Author: Chin-Cheng Chen, Chia-Ho Hua, Christopher J. Kutyreff, Thomas E. Merchant 👨🔬
Affiliation: St. Jude Children's Research Hospital 🌍
Purpose: A dosimetric study was conducted to determine the optimal field parameters for a commercially available small-field applicator (SFA) used in spot-scanning proton stereotactic radiosurgery, particularly for pediatric brain tumors.
Methods: The SFA is an extended aperture holder (~12 cm in diameter) that can be attached to the fixed snout (30×40 cm²), significantly reducing the air gap without causing a collision with the patient or the couch. Lateral penumbras for 70.3 to 150.3 MeV proton fields were calculated using 0.5 σ and 1.0 σ spot spacings at varying air gaps from 2 to 10 cm with the SFA in place. Optimized field parameters, such as treatment depth (proton energy), spot spacing, and air gap, were applied to simulate the treatment of a small brain tumor (prescribed 22 Gy(RBE) in a single fraction) within a RANDO phantom. The SFA-apertured plan was compared to open-field plans using two different spot sizes: standard (2.1–5.3 mm σ) and narrow (1.4–4.3 mm σ) in our hybrid beamline.
Results: The lateral penumbra can be significantly reduced with the SFA aperture for target volumes located at depths <10 cm in water, with 1 σ spot spacing and an air gap <5 cm. The SFA apertured plan showed a significant reduction in the gradient index to 2.4, compared to 4.6 and 4.2 for the plans using standard and narrow spot sizes, respectively. 2% of the normal brain (D2%) received only 14 cGy(RBE), compared with 209 cGy(RBE) and 177 cGy(RBE) for the plans using standard and narrow spot sizes, respectively. The homogeneity index showed a preference for using narrow spot sizes (1.39 vs. 1.61) over the aperture.
Conclusion: The SFA-apertured plan demonstrates improved normal tissue sparing, which is essential in pediatric radiotherapy for shallow-depth brain tumors.