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Validation of Dose Escalation in Hypofractionated Hemi-Larynx Irradiation of Early-Stage True Vocal Cord Cancer Based On Anthropomorphic Phantom Measurements


C Ding

C Ding*, S Chun, B Hrycushko, T Solberg, R Timmerman, UT Southwestern Medical Center, Dallas, TX

SU-E-T-431 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate the use of the CyberKnife delivery platform and Multiplan treatment planning system (TPS) to achieve acceptable target coverage and normal tissue sparing for dose escalation when treating the involved hemi-larynx and at risk paraglottic space for early-stage laryngeal cancer.

Methods:Specialized anthropomorphic phantoms containing a hollow larynx were constructed based upon patient CT images with early-stage glottic cancer. Contours of the hemi-larynx clinical target volume (CTV) as well as the thyroid gland, arytenoids, contralateral vocal cord, and anterior commissure were created in the Multiplan TPS. Treatment plans were created using the ray-tracing algorithm and were re-calculated using the Monte Carlo algorithm. Radiochromic EBT2 film was sandwiched in the phantom at the level of the vocal cords to determine the dose delivered to the target and critical structures. Phantoms were irradiated with multiple fractionation schemes including 50 Gy in 15 fractions, 45 Gy in 10 fractions, and 42.5 Gy in 5 fractions using 6 MV photons. Film dose distributions were compared with that predicted by the TPS.

Results:For plans generated using the Monte Carlo algorithm, greater than 95% PTV coverage by the prescription dose was achieved with conformality indices ranging from 1.4 to 1.5. Mean dose to the contralateral vocal cord and arytenoids was less than 40% and 25% of the prescription dose, respectively. A gamma passing rate of 90% was seen using a 3%/3mm criteria when comparing the Monte Carlo algorithm with film, while a 79% passing rate was seen when comparing the ray-tracing algorithm with film.

Conclusion:Adequate dose delivery with significant sparing of uninvolved normal tissue is feasible and practical with CyberKnife radiotherapy when using the Monte Carlo dose calculation algorithm. This technique has the potential to allow hypofractionation for the purpose of improving local control, voice quality outcome, and patient convenience.

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