Abstract
The purpose of the study was to investigate the potential of direct machine parameter optimization (DMPO) to achieve parotid sparing without compromising target coverage in IMRT of oropharyngeal cancer as compared to fluence modulation with subsequent leaf sequencing (IM) and forward planned two‐step arc therapy (IMAT). IMRT plans were generated for 10 oropharyngeal cancer patients using DMPO and IM. The resulting dose volume histograms (DVH) were evaluated with regard to compliance with the dose volume objectives (DVO) and plan quality. DMPO met the DVO for the targets better than IM, but violated the DVO to the parotids in some cases. DMPO provided better target coverage and dose homogeneity than IM and was comparable to IMAT. Dose to the parotids (23Gy) was significantly lower than for IMAT (48Gy), but somewhat higher than for IM (20Gy). Parotid sparing with IM was, however, only achieved at the cost of target coverage and homogeneity. DMPO allows achieving parotid sparing in the treatment of oropharyngeal cancer without compromising target coverage and dose homogeneity in the target as compared to two‐step IMAT. Better overall plan quality can be delivered with less monitor units than with IM.PACS number: 87.50.Gi
Highlights
For patients undergoing radiation therapy of oropharyngeal cancer, xerostomia due to irradiation of the parotid glands is a major limitation of quality of life
Radiation dose and the salivary gland volume irradiated strongly influence the amount of saliva produced.[1]. Severe effects can be avoided if at least 50% of the volume of the parotid glands is kept outside the radiation field.[2]. The Radiation Therapy Oncology Group RTOG 0022 recommends keeping the median dose to either parotid below 30Gy.[3]. This goal is difficult to achieve in conventional radiation therapy of oropharyngeal cancer with a prescription dose of up to 70Gy
Compliance to dose volume objectives (DVO) The minimum DVO to the PTVC, PTVLN left, and PTVLN right are met significantly closer for the plans optimized with Direct Step and Shoot” (DSS) than for the plans optimized with Intensity Modulation” (IM)
Summary
For patients undergoing radiation therapy of oropharyngeal cancer, xerostomia due to irradiation of the parotid glands is a major limitation of quality of life. Previous studies have reported that the use of intensity modulated radiation therapy (IMRT) spares the spinal cord and preserves the function of the parotid glands without compromising the dose to the target.[5,6,7,8,9,10,11,12,13,14,15,16] Most of these studies used beamlet-based fluence optimization with subsequent leaf sequencing. In this approach, each beam is divided into small pencil beams (beamlets). This is referred to as direct aperture optimization (DAO) if simulated annealing is used in the optimization, and direct machine parameter optimization (DMPO) in case of a gradient descent algorithm.[22,23,24,25,26,27,28]
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