Dosimetric Comparison: Volumetric Modulated Arc Therapy (VMAT) and 3D Conformal Radiotherapy (3D-CRT) in High Grade Glioma Cancer—Experience of Casablanca Cancer Center at the Cheikh Khalifa International University Hospital

Background: Intensity Modulated Radiation Therapy (IMRT) is currently employed as a major arm of treatment in multiforme glioblastoma (GBM). The present study aimed to compare 3D-CRT with IMRT to assess tumor volume coverage and OAR sparing for the treatment of malignant gliomas. Materials and methods: We assessed 22 anonymized patients datasets with High Grade Glioblastoma who had undergone post-operative Intensity Modulated Radiotherapy (IMRT) and 3D Conformal Radiotherapy (3D-CRT), This study will compare and contrast treatment plans Rapidarc and 3D-CRT to determine which technology improves significantly dosimetric parameters. Results: Plans will be assessed by reviewing the coverage of the PTV using mean, maximum and minimum doses while the OAR doses will be compared using the maximal doses for each, as set out in the QUANTEC dose limits. Conclusion: The use of IMRT seems a superior technique as compared to 3D-CRT for the treatment of malignant gliomas having the potential to in-crease the dose to the PTV while sparing OARs optimally.


Introduction
Intensity Modulated Radiation Therapy (IMRT) and Three Dimensional Conformal Radiation Therapy (3D-CRT) are both very promising techniques for the thhhe treatement of brain tumors. The standard of care for patients with multiforme glioblastoma is represented by a combination of surgical resection, adjuvant radiation therapy (RT), and chemotherapy, despite this aggressive multimodal strategy the prognosis remains poor.
Radiotherapy is usually delivered with Three Dimensional Conformal Radiation Therapy (3D-CRT) in 1.8 -2 Gy per fraction to a total dose of 59.4 -60 Gy. However, considering that GBM may lie in close proximity to several organs at risk, radiation treatment planning may lead to sub-optimal target coverage. In the attempt to improve clinical outcomes, Intensity Modulated Radiation Therapy (IMRT) has been increasingly evaluated and exploited for the treatment of GBM. Overall, from the dosimetric standpoint, Three Dimensional Conformal Radiation Therapy (3D-CRT) and IMRT seem to provide similar results in terms of target coverage, while IMRT, regardless of the employed technique, is better in terms of dose conformity, in reducing the maximum dose to the organs at risk (OARs) and in healthy brain sparing.
Purpose: We aimed to evaluate the dosimetric interest of Volumetric Modulated Arc Therapy (VMAT) using Rapidarc® the varian solution for the treatment of patients with multiforme glioblastoma near to organs at risk. We report the results of a retrospective study of 22 patients treated at the Casablanca Cancer Center of Cheikh Khalifa International University Hospital.

Materials and Methods
Through a retrospective study, we assessed 22 patients with High Grade Glioblastoma who had undergone post-operative Intensity Modulated Radiotherapy (IMRT) and 3D Conformal Radiotherapy (3D-CRT). The patients' characteristics are summarized in Table 1. We included patients with tumors in a variety of locations. The cases were selected to be representative of four dosimetric scenarios, there were no overlaps between OARs and the PTV, the second, the third and the last scenarios were characterized by the superposition with the PTV of 1, 2 and 3 OARs respectively. To improve delineation of target volumes and normal tissues, planning computed tomography (CT) and post-surgical magnetic resonance imaging (MRI) were automatically co-registered by using the dedicated treatment planning system (TPS). A visual check was performed at the end of the registration process: in case the results were not satisfactory, the radiation oncologist manually edited the co-registration. MRI were acquired for diagnostic purposes without the immobilization device. Gross tumor volume (GTV) was defined as the resection cavity plus any contrast-enhancing area on a postgadolinium T1-weighed MRI. The clinical target volume (CTV) was obtained by adding a three-dimensional 2 cm expansion to the GTV.
The physicians manually edited the CTV to respect natural anatomical barriers (bone, tentorium, falx). The CTV was then expanded by 0.5 cm to create

Results
Were

Discussion
Postoperative Radiotherapy with chemotherapy has been standard treatment for newly diagnosed glioblastoma as it had shown significant survival benefits after surgery.
Unfortunately HGG can develop in different sites of the brain, some lésions can be very proximal to several critical organs at risk (e.g. optical nerves, brainstem, chiasma and retina), that can cause late radiation toxicity including neurocognitive deficits and necrosis.
Therefore the potential for using the best technique to insure maximal coverage of the predicted target volume and simultaneously reducing radiation dose to OAR is discussed.
Our results indicate that, as compared with 3D-CRT, IMRT showed significant reductions in mean dose delivered to the brainstem, optic chiasma, normal brain and to the optic nerve, moreover IMRT also improved predicted target volume coverage and dose homogeneity over 3D-CRT.  [4] have been performed over the last years and nearly all, with few exceptions [4], suggest that IMRT techniques (static, volumetric, rotational) lead to a reduction of doses to OAR and to the healthy brain tissue [5] surrounding PTV, while maintaining target coverage without significant variations. MacDonald et al. [6] and Zach et al. [7] highlighted no differences in terms of PTV V95%. At the same time, in their comparative dosimetric study Wagner et al. [8] and Thilmann et al. [3] pointed out that IMRT achieved better target coverage with respect to 3D-CRT, scoring a V95% improvement of 13.5% and 13.1% respectively. This advantage was much more significant when PTV was in proximity of OAR [8].
MacDonald et al. [6] compared the dosimetry of Intensity Modulated Radiation Therapy and Three Dimensional Conformal Radiation Therapy techniques in patients treated for high-grade glioma. A total of 20 patients underwent computed tomography treatment planning in conjunction with magnetic resonance imaging fusion. Prescription dose and normal-tissue constraints were identical for the 3D-CRT and IMRT plans. As compared with 3D-CRT, IMRT significantly increased the tumor control probability (p < or = 0.005) and lowered the normal-tissue complication probability for the brain and brainstem (p < 0.033).
Intensity Modulated Radiation Therapy improved target coverage and reduced radiation dose to the brain, brainstem, and optic chiasma. With the availability of new cancer imaging tools and more effective systemic agents, IMRT may be used to intensify tumor doses while minimizing toxicity, therefore potentially improving outcomes in patients with high grade glioma. At the same time, in their comparative dosimetric study, Wagner et al. [8] and Thilmann et al. [3] pointed out that IMRT achieved better target coverage with respect to 3D-CRT, scoring a V95% improvement of 13.5% and 13.1%, respectively.
Recently most Radiotherapy technical platforms offer a choice among these different techniques, it is important to define the parameters which will guide the final decision adopted for the treatment, following a comparative dosimetric study. IMRT planning has demonstrated its superiority over Three Dimensional Conformal Radiotherapy with regard to the preservation of organs at risk.

Conclusions
IMRT seems a superior technique as compared to 3D-CRT, in our study it allows for a better target dose coverage and improves the homogeneity of the dose received by the predicted target volume while maintaining equivalent OARs sparing and reducing healthy brain irradiation.
What is already known on this topic:  The standard of care in GBM is represented by multimodal strategy consisting of surgical resection, adjuvant radiation therapy (RT), and chemotherapy.  Radiotherapy can be challenging as the target volume is surrounded by critical organs at risk.
What this study adds:  The goal of this study was to compare 3D-CRT and IMRT in GBM patients according to the dosimetric impact of the different scenarios on the critical structures irradiation.