Spot-optimization reduces beam delivery time in liver breath hold intensity modulated proton therapy

Liver irradiations with intensity-modulated proton therapy (IMPT) often require motion mitigation techniques that prolong treatment. A prototype spot-optimization algorithm was tested to evaluate whether plan delivery time could be reduced while preserving quality.

Fifteen patients previously treated with liver IMPT using breath-hold were re-planned with nominal treatment planning system (TPS) settings and using a prototype spot-optimization algorithm in which combinations of minimum Monitor Unit (MU) and layer-spacing settings were tested: 1MU/1MeV, 3MU/3MeV, 1MU/5MeV, 5MU/3MeV. Spot-optimized and nominals plans were compared using standard dose-volume histogram (DVH) metrics for targets and organs-at-risk. A Wilcoxon signed-rank test was applied (p < 0.05). Delivery time for all plans were measured by creating and delivering IMPT quality assurance (QA) plans. Gamma analyses were performed on all plans to test deliverability. Plans were considered deliverable if >90 % of points passed a gamma criterion of 3 %/3mm.

Minimal DVH differences were observed between nominal and spot-optimized plans. For the 3MU/3MeV setting, no DVH metrics were significantly different. Median and interquartile range (IQR) delivery times for these plans were 40 % (38 %-44 %) faster than nominal plans. 5MU/3MeV plans had median (IQR) delivery times 59 % (52 %-61 %) faster than nominal plans but had a small but significant increase in Liver_Eff D_mean with a median (IQR) difference of 0.2 Gy(RBE) (0.0-0.4 Gy(RBE)). QA analysis showed all spot-optimized plans were deliverable.

The spot-optimization algorithm produced clinically deliverable plans with negligible DVH differences to nominal plans and reduced delivery time of liver IMPT by over one-third.