Optimal dose/volume constraints of sensitive structures in inverse planning for nasopharyngeal carcinoma

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Purpose: The key to inverse planning is to provide a set of achievable and safe dose/volume constraints that spare the sensitive structures but still allows delivery of a high dose to tumor targets. A standard set of such constraints are not available for nasopharyngeal cancers. As a result, an optimal inverse plan involves numerous iterations of changing dose/volume constraints. The purpose of this study is to develop dose constraints for sensitive structures of the head/neck region that provide optimal intensity modulated radiation therapy (IMRT) plans while minimizing the treatment planning time. Materials and methods: Twenty-five nasopharyngeal carcinoma patients consecutively treated at our institution with IMRT were reviewed. Of these, 9 were T1/T2 and 16 were T3/T4 tuumors. The average dose given to the Gross Tumor Volume (GTV) and Planning Tumor Volume (PTV) was 74 Gy and 65.7 Gy respectively. The percentage of the GTV and PTV receiving <95% of the prescribed dose was 3.8% and 1.8%. The sensitive structure dose volume histograms (DVHs) were also reviewed. Different parameters were evaluated for structures organized in series versus those in parallel. Serial functional subunit parameters were maximum point dose and maximum dose to 5%, 10% of its volume. Parallel functional subunit parameters were mean dose and maximum dose to 50%, 80% of the volume. Because the clinical outcomes for these patients were excellent, these dose constraints can be used as a guideline. Results: The average doses with standard deviations to normal structures are summarized in the table below. When these average dose/volume parameters were applied to the optimization process as constraints, the number of iterations were significantly reduced. Conclusions: The efficacy of computer optimized inverse planning is dependent on a set of dose constraints that must be both clinically safe and achievable. Through our experience of the 25 nasopharyngeal carcinoma patients, we have established a series of normal tissue dose/volume constraints which provided high doses to target volumes as well as safe doses to surrounding sensitive structures. Significant reductions in planning time was also achieved using these dose constraints. Tabled 1Serial StructuresMean Maximum Point Dose (SD)Mean Maximum Dose to 5% VolumeMean Maximum Dose to 10% VolumeBrain Stem53.8 Gy (6.3)41.9 Gy (8.5)38.9 Gy (9.5)Spinal Cord40.5 Gy (10.6)32.0 Gy (12.3)26.0 Gy (16.5)Chiasm36.8 Gy (13.4)30.6 Gy (12.5)28.7 Gy (11.3)Optic Nerve34.8 Gy (17.0)29.5 Gy (14.4)26.4 Gy (14.2)Globe29.3 Gy (15.4)18.3 Gy (11.6)15.7 Gy (10.9)Parallel StructuresMean Average Dose (SD)Mean Maximum Dose to 50% VolumeMean Maximum Dose to 80% VolumeTMJ36.2 Gy (6.8)34.2 Gy (7.5)30 Gy (6.0)Middle/Inner Ear46.8 Gy (11.2)45.2 Gy (13.3)38.6 Gy (13.2)Parotid Gland27.2 Gy (5.8)24.7 Gy (5.6)18.2 Gy (6.0) Open table in a new tab