International Journal of Radiation Oncology*Biology*Physics
Clinical InvestigationDosimetric Study of Current Treatment Options for Radiotherapy in Retinoblastoma
Introduction
Retinoblastoma is a radiosensitive tumor that typically affects children less than 4 years of age. It is the most common intraocular malignant tumor in early childhood and the second most common tumor in all age groups after choroidal melanoma (1). Retinoblastoma affects approximately 1 child in 20,000. It is seen in hereditary (40%) and nonhereditary (60%) forms (2).
Management of retinoblastoma has changed considerably over recent years as a result of the availability of local treatment options such as photocoagulation 3, 4, 5, 6, cryotherapy 7, 8, and plaque therapy 9, 10, 11, 12, 13 and new approaches involving various combinations of chemotherapy agents 14, 15, 16, 17. External beam radiotherapy, although an established and extremely effective treatment for retinoblastoma, now is often regarded as a last resort because of the risk of late side effects, most especially the risk of second cancers.
All the new radiotherapy (RT) delivery techniques including intensity-modulated RT (IMRT) and fractionated stereotactic RT (FSRT) and new technologies such as volumetric arc therapy (VMAT) and helical tomotherapy (HT) and new modalities such as protons provide better sparing of normal structures than previously possible.
This study focused on the evaluation of the various radiotherapy techniques that could be used for treatment of retinoblastoma when the target volume was the whole globe. Conventional photon beam RT techniques including anterior and lateral wedged pairs (WP), multifield three-dimensional conformal RT (3D-CRT), and electron beam therapy, as well as dynamic multileaf IMRT, VMAT, HT, and FSRT, were compared. Dose-volume parameters and statistics were used to assess the ability of each technique to minimize the dose to normal structures while meeting target dose requirements.
Section snippets
Methods and Materials
Three children with retinoblastoma, each of whom required treatment to one eye, were anesthetized and immobilized in the supine position by using a thermoplastic mask system and scanned using an AcQSim (Philips, Cleveland, OH) computed tomography (CT) simulator with 3-mm-thick slices. The clinical target volume (CTV) was defined as the globe of the eye and proximal 0.5 cm of the optic nerve. The planning target volume (PTV) was defined as the CTV plus a uniform 3D 3-mm margin. Organs at risk
Target volume coverage
For the eight photon techniques, 100% of the PTV was covered by at least 95% of the prescription dose, i.e., V95% = 100%. For the two electron techniques, 95% of the prescription dose covered 95% and 97% of the PTV for the en face technique and the electron oblique technique, respectively. Supplementary Table E1 gives values of V95%, V99%, and Dmax and CI95% or CI100% averaged over the three cases for the 10 different techniques.
As expected for the WP, CRT 6F, and CRT 9F techniques, average V99%
Discussion
Use of systemic chemotherapy in conjunction with focal therapies such as cryotherapy, laser therapy, transpupillary thermocoagulation, and radioactive plaque brachytherapy has changed the role of external beam radiotherapy, which is now most commonly used as a treatment of last resort for ocular salvage in patients with extensive uncontrolled disease. Patients are typically very young (less than 3 years of age) and more likely to have a germ-line Rb mutation, placing them at risk for not only
Conclusions
Inverse planned image-guided radiotherapy delivered using HT or VMAT gives a better CI, improved orbital bone and brain sparing, and lower integral dose thanother techniques. With the use of sophisticated external beam radiation therapy delivery techniques such as IMRT, VMAT, HT, and FSRT, it is reasonable to anticipate a reduction in the risk of late effects in patients with retinoblastoma. Further improvements can be expected if the PTV margin can be safely reduced through the use of improved
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Conflict of interest: none.