Limited field adaptive radiotherapy for glioblastoma: changes in target volume and organ at risk doses

Abstract

Objective: This study aimed to investigate the tumor volume changes occurring during limited-field radiotherapy (RT) for glioblastoma patients and whether a volume-adapted boost planning approach provided any benefit on tumor coverage and normal tissue sparing. Materials and Methods: Twenty-four patients underwent simulation with magnetic resonance (MR) and computed tomography (CT) scans prior to RT (MR\_initial, CT\_initial) and boost treatment (MR\_adapt, CT\_adapt). For the boost phase, MR\_initial and MR\_adapt images were used to delineate GTV(2) and GTV(2\_adapt), respectively. An initial boost plan (Plan\_initial) created on CT\_initial for PTV2 was then reoptimized on CT\_adapt by keeping the same optimization and normalization values. Plan\_adapt was generated on CT\_adapt for PTV(2\_adapt )volume. Dose volume histogram parameters for target volumes and organs-at-risk were compared using these boost plans generated on CT\_adapt. Plan\_initial and Plan\_adaptive boost plans were summed with the first phase plan and the effect on the total dose was investigated. Results: Target volume expansion was noted in 21 0 /c, of patients while 791b had shrinkage. The average difference for the initial and adaptive gross tumor volume (G1V), clinical target volume (CTV), and planning target volume (PTV) volumes were statistically significant. Maximum dose differences for brainstem and optic chiasm were significant. Healthy brain tissue V-10 and ipsilateral optic nerve maximum doses were found to decrease significantly in Plan\_adaptive. Conclusion: Results of this study confirm occurrence of target volume changes during RT for glioblastoma patients. An adaptive plan can provide better normal tissue sparing for patients with lesion shrinkage and avoid undercoverage of treatment volumes in case of target volume expansion especially when limited-fields are used.