#1, Becky

Source:  #1, Becky    Tag:  portal dosimetry
Okay, so Becky and I are doing this project, so to speak... and we made a blog to keep track of our information. I think that about sums it up.

The things we think are important we'll highlight or something.

Here's some info that Becky found.

Introduction

Radiation oncology is a branch of medicine that uses various types of radiation to treat and control cancer. The foundation of radiation oncology is based on the interaction between matter and energy. Beginning with the discovery of x-rays in 1895 by Wilhelm Roentgen, the role of the physicist has been critical in understanding how radiation interacts with matter. With the discovery of radioactivity by Henri Becquerel in 1896 and the separation of radium by Marie and Pierre Curie in 1898, it became known that certain materials also emitted radiation. Almost immediately the hazards and biological effects of x-rays and radioactive materials became apparent. In 1903, Alexander Graham Bell was one of many who suggested that radiation might be used to treat malignancies. Since the turn of the 20th century, radiation has been a part of medicine. The medical application of x-rays and radioactivity has given rise to the discipline of Medical Physics.
X-rays are a form of electromagnetic energy which are light rays of very high frequencies. Gamma rays, which originate in the nucleus of the atom, have even higher frequencies. Electromagnetic energy with high frequency also has short wavelength, which means that it’s small and passes through matter. Very early on it was discovered that the use of radiation in medicine was revolutionary due to the fact that it passed through matter. When radiation does interact with matter, it produces ionization. When a cell gets enough ionization, it dies. Since the emission and absorption of radiation takes place on the atomic scale, the interaction between radiation and matter must be well understood for clinical application to be possible. The medical physicist helps translate the science of radiation physics into the clinical treatment of cancer. Radiobiologists also help us understand the biological effects of radiation.
The Department of Radiation Oncology and Molecular Radiation Sciences derives the second part of it's name from our experts in medical physics and radiobiology. As modern medical technology allows for the precise creation and application of radiation therapy, the medical physicist is called upon for treatment planning, quality assurance and the advancement of clinical procedures. This in turn continues to translate into the Johns Hopkins philosophy of excellence in research, teaching and patient care.

The Role of the Medical PhysicistClinical medical physicists are a very important part of the radiation oncology team. Their primary role is to assure that the highest level of quality care is maintained. The medical physics group design and implement the quality assurance program in radiation oncology. They are responsible for selecting and specifying the types of equipment that are used in radiation therapy. After new equipment is installed, the medical physicist assures that the equipment meets or exceeds specifications.
Once the equipment is accepted, the physicist is responsible for commissioning the equipment, which involves taking enough measurements so that the equipment can be used clinically. Measurement data must also be transferred to other computer systems so that patient treatments can be planned. The medical physicist is frequently consulted by the radiation oncologist to help design a treatment that is difficult or unusual. A physicist is responsible for doing the quality assurance of every treatment plan before it starts. He or she checks that the planned information has been correctly transferred to the machine, that the plan agrees with the physicians prescription, that beam-on times are correct for each treatment field, and that all information is consistent, understandable, and well-documented.
The medical physicists also instructs radiation oncology residents, physics residents and graduate students, dosimetrists, nurses, and radiation therapists on the subject of radiation physics. Most of the physicists are also involved in specific areas of research, some basic research, others clinical or translational research.

Dosimetry

Dosimetry is the section of Medical Physics that specializes in developing patient treatment plans. To begin, a patient must have three dimensional pictures taken of their anatomy. These pictures are created using technologies such as computer tomography (CT) which uses low energy x-rays to create images of their internal organs. Once a patient has been imaged their anatomy is highlighted within a computer program. This allows for the definition of the cancer or gross tumor volume (GTV), and the "critical structures". The critical structures are the sensitive areas of the body that must be limited in their exposure. These include the eyes, kidneys, lungs, and spinal cord among other organs.
Having defined the cancer a radiation oncologist will decide how much radiation needs to be delivered. The physician will make a prescription for the amount of radiation to be used and a dosimetrist will begin treatment planning. In treatment planning, the angles of delivery of radiation, and how much radiation each part of the body is receiving (called the dose distribution) must be calculated. Sophisticated computer programs are used to show levels of radiation within the body, and there are also many other facets of the treatment plan that the dosimetrist is responsible for.

Once a treatment plan is complete the dosimetrist performs hand calculations to verify accuracy and will have a medical physicist review the plan. When the physician is satisfied with the treatment approach, the patient can begin undergoing therapy. During this time a dosimetrist will review their charts regularly to monitor treatment delivery and check for any changes in the treatment plan.

