3045 - Impact of User-Defined vs. Automatic Static Angle Modulated Port Selections for VMATp Nasopharyngeal Carcinoma Treatment

Purpose/Objective(s):  

A new optimization solution within a commercial treatment planning system leverages dynamic collimation and combines VMAT with the modulation of IMRT-like static angle modulated ports (STAMP) in a single continuous delivery (VMATp).  VMATp offers automatic or user-selected options to balance the beam weight between the VMAT and STAMP portions of the field.  VMATp also allows either user-defined or automatic generation of STAMPs.  This comparative study evaluated the impact of user-defined versus automatic selections on VMATp plans, for a cohort of curative nasopharyngeal (NPC) cases.

Materials/Methods:  

Twenty patients with T3-4, any N, M0 NPC, treated at our institution between 1 July 2019, and 1 July 2023 were included.

Each approach was reoptimized using:

·        Single arc with 5 STAMPs (VMATp1)

·         Dual arc with 4 STAMPs per arc (VMATp2)

Each approach was reoptimized using:

·         Automatic weighting (aW)

·         Automatic weighting and automatic STAMP selection (aWaS)

Target and Organ at Risk (OAR) dose metrics were compared. Statistical differences were determined using a Wilcoxon signed-rank test with a Bonferroni adjusted significance level.

Results:  

One hundred and twenty plans were generated with VMATp. Table 1. summarises the mean difference from the VMATp approach when using automatic selections for a set of key clinical plan metrics.  Minimal statistically significant differences were noted.  For both approaches, except for VMATp1aWaS spinal cord, all metric differences were within 0.5Gy to those achieved with user-defined selections.

Conclusion:  

 This evaluation found that, for this NPC cohort, using VMATp automatic selections produced plans of equivalent quality, potentially reducing the learning curve when implementing into clinical practice.