Fractionated stereotactic radiotherapy for pediatric patients with retinoblastoma

In this report we discuss the application of a modified Gill-Thomas-Cosman (GTC) relocatable head frame to enable fractionated stereotactic radiotherapy (SRT) of infants under anesthesia. This system has been used to treat two infants, ages 12 and 18 months for bilateral retinoblastoma on a Varian 6/100 linear accelerator. The GTC head frame was used to reproduceably position and treat the orbits of these children to between 2520 and 3960 cGy in 180 cGy fractions. A standard head and neck tray, with accompanying thermoplastic mask, was adapted to mount to the head frame to enable these treatments. We found the maximum average deviation in the repeat fixations, as compared with the initial fitting data, to be +/- 2 mm. The overall average difference and standard deviation in measurement was 0.47 +/- 0.63 mm for the first case, and 0.19 +/- 0.94 mm for the second case with a combined average of 0.35 +/- 0.79 mm overall from a total of 381 point measurements. The stereotactic treatment plan (Radionics) incorporated a single isocenter for each orbit and 3-4 arcs per isocenter. Inter-comparisons have been made between this technique and a standard lateral field technique, designed using the SRS planning system. Dose-volume histograms and corresponding normal tissue complication probabilities (NTCP) based on pediatric bone growth inhibition have been calculated for each method for the orbital bone areas. We have found that the NTCP is reduced from 95-100% in the standard treatment method to 16% or less with SRT. Use of the modified head frame provides excellent setup reproducibility, facilitates access to patients for anesthesia and reduces the chances of a poor cosmetic result in these growing children.