.5 cm.
• Further improvements may be needed for ultra-small fields (below 1x1 cm).