Proton radiation therapy (PRT) for pediatric optic pathway gliomas: Comparison with 3D planned conventional photons and a standard photon technique

Purpose: Following adequate therapy, excellent long-term survival rates can be achieved for patients with optic pathway gliomas. Therefore, avoidance of treatment-related functional long-term sequelae is of utmost importance. Optimized sparing of normal tissue is of primary concern in the developmen t of new treatment modalities. The present study compares proton radiation therapy (PRT) with a three-dimensional (3D)-planned multiport photon and a lateral beam photon technique for localized and extensive optic pathway tum ors. Methods and Materials: Between February 1992 and November 1997, seven child ren with optic pathway gliomas underwent PRT. For this study, we computed p roton, 3D photon, and lateral photon plans based on the same CT data sets, and using the same treatment planning software for all plans. Radiation exp osure for normal tissue and discrete organs at risk was quantified based on dose-volume histograms. Results: Gross tumor volume (GTV) ranged from 3.9 cm(3) to 127.2 cm(3). Con formity index (relation of encompassing isodose to GTV volume) was 2.3 for protons, 2.9 for 3D photons, and 7.3 for lateral photons. The relative incr ease of normal tissue (NT) encompassed at several isodose levels in relatio n to NT encompassed by the 95% proton isodose volume was computed. Relative NT volume of proton plan isodoses at the 95%, 90%, 80%, 50%, and 25% isodo se level increased from 1 to 1.6, 2.8, 6.4, to a maximum of 13.3. Relative volumes for 3D photons were 1.6, 2.4, 3.8, 11.5, and 34.8. Lateral plan rel ative values were 6, 8.3, 11.5, 19.2, and 26.8. Analysis for small (< 20 cm (3)) and larger (> 80 cm(3)) tumors showed that protons encompassed the sma llest volumes of NT at all isodose levels. Comparable conformity and high-d ose gradient were achieved for proton and 3D photon plans in small tumors. However, with increasing tumor volume and complexity, differences became la rger. At the 50% isodose level, 3D photons were superior to lateral photons for small tumors; this advantage was equalized for larger tumors. At the l owest isodose level, 3D photons encompassed the highest amount of NT. Analy sis of organs at risk showed that PRT reduced doses to the contralateral op tic nerve by 47% and 77% compared to 3D photons and lateral photons, respec tively. Reductions were also seen for the chiasm (11% and 16%) and pituitar y gland (13% and 16%), with differences at clinically relevant tolerance le vels. Furthermore, reduced dose exposure of both temporal lobes (sparing 39 % and 54%) and frontal lobes was achieved with PRT. Conclusion: PRT offered a high degree of conformity to target volumes and s teep dose gradients, thus leading to substantial normal tissue sparing in h igh- and low-dose areas. It is expected that this will result in decreased long-term toxicity in the maturing child. Advantages of proton versus 3D ph oton plans became increasingly apparent with increasing target size and tum or complexity. Even in small tumors, conformity of 3D photon irradiation ca me at the expense of a larger amount of NT receiving moderate to low radiat ion doses. Lateral photons resulted in inferior dose distribution with high radiation exposure of clinically relevant normal tissues. (C) 1999 Elsevie r Science me.