The Impact of Normalization Isodose Line (NIDL) on Dose Distribution in Intracranial Tumors Treated with CyberKnife

Objective: Dosimetric characteristics and dose falloff variation of Cyberknife (CK) plans under different isodose lines (IDL) of head tumors were assessed in an attempt to provide reference for clinical radiotherapy treatment planning.Methods: Ten head tumor patients planned with stereotactic body radiotherapy (SBRT) were selected and replanned, uniformly meeting the same objective with 30Gy covering 95% of the planning target volume (PTV).Four separate plans normalized to 65%, 70%, 75%, 80% IDL respectively were generated for each case.Dose distribution outcomes were compared using dosimetric parameters, including dose-volume histogram (DVH), conformal index (CI), new conformal index (nCI), homogeneity index (HI) and dose gradient index (GI).Results: Plans normalized to a higher percentage provide a better dose conformation of the prescription dose envelop and PTV homogeneity.As the normalized percentage changes from 80% to 65%, CI changes from 1.16 to 1.24, nCI changes from 1.18 to 1.28, HI changes from 0.25 to 0.54.In addition, GI is defined as the difference between the effective radius of the package volumes of 50% and 100% IDL.For the plans normalized to 65%, 70%, 75%, and 80% IDL, GI values are 6.89 ± 2.60, 7.17 ± 2.72, 7.33 ± 2.84, and 7.57 ± 2.95 respectively.Moreover, it increases as the tumor volume increases. Conclusion:For SBRT treatment planning, a lower normalized percentage can achieve a better protection of normal tissue, while a higher normalized percentage will result in a greater conformity and homogeneity of dose distribution.Moreover, for different volumes of head tumor, comprehensive consideration should be given to the radiotherapy treatment planning, in order to improve the therapeutic ratio.