Room: Exhibit Hall
Purpose: Spot weight for intensity-modulated proton therapy is optimized using an inverse planning process to meet planning criteria. The accuracy of spot position and monitor unit (MU) values affects patient dosimetry, especially for highly-weighted spots. Beam interruption for highly-weighted spots presents a challenge for partial treatment delivery resumption, particularly for small fields with few MU.
Methods: We developed a three-step approach to investigate highly-weighted spots during patient-specific quality assurance (QA). First, a script was developed within the treatment planning system (TPS) to identify highly-weighted spot MU and the average MU per spot for each energy layer. Second, the delivered spot position and MU values from the treatment log DICOM file generated in the record and verify system were compared with the planned spot position and MU from the DICOM RT plan using in-house software. Lastly, the delivered spot position and MU information was used to generate an updated DICOM RT plan file which was imported to the TPS for dose recalculation. The updated dose distribution was compared with the original dose distribution. We simulated partial treatment delivery for highly-weighted spots and resumed treatment within one spot from where the beam was interrupted. The effect on dosimetry was studied using a phantom for fields with MU < 10 and highly-weighted spots.
Results: Our phantom study showed that the spot delivery position and MU value accuracy dosimetry effect can be quantified using our three step approach. For the test case there was small dosimetry difference at the center of SOBP (<1%) comparing the original plan to the recalculated plan based on the treatment log file.
Conclusion: Highly-weighted spots should be addressed in the planning and QA process when using spot scanning proton beams. We established the feasibility of using delivered spot positions and MU values to evaluate the effect of highly-weighted spots.
Not Applicable / None Entered.