SU-E-T-563: Which is More Important in IMRT Planning: The Number of Beam Angles or the Number of MLC Segments?

Rotational IMRT delivery has attracted considerable attention. Some people believe that rotational IMRT should provide better plan quality with more beam angles to irradiate the target. In this work, we will study the impact of the number of fields and MLC segments on the IMRT plan quality.

The RPC IMRT benchmark phantom was used in the IMRT planning. The number of gantry angles used ranges from 3 to 90 with various numbers of segments. All IMRT plans were created using Pinnacle3 DMPO module using the same objectives. The final objective value is used to quantify the plan quality.

When the number of beam angles is more than 3, the objective value decreases dramatically with increasing segments till the number reaches around 60. When the number of segments is small, the availability of more beam angles can clearly improve the IMRT plan quality. However, such a plan quality improvement becomes less dramatic as the number of total segments increases. For example, when the number of segments is 30, increasing the number of beam angles from 3 to 18 can reduce the objective value by 75%. Such a reduction reduces to 44% when the number of segments increases to 60. Single-arc VMAT plans showed less desirable plan quality comparing to fixed-field IMRT plans using the same number of beam-angles and segments.

The number of segments has dramatic impact on the IMRT plan quality when the number of fields is more than 5. If enough segments are used in IMRT planning, increasing the number of beam angles can only slightly improve the plan quality. The increase in plan quality led by increasing beam angles can be limited by the MLC motion constraint for rotational IMRT delivery.