1. Integrating Single and Multi-Field Optimization for Proton IMPT Head and Neck Plans
Karla Leach, BS, CMD Texas Center for Proton Therapy-Irving, TX
Introduction
Cancers of the Base of Tongue (BOT) are surrounded by critical organs at risk (OARs) in the oral cavity region. When treating with proton therapy, multi field optimization (MFO) is needed for ideal OAR sparing. With this technique, each beam has a unique dose distribution in order to spare OARs in the field path. MFO techniques are considered less robust because the individual dose distributions of each beam can be impacted by inter- and intra-fraction motions.
Ideal plan robustness utilizes single field optimization (SFO), where each treatment field covers the target with the total dose divided by the number of beams used. This technique is well suited for lymph node regions in the neck that are separated from critical OARs.
Integrating SFO and MFO planning techniques requires beam-specific PTVs in combination with beam-specific optimization objectives and robust optimization parameters.
Methods
Four fields with gantry 300o(RAO), 0o(AP), 60o (LAO), and 180o (PA) were used for treatment. All of the different dose level PTVs were combined and divided into superior and inferior targets at the level just below the oral cavity with an overlap region of 2cm (Fig. 1a). The superior target defines the PA beam treatment region and the inferior target defines the AP beam treatment region. The overlap region will aid in creating a gradient, or transition zone, between the AP and PA beam dose distributions.
Dose constraints were set to half of the prescribed dose for the ring structures for each beam. This prevents placement of hot spots outside the PTV on the individual beam dose distributions. Max dose constraints were set for each beam on the combined PTV to prevent too much contribution from a single beam. Other OAR objectives were added to achieve dose tolerances. Final robustness settings include 0.3cm shifts in all directions, independent beam motions and 4% range uncertainty. Plan was calculated using Monte Carlo ions/spot. a b c
Fig.1a Superior PA PTV: light blue, Inferior AP PTV: blue Fig.1b RAO PTV: green, LAO PTV: pink
Fig.1c Superior PA PTV: light blue, Inferior SFO AP PTV: blue, SFO RAO PTV: green, SFO LAO PTV: pink
Fig.5 AP and PA field dose distribution and gradient dose lines
The LAO and RAO beams treat
across midline offering additional parotid sparing on the distal end of the beam (Fig.4b). Overlap in the AP and PA PTVs yields a smooth gradient (Fig.5).
An analysis of 16 different plans shifting the CT in all directions 0.3cm in combination with +/-4% range uncertainty showed CTV and GTV V95% did not fall below 93% (Fig.6) with the average V95% at 97%.
Conclusions
Combining SFO and MFO techniques for BOT cancers allows for ideal OAR sparing in the superior oral region while maintaining optimal plan robustness in the inferior neck. Robust optimization and final plan analysis are necessary tools in conjunction with MFO to ensure plan reproducibility.
The LAO PTV excludes any isolated PTV on the right and the RAO PTV excludes any isolated PTV on the left (Fig.1b). To control the dose in the inferior SFO region, the RAO, AP and LAO PTVs were copied with the superior and overlap regions excluded (Fig.1c).
Beam specific spot avoidances were made for the superior oral region to minimize spot placement through the oral cavity, spinal cord and parotids. Each spot avoidance was edited to exclude areas that blocked coverage of the 70 Gy(RBE) PTV (Fig.2). Allowing contribution from each beam in the 70 Gy(RBE) region spreads out the RBE on the distal end of each beam.
Optimization PTVs were made for each dose level by subtracting each PTV from the other by 0.5cm. This allows for a dose fall
off region between a higher and lower dose PTV. Rings were
made around each dose
level PTV to help control the dose outside of the PTVs.
Energy layer and spot spacing were set to automatic 0.6cm with target margin set to constant 0.5cm. A min dose objective with zero weighting was used for each PTV and its respective beam to place spots. Uniform dose objectives were used for each SFO PTV and its respective beam. Robust min dose objectives were set to the CTVs for each dose level. Uniform dose objectives were used for each optimization PTV (Fig.3).
Fig. 6 Plot of V95 for each target for 16 CT shift combinations, Shift key: O/U=over/underrange, R/L= Right/Left, A/P= Anterior/Posterior, S/I=Superior/Inferior, Units=mm
Results
The inferior neck has an SFO dose distribution with good sparing of the larynx and esophagus (Fig.4a) The superior region in the oral cavity utilizes the PA beam to spare the oral cavity and parotids. a
Fig.3 Target optimization objectives b
Fig.4a SFO distribution for inferior neck fields RAO, AP, LAO
Fig.4b MFO distribution for superior oral cavity beams RAO, PA, LAO
Fig.2 PTV 70Gy(RBE):Maroon, Left: PA spot avoidance(pink), Middle: LAO spot avoidance(green), Right: RAO spot avoidance(light blue)