Advanced Mixed Beam Therapy using MERT and IMRT
Jinsheng Li, Ph.D (Jinsheng.Li@fccc.edu) and Chang-Ming Ma, Ph.D.
Fox Chase Cancer Center, Philadelphia, PA 19111
Radiation therapy, the treatment of cancer using radiation, has proven to be one of the most efficient methods to control tumor growth. The most common radiation used for cancer treatment is electrons and photons (particles of light). They operate on the same principle: to kill the cancer cell by damaging its DNA. The difference between them is that a photon beam can penetrate tissue well, while an electron beam has a limited treatment range. Its radiation dose falls off rapidly as one moves farther along the direction of the beam. Utilizing this special property of the electron beam and combining it with photon beams benefits the treatment of shallow tumors in the sense that it spares the normal tissue and healthy organs around the tumor. An electron beam acts over a range that is proportional to its energy. Therefore, in order to have a uniform dose distribution in the tumor and as little dose as possible to the surrounding organs, the energy spectra of electron beams are optimized to provide a radiation dose that fits, or conforms to, the depth of the tumor. The intensities for both electron and photon beams are also optimized to provide a dose that conforms to the shape of the tumor in the lateral (side-to-side) direction.
Disease sites such as a post-mastectomy chest wall, breast, and head-and-neck region are better suited for mixed photon and electron beam therapy (MBRT). Feasibility studies on this topic have been performed. Calculated radiation doses, optimization methods and beam delivery systems using prototype electron multi-leaf collimators (devices that shape the beam; abbreviated as MLCs) or existing photon MLCs have been investigated. Smaller source-to-surface distances (60–70cm) are recommended when using existing photon MLCs for beam delivery on Siemens or Varian accelerators. Surface dose and beam properties are studied using the "Monte Carlo" method, which employs the element of chance, or statistical probability, to determine the information of interest. By simulating billions of interactions between the beam and its surroundings (including the tumor), with the outcome of each interaction determined by statistical probabilities dictated by basic physics, one can obtain accurate estimates of the dose to the tumor surface and properties of the beam. Treatment plans for head and neck and breast are generated with advanced MBRT. MBRT beam delivery accuracy and efficiency are evaluated with phantom measurements.
Results show that the Monte Carlo method can provide accurate (2% or 2mm) dose distributions for MBRT. MBRT plans show great advantages that provide excellent dose conformity for treatments involving shallow target volumes such as breast and head and neck. Our results of MBRT for 78 breast patients showed that the acute skin complications were significantly reduced in a "hypofractionated" breast trial, in which patients received fewer, but more potent, doses of radiation to treat breast cancer.
MBRT can provide much improved target dose conformity and uniformity, adequate skin coverage/avoidance and significant reduction in the dose to the adjacent normal organs and critical structures for shallow tumor treatment. Preliminary results have shown great potential of this technique for treating breast, chest wall and head-and-neck cancers.