1. A Multi-Institutional Dosimetric Evaluation of Proton Versus Photon Stereotactic Body Radiation Therapy for Hepatocellular Carcinoma
F. Khan, B. Nguyen, M. Perez, S. Leuthold, N. Schreuder, T. Hyun, T. Kim, S. Yoon, C. McKenzie, A. Gutierrez, S. Apisarnthanarax, M. Chuong
Department of Radiation Oncology, Miami Cancer Institute, Miami, FL; Proton Therapy Center, Seattle Cancer Care Alliance, Seattle, WA; Department of Radiation Oncology, University of Washington School of Medicine, Seattle, WA; Department of Radiation Oncology, Moffitt Cancer Center, Tampa, FL; Provision Proton Therapy Center, Knoxville, TN; Department of Proton Therapy Center, National Cancer Center, Seoul, Korea; Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea
Introduction
Hepatocellular carcinoma (HCC) continues to remain one of the deadliest cancers, accounting for nearly 30,000 deaths in Historically, radiation therapy has played a limited role in the management of HCC due to poor dose conformity and the risk of hepatotoxicity. With the use of stereotactic body radiation therapy (SBRT) and intensity modulated radiation therapy (IMRT), photon therapy gained popularity in the treatment of liver tumors. However given the large low dose cloud often produced by IMRT and VMAT, external beam therapies with lower integral doses have been sought. Several previous works on proton therapy for HCC have shown advantages over photons, however many of these studies have been limited to a single institution. The purpose of our abstract is to provide a dosimetric study on the comparison of protons and photons for HCC across multiple institutions.
Methods & Materials
Five computed tomography (CT) scans of previously treated HCC patients were selected and de-identified. Artificial gross tumor volumes (GTVs) of the following sizes and locations were contoured: small solitary, large solitary, portal vein thrombus, multi-focal unilobar and multi-focal bilobar. Planning target volumes (PTVs) were generated using a uniform 5mm expansion of the GTV. These five CTs and structure sets were distributed to several proton and photon based institutions nationally and internationally. Planning guidelines were based on the RTOG 1112 protocol. Following the protocol, each institution attempted the plan at the highest dose level (50Gy) then de-escalated the prescription (RX) dose as necessary based on the mean liver dose (MLD). A wide variety of techniques for both modalities were permissible and breath hold was assumed for all cases. The final RX, number of de-escalations, MLD, liver volume receiving < 15Gy and a ratio of GTV volume/Liver minus GTV volume (GTV/Liver Ratio) were analyzed.
Figure 1. Categorical bubble plot representing the final RX (PTV D95) and MLD (diameter of bubble) for all plans in the study. Light red and blue are used to represent photon and proton plans, respectively. Cases are arranged from left to right in order of decreasing GTV/Liver ratio. The red outlined box highlights plans which did not meet the 700cc of liver < 15Gy threshold. A slight lateral jitter was used to prevent excessive overlapping of data points.
Results
For each case, proton plans demonstrated lower MLD values than photon plans. Among all plans, only one de-escalation was performed for protons while fifteen were performed for photons. The largest number of de-escalations in a single case was for the large solitary mass, totaling eight with photons alone. Additionally, the photon plans of the large solitary mass were the only plans which failed to meet a 700cc vol. of liver receiving < 15Gy threshold. This case’s volume was neither the largest nor the most complex of the study, however, it did exhibit the largest GTV/Liver ratio (see table 1). The portal vein thrombus, the largest target volume of the study, required only minor de-escalations.
Table 1. Numerical data for GTV Volume, Liver Volume, PTV D95, Number of De-escalations, MLD, Liver Volume < 15Gy and GTV/Liver Ratio.
Conclusions
Both protons and photons can produce highly conformal plans with relatively low OAR dose for liver tumors. Proton therapy has the additional potential to deliver higher doses without the need to de-escalate. While 50Gy was the highest allowable dose in this study, our results suggest dose escalation beyond 50Gy is possible with protons. With photon therapy, the need to de-escalate as well as the volume of liver receiving < 15Gy can be a concern in patients with high GTV/Liver ratios.