Room: Exhibit Hall | Forum 1
Purpose: To compare the dosimetry from volumetric modulated arc therapy (VMAT), non-coplanar intensity modulated radiation therapy (nc-IMRT), and intensity modulated proton therapy (IMPT) for glioblastoma multiforme (GBM).
Methods: Thirteen GBM patients were divided in two groups. The first, included six patients with tumor overlapping the brainstem and optic chiasm; and the second, consisted of seven patients without overlapping normal structures. The prescription was 4600cGy in 23 fractions and 1400cGy boost in 7 fractions with VMAT as the original technique. Non-coplanar-IMRT and IMPT plans were then created for comparison. Photon plans were calculated with Eclipse AAA v11.0.31 (Varian Medical Systems) and IMPT used RayStation 8A Monte Carlo 4.2v. Organs at risk (OAR) were brainstem (with tumor subtracted), optic chiasm, optic nerve and cochlea. The OAR comparison metrics were maximum doses (QUANTEC). Two GTVs were contoured: GTV4600 and GTV6000 (smaller and inside GTV4600). The metrics to evaluate the PTV6000 for photon techniques and CTV6000 for IMPT coverage were conformity index (CI), heterogeneity index (HI), uniformity index (UI), minimum and maximum doses.
Results: The VMAT technique resulted in better tumor coverage than nc-IMRT and IMPT for overlapping and non-overlapping cases based on CI, HI, and UI. No meaningful statistical differences in dose sparing to brainstem and optic chiasm were found in overlapping cases among all techniques (p>0.083). In non-overlapping cases, IMPT had lowest maximum doses especially for the brainstem and optic chiasm compared to other techniques. Average maximum doses in brainstem and optic chiasm with IMPT were 2643.57cGy and 1809cGy, 3076.69cGy and 2314.21cGy with nc-IMRT, and 3739.07cGy and 2628.26cGy with VMAT, respectively.
Conclusion: Proton plans resulted in less average maximum doses than photon techniques for brainstem and optic chiasm. The nc-IMRT technique can improve normal tissue dosimetry if the tumor is not proximal or invading normal structures compared to VMAT.
Not Applicable / None Entered.