External Beam Radiation Treatment of Retinoblastoma with Volumetric Modulated Arc Therapy Technique

Purpose: Volumetric modulated arc therapy (VMAT) has been successfully demonstrated with treatments of many disease sites. However, it is unknown how effective this technique could be used for the treatment of retinoblastoma. This study investigates clinical and dosimetrical feasibility of using VMAT in comparing to intensity modulated radiation therapy (IMRT) for the treatment of retinoblastoma.Method and Materials: Two patients with retinoblastoma previously receiving radiotherapy were selected for the retrospective study: one with unilateral retinoblastoma; the other with bilateral retinoblastoma. A treatment planning computed tomography (CT) scan with a slice thickness of 2.5 mm was acquired from each patient. A radiation oncologist contoured clinical and planning target volumes (CTV, PTV), and organs at risk (OARs), which includes lens, lacrimal gland, optic nerve, chiasm, and brainstem. The PTVs for both patients received 41.4 Gy in 1.8 Gy per fraction. Treatment plans were generated using a Varian Eclipse planning system. Two plans were generated with 6MV photon beams for each patient: one plan using IMRT and another plan using VMAT. For the IMRT plan, a six-field non-coplanar beam arrangement was used. The beam orientations were selected to avoid angles directed toward critical normal structures. For the VMAT plan, a double-partial-arc arrangement was selected: one arc from 150 degree to 320 degree delivered with no couch rotation, and the other arc from 50 degree to 181 degree delivered at a couch angle of 320 degree. The VMAT plan was obtained through a computerized arc optimization algorithm. To evaluate the VMAT technique, the following parameters were analyzed: maximum dose (Dmax), mean dose (Dmean), and minimum dose (Dmin) within the PTV, and the conformity index, which is defined as the ratio of volume covered by the prescribed dose over the volume of the target receiving the prescribed dose. A dose-volume-histogram analysis for the OARs was performed to assess dosimetric characteristics in both the VMAT and IMRT plans.Results: Both IMRT and VMAT plan offers comparable target coverage and OAR sparing. More specifically, both CI values of IMRT and VMAT vary from 1.03 to 1.04 for the two patients. The differences in the Dmean to PTVs were less than 1% of the prescribed dose. VMAT shows greater Dmax and Dmin to PTVs than IMRT. The differences of maximal doses to OARs between VMAT and IMRT are less than 3 Gy. Assuming the time for each couch rotation comparable to one beam delivery, the total treatment time for VMAT is only about a quarter of that of IMRT.Conclusions: The VMAT technique allows an efficient treatment of patients with either unilateral or bilateral retinoblastoma, while offering comparable dosimetric characteristics to those of a standard 6-field non-coplanar IMRT.

LL-ROS-TH2B

External Beam Radiation Treatment of Retinoblastoma with Volumetric Modulated Arc Therapy Technique