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An Investigation of the Impact of Spot Spacing On Plan Quality Using IMPT Optimization Incorporating Deliverable Monitor Unit Constraints

W Cao

W Cao*, X Li, G Lim, Y Li, X Zhu, X Zhang, MD Anderson Cancer Center, HOUSTON, TX

TH-A-213AB-9 Thursday 8:00:00 AM - 9:55:00 AM Room: 213AB

To investigate the impact of spot spacing on intensity modulated proton therapy (IMPT) plan quality based on a new IMPT optimization method which incorporates deliverable monitor unit (MU) constraints and therefore eliminates the dose distortion effect of post processing while using small spot spacings in the current treatment planning system (TPS).

We introduce a two-stage linear programming (LP) based model to solve the deliverable spot intensity optimization (DSIO) problem by incorporating deliverable minimum MU constraints. The first-stage LP solve selects a sub set of pre-arranged spots without considering MU constraints, and the second-stage LP solve optimizes intensities of selected spots with forced MU constraints. A quadratic programming (QP) based model to simulate the conventional spot intensity optimization (SCIO) without MU constraints is used for comparison. Four prostate cancer cases were studied in this work. Plans optimized by both the DSIO and CSIO were evaluated for different settings of spot spacing of 3, 4, 5, 6 and 7 mm.

Patient studies demonstrated improved dose-volume metrics of DSIO optimized IMPT plans, i.e., better rectum and bladder sparing, compared with CSIO optimized plans with the same spot spacing. With decreased spot spacings, improvements on dose-volume metrics were relatively marginal. There was no pronounced improvement achieved when spot spacing reduced below 4 mm. In addition, the DSIO model can automatically create a non-uniform spot arrangement using only a fraction of scanning spots required by plans optimized by CSIO in our study.

The proposed DSIO method can firstly avoid the discrepancy between optimized and delivered plans in the current IMPT TPS, and secondly, allow the use of small spot spacings to improve target coverage and normal tissue sparing. Evaluations on four prostate cancer cases found that spot spacing less than 4 mm could not achieve pronounced dosimetric improvements.

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