Improving Health Through Medical Physics

AAPM Newsletter — Volume 42 No.2 — March | April 2017

WORK GROUP ON IMRT REPORT A Report of the AAPM Working Group on IMRT for the 4th Quarter of 2016 Written By WGIMRT

Kyroudi et al. investigated the discrepancies between Pareto optimal plans and final deliverable plans in Raystation's multi-criteria optimization (MCO). MCO allows you to interactively compare near-optimal plans created in fluence space, however discrepancies can arise during final dose calculation when machine restrictions and heterogeneities are applied. The largest differences were found for small targets surrounded by low density tissue. The lack of systematic differences between plans made it difficult to predict the direction and magnitude of dosimetric changes. Radiother Oncol. 2016 Aug; 120(2):346-8.

Hussein et al. built a knowledge-based planning (KPB) model for prostate and cervix uteri cancer cases using Varian's RapidPlan. Forty IMRT clinical cases were used for prostate, and thirty-seven VMAT cases were used for the cervix uteri cases. Initial plans generated using KPB had poor superior-inferior target coverage. Adjusting the PTV parameters in the model resulted in the generation of acceptable clinical plans, requiring no planner intervention, in 9 out of 10 cases. Their model has since been used on 100 clinical cases. Radiother Oncol. 2016 Sep;120(3):473-479.

Henry Wang et al. reported an interesting IMRT planning approach of incorporating planners' actual operation (mouse and keyboard interactions) on a commercial TPS and then use the information acquired to improve efficiency of IMRT planning of similar clinical cases. The knowledge is gained by using C# programs to automatically record planners' operations during the actual planning process. The knowledge is used to help modify TPS optimization parameters subsequently on other similar cases. They demonstrated the method on a head and neck VMAT case and a prostate IMRT case, where significant improvement on planning efficiency is observed. J Appl Clin Med Phys. 2016 Nov 8;17(6):6425.

Van Kranen et al. investigated the loss of target coverage from anatomy changes in head and neck cancer patients as a function of applied safety margins. A population of 19 oropharyngeal cancer patients was used in their study. They reported that VMAT for HN cancer treatment plans with 5-mm margins are robust for anatomy changes and show a modest increase in OAR dose. Margin reduction improves OAR sparing at a rate of approximately 1 Gy/mm at the expense of target coverage in a subgroup of the patients they studied. Int J Radiat Oncol Biol Phys. 2016 Nov 1;96(3):653-60.

Henke et al. proposed how to characterize potential advantages of online-adaptive magnetic resonance (MR)-guided stereotactic body radiation therapy (SBRT) to treat oligometastatic disease of the non-liver abdomen and central thorax. They used a set of ten patients and demonstrated that online-adaptive MRI-guided SBRT may allow PTV dose escalation and/or simultaneous OAR sparing compared with nonadaptive SBRT. They have introduced a prospective clinical trial to evaluate clinical outcomes of this technique. Int J Radiat Oncol Biol Phys. 2016 Dec 1;96(5):1078-1086.

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