Proton radiotherapy for orbital rhabdomyosarcoma: clinical outcome and a dosimetric comparison with photons.
BACKGROUND: Over 85% of pediatric orbital rhabdomyosarcoma (RMS) are cured with combined chemotherapy and radiation. However, the late effects of photon radiation compromise function and cosmetic outcome. Proton radiation can provide excellent tumor dose distributions while sparing normal tissues better than photon irradiation. METHODS AND MATERIALS: Conformal 3D photon and proton radiotherapy plans were generated for children treated with proton irradiation for orbital RMS at Massachusetts General Hospital. Dose-volume histograms (90%, 50%, 10%) were generated and compared for important orbital and central nervous system structures. Average percentages of total dose prescribed were calculated based on the 3 dose-volume histogram levels for normal orbital structures for both the proton and photon plans. The percent of normal tissue spared by using protons was calculated. RESULTS: Seven children were treated for orbital rhabdomyosarcoma with proton irradiation and standard chemotherapy. The median follow-up is 6.3 years (range, 3.5-9.7 years). Local and distant controls compare favorably to those in other published accounts. There was an advantage in limiting the dose to the brain, pituitary, hypothalamus, temporal lobes, and ipsilateral and contralateral orbital structures. Tumor size and location affect the degree of sparing of normal structures. CONCLUSIONS: Fractionated proton radiotherapy is superior to 3D conformal photon radiation in the treatment of orbital RMS. Proton therapy maintains excellent tumor coverage while reducing the radiation dose to adjacent normal structures. Proton radiation therapy minimizes long-term side effects.