Pencil Beam Scanning Proton Therapy as Single Vocal Cord Irradiation for Early-Stage Glottic Cancer

Abstract

Purpose: Single vocal cord irradiation (SVCI) is a promising technique to maintain excellent oncologic control and potentially improve upon toxicities for treatment of early-stage glottic squamous cell carcinomas. We sought to investigate whether pencil beam scanning (PBS) proton therapy could improve upon the already favorable dose gradients demonstrated with volumetric modulated arc therapy (VMAT) SVCI.

Patients and methods: A 64-year-old gentleman was treated in our department with 6X-flattening filter-free VMAT SVCI to 58.08 Gy in 16 fractions for a T1a well-differentiated squamous cell carcinoma of the left true vocal cord and tolerated it well with good local control. Comparative PBS plans were created in Raystation for the Varian ProBeam with clinical target volume (CTVs) generated to mimic the prescription target volume extent of the VMAT planning target volumes when accounting for PBS plan robustness (±3 mm translational shifts, 3.5% density perturbation). A 3-field single-field optimization plan was selected as dosimetrically preferable. Dosimetric variables were compared.

Results: Several organs at risk doses improved with PBS, including the maximum and mean dose to ipsilateral carotids, maximum and mean dose to contralateral carotid, maximum dose to the spinal cord, maximum and mean dose to inferior constrictor/cricopharyngeus, maximum and mean dose to the uninvolved vocal cord, and mean dose to the thyroid gland. There are tradeoffs in skin dose depending on location relative to the target-with the highest and lowest isodoses extending more into the skin with the VMAT plan but with the moderate isodose lines covering a wider area with the PBS plan, but we deemed it tolerable regardless.

Conclusion: SVCI is a promising strategy for maintaining the oncologic effectiveness of whole-larynx photon radiation while potentially improving upon the historic toxicity profile. The favorable dose distribution with PBS with respect to organs at risk dosimetry for PBS may allow for further improvements upon VMAT SVCI strategies. Clinical implementation of PBS SVCI may be considered.

Keywords: Early-stage glottic; Proton therapy; Single vocal cord irradiation.

Figure 1

Flexible fiberoptic laryngoscope view of the treated lesion at initial presentation.

Figure 2

A (upper left): Volumetric modulated arc therapy plan (6 MV flattening filter-free beam). B (upper right): Unused 2-field pencil beam scanning (PBS) plan. C (lower left): Another unused 2-field PBS plan. D (lower right): The preferred 3-field single-field optimization PBS plan.

Figure 3

Comparison of pencil beam scanning versus volumetric modulated arc therapy dose distributions. Red and gold colors are areas increased dose with pencil beam scanning plan (+25% removed since all outside body, as is the majority of the +10% except in the posterolateral portion of the neck). Blue and green colors are increased dose for the volumetric modulated arc therapy plan, including the green patch in vicinity of the right arytenoid.