ABSTRACT
Cranio-spinal irradiation (CSI) treatment of embryonal tumors is associated with long-term endocrine and neuro-cognitive sequelae. As an example, the radiation regiment for standard risk medulloblastoma is 23.4 Grays (Gy) CSI followed by a boost of 30.6Gy to the tumor bed. We hypothesize that a novel CSI technique, which we named "Functional" CSI (F-CSI) can reduce the dose to the hypothalamic-pituitary axis (HPA) and hippocampi compared to standard CSI (S-CSI) without sacrificing coverage. In this study, we compared the efficacy of Volumetric Modulated Arc Therapy (VMAT) and Helical Tomotherapy (HT) in delivering this novel CSI technique. Plans were constructed from 10 patients with embryonal tumors previously treated at our institution. Target volumes and organs at risk were delineated as per our local protocol and the ACNS0331 Atlas. The HPA and hippocampi contours were verified by an experienced neuro-radiologist. Primary objective was to achieve a D95% to the prescribed dose of 23.4Gy for CSI and 30.6Gy for the boost. Dmean ≤18Gy was assigned to the HPA and hippocampi. A two-sided t-test was used for comparison. F-CSI in both modalities were able to achieve the D95% target coverage. Hot spots (D2%) were lower with HT for both the CSI component (p = 0.03) and boost component (p < 0.01). VMAT was able to achieve better conformality (p < 0.01). Compared to S-CSI, both F-CSI modalities were able to achieve a significant decrease in dose to the HPA and Hippocampi. The average S-CSI HPA and Hippocampi Dmean were 23.9Gy and 23.8Gy. In contrast, respective F-CSI Dmean were 13.9Gy and 17.2Gy in VMAT and 15Gy and 15.9Gy in HT. The average composite (F-CSI plus boost) Dmean to the HPA and hippocampi often exceeded 18Gy. Compared to S-CSI, F-CSI with VMAT and HT were capable of achieving acceptable coverage while sparing the HPA and hippocampi. However, the addition of the boost component often exceeded the mean dose of 18Gy. This may be overcome with more conformal modalities for the boost phase such as stereotactic radiotherapy or proton therapy.
Subject(s)
Craniospinal Irradiation/methods , Neoplasms, Germ Cell and Embryonal/radiotherapy , Radiotherapy Planning, Computer-Assisted/methods , Radiotherapy, Intensity-Modulated/methods , Hippocampus , Humans , Hypothalamo-Hypophyseal System , Organs at Risk , Retrospective StudiesABSTRACT
PURPOSE: The aim of this study is to compare the dosimetric differences between four techniques for spine stereotactic body radiotherapy (SBRT): CyberKnife (CK), volumetric modulated arc therapy (VMAT), and helical tomotherapy (HT) with dynamic jaws (HT-D) and fixed jaws (HT-F). MATERIALS/METHODS: Data from 10 patients were utilized. All patients were planned for 24 Gy in two fractions, with the primary objectives being: (a) restricting the maximum dose to the cord to ≤ 17 Gy and/or cauda equina to ≤ 20 Gy, and (b) to maximize the clinical target volume (CTV) to receive the prescribed dose. Treatment plans were generated by separate dosimetrists and then compared using velocity AI. Parameters of comparison include target volume coverage, conformity index (CI), gradient index (GI), homogeneity index (HI), treatment time (TT) per fraction, and monitor units (MU) per fraction. RESULTS: PTV D2 and D5 were significantly higher for CK compared to VMAT, HT-F, and HT-D (P < 0.001). The average volume of CTV receiving the prescription dose (CTV D95) was significantly less for VMAT compared to CK, HT-F and HT-D (P = 0.036). CI improved for CK (0.69), HT-F (0.66), and HT-D (0.67) compared to VMAT (0.52) (P = 0.013). CK (41.86) had the largest HI compared to VMAT (26.99), HT-F (20.69), and HT-D (21.17) (P < 0.001). GI was significantly less for CK (3.96) compared to VMAT (6.76) (P = 0.001). Likewise, CK (62.4 min, 14059 MU) had the longest treatment time and MU per fraction compared to VMAT (8.5 min, 9764 MU), HT-F (13 min, 10822 MU), and HT-D (13.5 min, 11418 MU) (P < 0.001). CONCLUSION: Both CK and HT plans achieved conformal target coverage while respecting cord tolerance. Dose heterogeneity was significantly larger in CK. VMAT required the least treatment time and MU output, but had the least steep GI, CI, and target coverage.
