Room: Exhibit Hall | Forum 4
Purpose: Evaluate if the MU-weighted sum of the energy of all pencils in a proton pencil-beam-scanning (PBS) plan can be used to identify plans that deliver more or less dose to a target volume than intended.
Methods: The deposited energy of a PBS plan is determined by summing the energy of all spots in the spot map, excluding the energy deposited in the proximal region. The excess of this deposited energy over the nominal target energy (target volume x prescribed dose) is termed the spill-factor and depends on the beam penumbrae as well as planning margins and strategies. The spill-factor is parameterized per treatment site as function of target volume based on clinical plans. We evaluate the variation of the clinical spill-factor from this parameterization to determine spill-factor predictability. Next we introduce various deliberate planning errors (wrong margins, no optimization objectives on dose fall-off, wrong normalization, â€¦) and evaluate the change in spill-factor to determine the sensitivity to these errors. This analysis is performed for 125 clinical plans for 11 different treatment sites.
Results: Overall 89% of the clinical plans had a spill-factor within Â±10% of predicted. Change in spill-factor under the investigated planning errors varied from <2% to 40%. For sites with regular-shaped targets and standardized planning protocols (spinal axis, brain, prostate) the agreement with the parameterization was best and most deliberate planning errors could be detected.
Conclusion: For select sites it is feasible to predict the spill-factor from the PBS-plan energy and target volume and detect (gross) planning errors based on spill-factor deviation.