First Report of Stereotactic Conformal Radiotherapy With Micromultileaf Collimator for Retinoblastoma

Abstract

Abstract: : Purpose: To describe stereotactic fractionated conformal radiotherapy with micromultileaf collimator as a new treatment modality for retinoblastoma. Methods: A 3–year–old girl was referred with a recurrent unilateral retinoblastoma which was initially treated by chemoreduction for a group D retinoblastoma, according to the ABC classification (Reese–Ellsworth Vb). The primary tumor was located in the inferior temporal quadrant and associated with a subtotal retinal detachment and multiple intra and subretinal seeds but no vitreous seeding. The patient underwent a CT–scan simulation for external radiotherapy treatment planning. The planned target volume was defined as the visible tumor on the CT image with an added margin of 2,5 mm, using treatment planning system BrainSCAN version 5. The treatment plan was defined to minimize irradiation of normal surrounding tissues, including the lacrimal gland and the bony structures. The relative sparing of normal tissues was achieved using a micro–multileaf collimator and 5 non coplanar conformal beams delivered by a 6MV photons linear accelerator (Siemens PRIMUS). The reproducible positioning of the globe utilized two immobilization devices consisting of a Brainlab stereotactic mask and a vacuum corneal contact lens. Results: A fractionated irradiation of 50.4Gy (1.8Gy/fraction) was delivered to the planned target volume. The volumes of ipsilateral bony orbit and lacrimal gland treated above 40 Gy were 2% and 26% respectively for stereotactic radiation therapy, compared to 11% and 59% for the lens sparing single lateral D–shaped technique. Conclusions: External beam radiotherapy has so far been used exclusively for selected advanced cases of retinoblastoma due to its major known complications. Stereotactic conformal radiotherapy can deliver the reported effectiveness of external radiotherapy while minimizing the dose received by surrounding normal tissues by a substantial factor. This relative bone–sparing technique is expected to significantly reduce both short and long term radiation–induced side effects, and may thus replace conventional external beam radiotherapy for the treatment of retinoblastoma. Further studies are needed in order to evaluate its role in the conservative management of retinoblastoma.

Keywords: retinoblastoma • oncology • radiation therapy