Multi-Criteria Hybrid Proton-Photon Inverse Planning for Oropharyngeal Cancer

Abstract

To evaluate a recently developed hybrid proton-photon inverse planning technique (HYBRID) with conventional photon-only (PHOTON) and proton-only (PROTON) planning for HN cancer patients using multi-criteria optimization (MCO). The hypothesis is that HYBRID will provide more favorable CTV coverage and OAR sparing than PHOTON or PROTON for HN cancer. HYBRID simultaneously optimizes proton and photon variables in addition to their numbers of fractions, during which the photon dose covers PTV that implicitly accounts for setup uncertainty, and the proton dose and the total dose cover CTV that explicitly account for range and setup uncertainty via robust optimization. HYBRID optimally allocates protons and photons to improve total planning objective: the coverage of CTV is fine-tuned to minimize hot and cold spots and improve dose uniformity by photons owing to its robustness to delivery uncertainties; the sparing of OARs is improved using both protons and photons of complementary characteristics with more degrees of freedom for dose shaping. HYBRID was compared with PHOTON and PROTON for 10 HN patients treated with definitive radiation for locally advanced oropharyngeal cancer. The highest CTV was treated to 70Gy in 35 fractions using SIB with 2-3 CTVs. All three planning methods were solved with the same objectives and the same normalization (D98=100% to CTV for the worst case). The following table summarizes optimization objectives (normalized to the total of HYBRID (100.0 in the table)) and dosimetric quantities from MCO: better CTV coverage vs more OAR sparing by adjusting objective weights, where the best values are highlighted in bold. HYBRID provided the best total objectives (the sum of CTV and OAR objectives), and also the best OAR and CTV objectives for almost all cases (CTV objective slightly worse than PHOTON for 1 case); under the similar CTV coverage (D98), HYBRID had the lowest maximum dose (Dmax); in terms of mean or integral OAR dose (Dmean), HYBRID was the best for submandibular and larynx, the best for parotid under MCO with more OAR sparing, and in between PHOTON and PROTON for oral cavity and body. In average, HYBRID had 10 proton and 25 photon fractions. HYBRID offered the lowest total minimization objectives for all 10 oropharyngeal cancer patients, and the lowest OAR and CTV objectives as well for 9 patients. In particular, MCO-based HYBRID had the lowest mean dose for parotid, submandibular gland and larynx.Abstract 2970; Table 1QuantityBetter CTV CoverageMore OAR SparingPHOTONPROTONHYBRIDPHOTONPROTONHYBRIDObjectiveCTV51.5237.642.134.9194.330.3OAR72.7126.557.999.8130.769.7TOTAL124.3364.1100.0134.6325.0100.0CTVD98 (%)100.0100.8100.2100.0100.9100.4Dmax (%)107.5109.6105.7110.0120.0108.1ParotidDmean (Gy)23.520.421.717.216.714.7Oral CavityDmean (Gy)30.322.126.723.217.118.4SubmandibularDmean (Gy)31.029.327.419.521.916.4LarynxDmean (Gy)30.034.128.318.924.616.7BodyDmean (Gy)6.53.75.66.63.75.7 Open table in a new tab