Single-field Optimization Research Articles

SU-E-T-492: Spot Scanning Proton Therapy: Single Field Simultaneous Integrated Boost

Purpose: For spot scanning proton therapy (SSPT), treatment plans generated using single field optimization (SFO) are more robust than plans generated with multi‐field optimization (3D intensity modulated proton therapy). The purpose of this work is to present a single field (simultaneous) integrated boost (SFIB) technique for SSPT. Methods: The SFIB plans were designed to deliver multiple level doses to multiple targets using SFO for each field. Dose constraints were used to achieve the target coverage and normal tissue sparing. Multiple SFO fields are normally used for each fraction of treatment. The total and fraction doses for each dose level were based on the current clinical practice using x‐ray IMRT in our clinic. Fourteen patients, nine with brain lesions, one with spine lesions and four with prostate cancer, were treated with SSPT using SFIB. All patients were treated with two different dose levels, except the spine patient treated with three dose levels. All lesions were treated with two or three fields except the spine lesion treated with a single PA field. Results: The SFIB creates more conformal dose distributions. Examples of dose distributions and results of quality assurance (QA) measurements will be presented. The applicability and logistics of SSPT using SFIB will be discussed. Conclusions: Using SFO, we have successfully implemented a SFIB technique for SSPT and proved that we are not limited to create uniform dose within target volumes. Compared to the conventional sequential delivery, the SFIB technique offers advantages, including seamless treatment delivery and more efficient treatment planning and patient specific QA. Compared to IMPT, SFIB plans are more robust by definition and but less versatile. The biological benefits of SFIB could also be explored.

MO‐FF‐A3‐03: The Impact of Monitor Unit Constraints on Plan Quality of IMPT Planning Using Single‐Field Optimization

Purpose: To investigate the effect of monitor unit (MU) constraints on the dose distribution created by intensity modulated proton therapy (IMPT) treatment planning using single‐field optimization (SFO). Methods and Materials: Ninety‐four energies between 72.5 and 221.8 MeV are available for scanning‐beam IMPT delivery at our institution. The minimum and maximum MUs for delivering each pencil beam (spot) are 0.005 and 0.04, respectively. These MU constraints are not considered during optimization by the treatment planning system; spots are converted to deliverable MUs during post‐processing. Treatment plans for delivering uniform doses to rectangular volumes with and without MU constraints were generated for different target doses, spot spacings, spread‐out Bragg peak (SOBP) widths, and ranges in a homogenous phantom. Four prostate cancer patients were planned with and without MU constraints using different spot spacings. Rounding errors were analyzed using an in‐house software tool. Results: From the phantom study, we have found that both the number of spots that have rounding errors and the magnitude of the distortion of the dose distribution from the ideally optimized distribution increases as the field dose, spot spacing, and range decrease and as the SOBP width increases. From our study of patient plans, it is clear that as the spot spacing decreases the rounding error increases, and the dose coverage of the target volume becomes unacceptable for very small spot spacings. Conclusions: Constraints on deliverable MU for each spot could create a significant distortion from the ideally optimized dose distributions for IMPT fields using SFO. To eliminate this problem, the treatment planning system should incorporate the MU constraints in the optimization process and the delivery system should reliably delivery smaller minimum MUs.

Intensity modulated proton therapy treatment planning using single‐field optimization: The impact of monitor unit constraints on plan quality

To investigate the effect of monitor unit (MU) constraints on the dose distribution created by intensity modulated proton therapy (IMPT) treatment planning using single-field optimization (SFO). Ninety-four energies between 72.5 and 221.8 MeV are available for scanning beam IMPT delivery at our institution. The minimum and maximum MUs for delivering each pencil beam (spot) are 0.005 and 0.04, respectively. These MU constraints are not considered during optimization by the treatment planning system; spots are converted to deliverable MUs during postprocessing. Treatment plans for delivering uniform doses to rectangular volumes with and without MU constraints were generated for different target doses, spot spacings, spread-out Bragg peak (SOBP) widths, and ranges in a homogeneous phantom. Four prostate cancer patients were planned with and without MU constraints using different spot spacings. Rounding errors were analyzed using an in-house software tool. From the phantom study, the authors have found that both the number of spots that have rounding errors and the magnitude of the distortion of the dose distribution from the ideally optimized distribution increases as the field dose, spot spacing, and range decrease and as the SOBP width increases. From our study of patient plans, it is clear that as the spot spacing decreases the rounding error increases, and the dose coverage of the target volume becomes unacceptable for very small spot spacings. Constraints on deliverable MU for each spot could create a significant distortion from the ideally optimized dose distributions for IMPT fields using SFO. To eliminate this problem, the treatment planning system should incorporate the MU constraints in the optimization process and the delivery system should reliably delivery smaller minimum MUs.