A Dosimetric Study of Intensity Modulated Radiation Therapy (IMRT) Plans for Oropharyngeal Carcinoma (OPC)-Comparing Detailed vs. Simple Cost Functions and Their Effect on Dose-volume Parameters

Results: In comparison of exposed dose distribution for the conventional radiotherapy system in the static state of tumor motion simulator,penumbrawidth(20%80%ofmaximumdose),effectivetreatmentarea(above80%ofmaximumdose),andfieldarea (above 50% of maximum dose) were changed to 49.06% increase, 3.40% decrease, and 7.94% increase, respectively. Meanwhile, the penumbra width, effective treatment area, and field area in the experiments using the MLC module were changed to 4.34% increase, 1.16% decrease, and 0.86% increase, respectively. Conclusions:In this study, the dose characteristics of the target-tracking MLC module developed in the KIRAMS were evaluated. For comparison with conventional radiotherapy treatment, beam exposure using the MLC module showed the decrease of penum- bra width, the increase of effective treatment area and the decrease of field area. Future works are needed to evaluate dose char- acteristics of MLC modules in consideration of target movements of LR, and AP direction as well as SI direction to apply these modules to clinic. Purpose/Objective(s): To perform a dosimetric comparison of IMRT plans for patients with OPC with indications for contralat- eral parotid sparing. The comparison will be made between a detailed cost function vs. a simplistic cost function. Materials/Methods: After IRB approval, 15 patients with OPC who were eligible for definitive radiotherapy with contralat- eral parotid sparing were identified. Two separate IMRT plans were developed for each patient. The first plan was performed on a detailed view of patient anatomy. A cost function optimized dose to the planning target volumes (PTvs.) while mini- mizing dose to numerous non-target structures. The second was performed on a simplistic view of patient anatomy and op- timized dose to the PTvs. while attempting to minimize dose to only the contralateral parotid, cord, brainstem and tissue outside the PTV. The treatment plans were generated using Varian's Eclipse planning system with a co-planar nine field IMRT technique. The dose calculated was performed utilizing the estimated deliverable fluence of both plans using a 3D su- perposition/convolution algorithm. The two plans were compared according to clinically relevant dose-volume parameters us- ing a paired t-test. Results:With a prescribed dose of 70 Gyto the high risk PTV,the meanretropharyngeal constrictordose was 57.2 Gy(+/- 4.5 Gy) in the detailedplans and 61.8 Gy(+/- 4.3Gy) inthe simplified plans(p\0.01).Themaximumdose to 1% ofthe ipsilateral cochlea was 28.4 Gy (+/- 4.9 Gy) in the detailed plans and 42.5 Gy (+/- 11 Gy) in the simplified plans (p\0.01). The maximum dose to 1% of the ipsilateral brachial plexus was 55.9 Gy (+/- 10 Gy) in the detailed plan and 62.6 Gy (+/- 11.8 Gy) in the simplified plan (p\0.01). There was not a statistically significant difference in dose to the high risk PTV. The dose covering 95% was 69.3 Gy (+/-1.3 Gy) in the detailed plan and 70.4 Gy (+/- 2.2 Gy) in the simplified plan (p = 0.06). The dose to 95% of the intermediate risk PTV (60 Gy) was greater than the prescription dose in both plans. There were no significant differences to the contralateral parotid, cord or brainstem. Conclusions: A detailed cost function to create IMRT plans for patients with oropharyngeal carcinoma may provide clinically significant dosimetric benefits when compared to a simplistic cost function. While a more complex cost function requires attention to detail and additional time for contouring, we found statistically significant differences in these normal structures with minimal differences in PTV coverage. Additional studies are required to determine if the fluence delivered resembles the estimated deliver- able fluence generated by these complicated treatment plans.