Intracranial Hypofractionated Stereotactic Radiotherapy with Different Head Positions Utilizing a New Positioning Device

Abstract

Materials/Methods: Based on planning target volumes (PTVs) of one, two, and three consecutive thoracic vertebral bodies, and contoured organs at risk (esophagus and spinal cord), treatment plans were independently generated for CyberKnife, intensity modulated protons, fixed field IMRT with 5 mm (11 field), 4 mm (9 field), and 2.5 mm (both 8 and 9 field based plans) leaf width MLC, and intensity modulated volumetric arc (RapidArc) with a 2.5 mm MLC. Treatment planning objectives included: a prescribed dose of 24 Gy in 3 fractions to the PTV and 95% of the PTV to be encompassed by 95% of the prescribed dose (V95%). For the organs at risk: the spinal cord limit was 10 Gy to = 0.1 cc and 12 Gy to the point maximum, and the esophagus limit was 18 Gy to\= 0.1 cc and 20 Gy to the point maximum. For analysis, all plans were normalized such that the dose to 95% of the PTV (D95) was 24 Gy. The maximum dose divided by the prescribed dose (MDPD) was used to compare the target dose inhomogeneity. Results:All modalities fulfilledthetargetvolumecoverageconstraintonV95%.Asatrade-off,CyberKnifeyieldedsystematically higher dose heterogeneity, and became extreme as the PTV volume increased to two and three vertebral body PTVs with MDPD of 2.44 and 3.13, respectively. IMRT with 2.5 mm MLC, RapidArcand proton plans yielded mutually equivalent and the best MDPD for one-, two-, and three-vertebral body PTVs (ranging from 1.1-1.16). In addition, protons, RapidArc or IMRT exhibited no increase in target dose heterogeneity as the PTV increased in number of vertebral bodies. For the spinal cord and esophagus of 0.1 cc volumes, the 2.5 mm MLC 9-field IMRT, RapidArc, and CyberKnife plans resulted optimal sparing; the most significant trade-off was observed for the CyberKnife at the expense of significantly overdosing of the target volume. Conclusions:Thestudyprovedthat:i)alltechniquesmetmajorityoftheplanningobjectives,ii)CyberKnifemettargetcoverageat the expense of high dose heterogeneity in the target, iii) RapidArc and IMRT with 2.5 mm MLC avoided high dose heterogeneity and achieved comparable results as protons, iv) a 2.5 mm MLC proved to be advantageous to respect the tight objectives of this study.