Advantages of intensity modulated proton therapy during hippocampal avoidance whole brain radiation therapy

Abstract

Background and purpose: Intensity modulated proton therapy (IMPT) allows for modulation parameterized for individual beamlets by position, intensity, and depth. This modulation capability is ideally suited for sparing organs at risk intermediate of the radiation target, such as hippocampal volumes within the whole brain. This work compared IMPT relative to volumetric modulated arc therapy (VMAT) during hippocampal avoidance whole brain radiation therapy (HA WBRT). Materials and methods: Ten adult and ten pediatric patients previously treated for central nervous system malignancies were identified. IMPT and VMAT treatment plans employing HA WBRT were generated for each patient, delivering 30 GyE (Gray Equivalent) in 10 fractions for adults and 36 GyE in 20 fractions for pediatrics. Dose indices, including dose volume histogram metrics and homogeneity index HI = [D5% − D95%]/ [D mean ] × 100, were used to assess plan quality and describe target coverage and normal-tissue sparing. Results: IMPT offered significant benefits relative to VMAT for hippocampal sparing. Hippocampal mean dose was reduced from 13.7 ± 0.8 Gy with VMAT to 5.4 ± 0.3 GyE using IMPT for pediatrics, and was reduced from 11.7 ± 0.9 Gy with VMAT to 4.4 ± 0.2 GyE using IMPT for adults. IMPT similarly lowered left hippocampal mean dose. Dose to 95% of the clinical target volume was statistically equivalent for both groups; however IMPT reduced the homogeneity index by roughly half. Conclusion: This manuscript demonstrates that HA IMPT can match or exceed dosimetric benefits offered with modulated X-rays. Inclusion of IMPT in future prospective studies is warranted.

Proton versus Photon Radiotherapy for Pediatric Central Nervous System Malignancies: A Systematic Review and Meta-Analysis of Dosimetric Comparison Studies

“…Globally, three studies [11, 14, 17] provided useful dosimetric data for extracranial OARs of the cephalic district: mean left lens dose, maximum left lens doses, and mean doses to lacrimal glands were significantly improved in PBT plans (Figure 5). Three supplementary studies [15, 18, 19] assessed dosimetry of cervical, thoracic, and abdominal OARs: mean doses to the esophagus, thyroid, lungs, and liver were significantly improved with PBT, while no significant overall advantage for the kidneys was observed ( P =0.11) (Figure 5).…”

“…Single studies reporting dosimetric comparisons for the heart mean dose [19], right lens mean dose [14], and optic chiasm and pituitary and lacrimal gland maximum doses [11] substantially confirmed the advantages of PBT over photon RT (see Supplementary Materials) (), with the exception of pituitary gland Dmax in Correia's study [11]. Boehling compared mean and maximum doses to vascular structures of the circle of Willis between PBT (with both 3D-PBT and IMPT) and IMRT, confirming the dosimetric advantages of protons [10].…”

“…In particular, besides the studies on CSI [15, 18, 19], Takizawa et al [31] and Correia et al (who also planned a boost to the tumor bed) [11] analyzed whole-ventricular (WV) RT (24–30 Gy) for patients affected by germ-cell tumors, Stoker et al [14] analyzed hippocampal-avoidance whole-brain RT (total dose: 36 Gy), while other authors analyzed RT to the resected intracranial tumor bed with total target doses in the range between 50.4 and 55.8 Gy (Table 2).…”

Stereotactic radiosurgery optimization with hippocampal-sparing in patients treated for brain metastases

“…Hippocampal-sparing WBRT with helical tomotherapy had a median dose of 5.5 Gy and maximum dose of 12.8 Gy, whereas with LINAC-based IMRT, these were 7.8 Gy and 15.3 Gy, respectively [9] . Similar evaluation of hippocampal-sparing WBRT with intensity modulated proton therapy demonstrated mean hippocampal doses of 4.4 +/− 0.2 GyE in adults [24] . With all these techniques, the dose to the hippocampi are far greater than what we have demonstrated with SRS.…”

The Advantage of Proton Therapy in Hypothalamic-Pituitary Axis and Hippocampus Avoidance for Children with Medulloblastoma

“…Beyond photon-based techniques, proton therapy's unique spatial properties have recently garnered increased attention in HA. Stoker et al [ 24 ] performed an IMPT dosimetry analysis comparing HA IMPT versus VMAT in 10 pediatric patients previously treated with WBRT for high-grade embryonal tumors to 36 Gy in 20 fractions. The IMPT was found to decrease the mean hippocampal dose from 13.7 Gy seen in VMAT to 5.4 Gy [ 24 , 25 ].…”