Room: ePoster Forums
Purpose: Uncertainties associated with linear energy transfer (LET) and resultant variation in biological dose using proton therapy may lead to toxicities which are unexpected at the physical doses prescribed. Many centers evaluate for potential LET effects during planning to limit the potential for untoward toxicities. We sought to validate the practice of limiting acute angles between beams to reduce high LET regions in patient treatment plans with a clinical outcomes data set.
Methods: A prospective registry study of all pediatric proton therapy patients treated from 2015 yielded 452 patients. Analysis was restricted to patients with primary central nervous system (CNS) tumors that received greater than 36 Gy of their treatment from protons and were treated with only two beams utilizing single field optimization (SFO). A query of the record and verify system was performed to pull all treatment planning parameters. Univariate logistic regression was used to evaluate the impact beam intersection angle on presence or absence of radiation associated CTCAE G1 or greater CNS toxicities.
Results: CTCAE G1 or higher toxicity was observed in 10.9% (N = 12) of patients. The median angle was 147 degrees (IQR = 65 degrees). A positive, insignificant trend between decreasing intersection angle and CNS toxicities of G1 or higher was observed (p = 0.69).
Conclusion: While decreasing intersection of beam angles can result in increasing LET in two field treatment plans, this does not translate to a clinically significant increase in the frequency of CNS toxicities in pediatric patients treated with protons. Altering beam angles should not be done at the expense of more favorable physical dose distributions facilitating avoidance of critical structures.