A Dosimetric Comparison between RapidArc and Conventional 3D Conformal Radiotherapy in Post-Operative Irradiation of Low Grade Gliomas: Any Added Benefit?

Abstract
Background: Low-Grade Gliomas (LGG) comprise a rare group of central nervous system tumors and clinically chal-lenging cases to manage. They account for approximately 15% of all primary brain neoplasms. Radiotherapy (RT) remains an important component in the treatment of low grade gliomas and Three-Dimensional Conformal RT (3DCRT) is the current state of art for RT delivery. The main gain of modern RT technology is more likely reducing treatment related toxicity rather than an improvement in overall survival. RapidArc (RA) technology (Varian Medical Systems, Palo Alto, CA) is a novel radiation therapy technique integrating 3 important factors for treatment delivery.
Aim of the Study: The aim of the study is to investigate the potential dosimetric benefit of RapidArc (RA) in compar-ison to conventional 3D-CRT for low grade gliomas of the brain regarding target coverage and doses received by organs at risk (OAR).
Material and Methods: Twenty patients diagnosed with low grade glioma (WHO grade I-II) were referred to our center for postoperative irradiation using 3D-CRT. The pre-scribed dose was 54 Gy/30F/6weeks. CT scans of the 20 patients was done and delineation of the target volumes and organs at risk was performed. Two set of plans were done for each patient, one using conventional 3D-CRT and other using a RA plan. Dosimetric parameters regarding target coverage and dose received by OAR were evaluated and compared. Monitor units was also calculated.
Results: Target coverage in terms of V95% was signifi-cantly better in the RA plans with a value of 98.0±1.9 versus 96.0±2.6 for the 3D-CRT plans (p-value of 0.006). The RA plans gave a better conformity with a CI95% of 1.01±0.012 compared to1.07±0.024 achieved with the 3D-CRT plans (p-value of 0.678). The Homogeneity Index (HI) was higher for the 3D-CRT plans with a value of 0.131±0.112 compared to 0.097±0.033 for the RA plans (p-value of 0.114). The OAR received less dose in RA than 3D-CRT except for both lenses which received higher doses in the RA plans with a significant p-value of 0.002 and 0.001 for the right and left lens, respec-tively. The maximum dose and D1% for the healthy brain tissue was significantly lower in the RA plans when compared to 3D-CRT (p-value of 0.001). With respect to the average MUs ±SD needed per fraction, it was found to be 257.6±16.1 for the 3D-CRT plans as opposed to 355.6±44.4 RA plans (p-value of 0.001).
Conclusion: RA plans dosimetrically achieved a better PTV coverage, dose conformity, more homogenous dose distribution and better OAR sparing when compared to con-ventional 3D-CRT plans except for both lenses. The 3D-CRT plans utilized a lower number of MUs than RA.