Protons or Photons in Pituitary Neuroendocrine Tumors-That Is Not the Question

With the long-life expectancy of patients with pituitary neuroendocrine tumors (PitNETs), reducing long-term toxicity is essential to preserve quality of life. We aimed to investigate whether 2- or 3-field proton beam therapy (PBT) plans better spare organs at risks (OARs), including healthy tissue, than volumetric modulated arc photon therapy (VMAT).

Fifteen consecutive patients who had received radiation therapy for PitNETs were included. We made 4 treatment plans for each patient: one 3-field PBT plan (3F), two 2-field PBT plans (2F-L and 2F-IV), and 1 VMAT plan. Dosing was set to 1.8 Gy × 30 to allow for dosimetric comparisons.

All 4 treatment techniques showed excellent target volume coverage. Compared to VMAT, the 3 PBT techniques had lower doses to OARs. Median D_40% to the hippocampi varied from 0.4 to 1.1 Gy (relative biological effectiveness [RBE]) with PBT and was 5.2 Gy for both hippocampi for VMAT. Median mean dose to brain-CTV and head-CTV were lowest for 2F-L (3.1 and 2.3 Gy [RBE]), followed by 3F (3.9 and 2.5 Gy [RBE]), 2F-IV (4.1 and 2.6 Gy [RBE]), and highest for VMAT (4.8 and 4.7 Gy). A relative reduction in integral dose compared to VMAT was seen for all PBT techniques, with a median reduction of 38.5% to 45.9% for head-CTV.

Proton techniques spared OARs better than VMAT; however, absolute dose sparing was relatively small, and the clinical value remains uncertain. Nonetheless, the dose reduction, especially for the hippocampi and integral dose, is probably clinically meaningful. We argue that head-CTV should be assessed, as much of the volume receiving dose is located outside the brain. The size and location of the target volume impacted which proton plan was preferable. We conclude that individualized planning is more important than a mere question on protons or photons in PitNET patients.