Intensity Modulated Radiation Therapy (IMRT) Versus Intensity Modulated Proton Therapy (IMPT) in Low-Grade Gliomas: A Dosimetric Study

Abstract

To compare intensity-modulated proton therapy (IMPT) and intensity-modulated radiation therapy (IMRT) in patients (pts) with low-grade gliomas (LGG). Eleven pathologically confirmed previously irradiated LGG pts were retrieved and re-planned both with IMRT and IMPT. The dose prescription was 54 Gy in 27 fractions with the following goals: first, compliance of maxDose constraints for primary organs at risk (OARs) (optic nerves, chiasm, retinas, and brainstem); second, ensure target coverage: the percentage of planning target volume (PTV) receiving at least 95% of the prescribed dose (V95) should be ≥99%. Once the previous goals were met, a further effort was made to reduce the dose to secondary OARs: cochlea (meanDose), lens (maxDose), pituitary gland (volume receiving 50 Gy - V50), brain, temporal lobe, and hippocampus (volume receiving 30 and 40 Gy - V30 and V40, respectively). Target conformity and homogeneity were evaluated via conformity index and homogeneity index. Wilcoxon Rank Sum test was performed to determine statistical differences between the techniques and p < 0.05 was considered significant. Both techniques always fulfilled the established constraints for primary OARs. IMPT and IMRT provided same target coverage and conformity while homogeneity was significantly better with IMPT (p = 0.01). With a few exceptions, IMPT systematically provided a lower maxDose to primary OARs with respect to IMRT. However, the differences were statistically significant only for the contralateral retina (p = 0.03). IMRT and IMPT achieved comparable results regarding pituitary gland V50. IMPT significantly reduced the maxDose to omo- and contralateral lens (mean reduction 5.2 and 6.3 Gy, respectively) as well as mean dose to omo- and contralateral cochlea (mean reduction 5.3 and 3.8 Gy). Moreover, IMPT scored significantly better results in terms of brain V30 and V40 (mean reduction 8.9% and 4.1%, respectively) as well as omolateral temporal lobe V30 (mean reduction 19.1%). Even though results were not statistically significant, IMPT performed better than IMRT also for omolateral temporal lobe V40, contralateral temporal lobe V30 and V40 as well as omo- and contralateral hippocampus V30 and V40. IMRT and IMPT provide same target coverage and primary OARs sparing so that a better tumor control and lower primary OARs toxicity cannot be expected with IMPT. IMPT achieves a significant better sparing of secondary OARs and healthy brain. Hence, IMPT could potentially reduce the secondary OARs toxicity and radiation related neurocognitive deficits.