Shahed N. Badiyan1, Christopher L. Hallemeier2, Steven H. Lin3, Matthew D. Hall4, Michael D. Chuong4
Despite the conformality of modern X-ray therapy limiting high dose received by normal tissues the physical properties of X-rays make it impossible to avoid dose being delivered distal to the target. This “exit dose” is likely clinically significant especially for patients with gastrointestinal (GI) cancers when considering that even low dose received by the heart, lungs, bowel, and other radiosensitive structures can lead to morbidity and even may affect long-term tumor control. In contrast, proton beam therapy (PBT) delivers no “exit dose” and a growing body of literature suggests that this may improve clinical outcomes by reducing toxicity and even allowing for safe dose intensification to enhance tumor control. While there are not yet robust prospective data demonstrating the role of PBT for GI cancers, emerging retrospective data provide a strong rationale for continued study of how PBT may improve the therapeutic ratio for these patients. Here we review these data as well as discuss ongoing clinical trials of PBT for GI cancers
Figure 1 Significant normal tissue sparing with proton beam therapy (PBT) compared to intensity modulated radiation therapy (IMRT) for treatment of esophageal cancer. Dmax, maximum point dose; Vx, volume getting x dose in Gy; MLD, mean lung dose; MHD, mean heart dose; MLivD, mean liver dose; MKD, mean kidney dose.
Figure 2 A patient with a 20-cm localized hepatocellular carcinoma replacing the entire right lobe of the liver who was treated with proton beam therapy (PBT) (left images) to a dose of 58.05 Gy (RBE). In the right images, a comparison plan with X-ray intensity modulated radiotherapy (IMRT) is shown. PBT (vs. IMRT) resulted in significant reduction in dose to the uninvolved liver (mean dose, 13 vs. 27 Gy), stomach (mean dose, 0 vs. 18 Gy), and right kidney (volume receiving 20 Gy, 26% vs. 48%).
Figure 3 Significant sparing of abdominal organs with proton beam therapy (PBT, right) compared to X-ray therapy (left) in a patient with pancreatic
Given the accruing data showing a strong relationship between clinical outcomes and low dose received by organs at risk, there is a strong rationale to consider Proton Beam Therapy for patients with cancers of the foregut. While not all patients likely benefit from PBT, mounting retrospective data indicate that ongoing and future clinical trials may demonstrate that PBT provides clinically meaningful benefit for a subset of patients with Gastro Intestinal cancers.
Journal of Gastrointestinal Oncology Vol 9, No 5 (October 2018)