In last month’s proton therapy blog, we covered the history of Carbon Ion Radiotherapy (CIRT). Discovered in California during the 1940s, clinical CIRT use began with the construction of the National Institute of Radiological Sciences (NIRS) in 1984. Since the completion of NIRS, there are 13 CIRT centers located throughout Europe and Asia. Recently, the MAYO clinic announced the construction of the first CIRT center in the western hemisphere — in Jacksonville, FL. Prior to the opening of this center, it is vital to understand the key similarities and differences between CIRT and Proton Therapy. In this blog, we will discuss the physical properties of these charged particles, the treatment delivery method, and the cost differences between the two therapies.
Similarities of Proton Therapy and CIRT
Protons and carbon ions are both charged particles, which means they both exhibit a property called the Bragg-Peak. The Bragg Peak allows for the absorbed dose to be gradual initially then it suddenly rises and peaks, after the peak the dose drops dramatically. This peak can be manipulated to occur entirely in the tumor, which means there is little to no dose deposited in the surrounding tissues. Additionally, both protons and carbon ions can be delivered to the patient using Pencil-Beam Scanning. This delivery method allows for the protons to deposit their energy in the tumor layer by layer, allowing the beam to perfectly fit the tumor.
Differences of Proton Therapy and CIRT
Treatment with carbon ions provides several unique physical and radiobiologic properties. The Bragg-Peak in carbon ions differs from the Bragg-Peak of protons due to nuclear fragmentation. Nuclear fragments contribute to dose distal to the target, creating uncertainty to tissues beyond the target to a greater degree than proton therapy. This is an important dosimetric principle to consider because it significantly complicates treatment planning. Additionally, a steeper lateral dose penumbra is observed at greater depths than with heavy ions, such as carbon, than with protons. Lateral penumbra is a measure of the sharpness of the shadow cast by a patient’s aperture. It depends on depth in patient, air gap, and beam line design, as well as prescribed depth and modulation.
Carbon ions exhibit a higher linear energy transfer (LET) than protons. This leads to a higher relative biological effectiveness, where the damage caused by carbon ions overwhelms the cellular repair systems. With a higher LET than other methods of radiation, CIRT provides a promising treatment choice for providing higher doses to targets than protons. Additional advantages of CIRT include better physical dose distribution because lateral scattering is lessened; and they have a lower oxygen enhancement ratio, which is a desirable feature for the eradication of radioresistant, hypoxic tumors. Furthermore, research has indicated that CIRT can create a stronger immunological response.
Despite the potential advantages of CIRT, the cost of developing and maintaining a heavy-ion center has been prohibitive for adoption in the United States. The cost of developing a center with a capacity of 1,000 patients per year is roughly twice as expensive as a proton center of the same size. Furthermore, due to the size and expense associated with CIRT, the majority of centers are treating with fixed-beam gantries, limiting the treatment positions available and requiring changes in patient setup before irradiation with multiple beams. In contrast, Proton Centers can afford a 360-degree rotating gantry that allows for a wide range of treatment positions. Below are schematics of a CIRT center and Proton Therapy Center. Take note of the differences with the gantry systems.
Figure 1- CIRT treatment center
Figure 2- Proton Therapy Center
This blog is the second in a series of three covering CIRT. Please stay tuned for next month’s
blog that will cover the current and future research of this treatment. In the meantime, check out
the resources below for more information on the development and adoption of CIRT.