Comparison of Biological-Based and Physical-Based Optimization Algorithm in Treatment of Hypopharyngeal Tumors Using VMAT technique

10 Hypopharyngeal patients are retrospectively selected for this study. VMAT plans were generated using two different planning system, TPSe and TPSm. TPSe uses physical based cost function, whereas TPSm uses biological based cost functions. The total dose of 66Gy, 60Gy were prescribed to the primary and nodal target volumes. Clinical planning objectives were achieved by both the optimization techniques Dosimetric parameters such as Homogeneity Index (HI) and conformity Index (CI97%) were calculated for PTV. For critical organs, Spinal cord (V1%), brain stem (V1%) and parotids (mean dose) were used for plan evaluation. Monitor units were also compared between two treatment planning systems to understand its impact on treatment delivery. An unpaired student t–test values were used for plan comparison. PTV Conformity index (CI97%) was found to be 1.2±0.2, 1.1±0.1 (p-0.097) and Homogeneity index was found to be 1.07±0.02, 1.10±0.04 (p-0.096) for TPSe and TPSm respectively. PTV mean dose difference was observed to be less than 0.5% (p-0.922) between TPSe and TPSm . Dose received by 1% volume of spinal cord(V1%) is 4003.9±189.2cGy, 3597.3±262.6cGy and brainstem(V1%) is 4729.6±277.3cGy, 3913.8±293.6cGy, Mean parotid dose was noted as 2339.8±587cGy, 2817±722cGy. Monitor Units(MU) required to deliver same dose is 490.4±59.4MU, 791.6±122.8MU for TPSe and TPSm. TPSm was restricting the spinal cord and brainstem dose by 10.16% (p-0.008) and 17.25% (p-0.00002) lesser than TPSe. On the other hand TPSm was restricting the mean parotid dose and MU’s by 20.4% (p-0.002) and 61.42% (p-0.000057) lesser than TPSm. VMAT plans of TPSe and TPSm offered clinically acceptable dose distributions. Biological based optimization (TPSm) showed enhanced OAR sparing for serial structures (Cord and Brain stem) whereas physical based optimization (TPSe) showed enhanced OAR sparing for parallel structures (Parotids). TPSe treatment delivery times were much better than TPSm. The result pertaining to serial, parallel structures and treatment time were statistically significant. Considering the uncertainty in the biological parameter used in the biological optimization and the limitation in single scoring optimization function an elaborative study has been initiated for further analysis.