Medical Physics and Radiation Therapy

Medical Physics is based on the understanding of the scientific principles of the interaction between radiation and the human body. In the case of radiation oncology, cancer cells are acted upon by high energy x-rays. The x-rays deposit energy in tissue, which is called absorbed dose. This energy causes cancer cells to die, but the precise application of such energy must be exact so as to avoid damaging normal healthy cells. By treating a tumor from a number of different directions and avoiding normal tissue, we can destroy the tumor without causing serious side effects.

Types of External Beam Radiation Therapy

Conventional external beam radiation therapy - The science of radiation oncology and medical physics has developed standard approaches to dose delivery. In many cancer cases the treatment approach may be very similar and allows for conventional treatment.For example, many tumors can be treated with a single field from the front and a single field from the back or with two fields from the opposite sides. These are examples of parallel opposed fields. The combination of fields helps to uniformly deliver dose across the tumor. Sometimes 3 or 4 fields will be used. Occasionally, the gantry of the linear accelerator will rotate during treatment using what is called arc therapy.

3-D Conformal Radiation Therapy - Through the advancement of imaging technology enhanced images of the body allow for programming of treatment beams to conform better to the shape of a tumor. Hence treatment is more effective and side effects are reduced. By treating with large numbers of beams each shaped with a multileaf collimator (MLC) or cerrobend block, radiation dose is delivered uniformly and conformally to the tumor

Intensity Modulated Radiation Therapy (IMRT) - IMRT is one of the latest advancements in radiation therapy. This new approach to treatment allows for dose sculpting and even distribution of delivery to avoid critical structures while delivering precise uniform treatment. In this technique, the multileaf collimator (MLC) moves and modulates the radiation as the linac treats the patient.

Linear Accelerators for External Beam Radiation Therapy
The transmission of radiation in the clinical environment depends on very sophisticated technology. One of the primary types of treatment devices is called a linear accelerator. These linear accelerators, or Linacs, create the x-ray treatment beams. These beams consist of much higher energies then a standard x-ray machine and must be meticulously maintained in order to guarantee patient safety. Physicists are responsible for regular quality assurance measurements on all the equipment in the department that is used for patient treatment.

Linacs at Johns Hopkins:

600C -The 600C model is a lower energy (only 6 million volts effective energy, i.e. 6 MV) linear accelerator that is used mostly for treating areas of the head and neck, breasts, and lungs. The 600C has a 52 leaf MLC, which restricts the size of tumors that can be treated on this machine. The single X-ray energy also makes it less useful for treating the abdomen or pelvis. The 600C also has a special micro-multileaf collimator (mMLC) that can be used to treat small lesions in the brain.

6EX - This unit is also a single energy linac (6 MV) similar to the 600C, butnewer and more flexible for a variety of treatment plans The 6EX has an 80 leaf MLC allowing beam shaping for a larger range of beams. A special mMLC can also be attached to this linac and provides support for IMRT and stereotactic radiosurgery (see below). This linac is mostly used for brain, head and neck, and breast treatments.

2300 - This type of accelerator provides a dual energy system which allows for two X-ray energies (6MV and 15MV). In addition, this machine allows for treatment with electron beams of 6 different energies (from 4 MeV to 20 MeV) The higher energy X-ray beam can be used for treating larger regions of the body, such as the abdomen or the pelvis.

21EX - This linac is also a dual energy system (6MV and 15MV X-rays). It is capable of treating with electrons of 5 different energies. The 21EX is combined with a "BAT" ultrasound system. An ultrasound is used to align a patient properly for treatment of the prostate. In addition this unit incorporates a gating system used for lung cancer treatment. The gating system records the patient's respiratory pattern and treats according to their breathing position.

Films and Electronic Portal Imaging

Correct positioning is verified and documented regularly. On the 600C and 2300C, positioning is verified with X-ray film. On the 6EX and 21EX, films may be periodically taken, but most verification is done with electronic portal imaging (EPI).

Stereotactic Radiosurgery
This form of treatment is extremely precise and literally means to probe in three dimensions. It is most often referred to treatments of the brain as computer defined coordinates are used to precisely locate the target point.

Treatment modalities :

Stereotactic Radiosurgery (SRS) - Single treatment, high dose radiation, very accurate, patient's body position must be exact.

Stereotactic Radiotherapy (SRT) - Localized treatment using 3-d images, treatments will be repeated over time using lower dosages. The patient's position will be recorded using a stabilization device.

Fractionated Radio surgery (FSR) - Uses intermediate levels of radiation that are delivered over a small number of treatments.

Stereotactic treatment planning relies heavily on precise data acquisition and imaging technologies. Computer Tomography (CT Scannning), Magnetic Resonance Imaging (MRI), angiograms, and the fusion of CT and MRI scans are relied upon to provide the accuracy of anatomy necessary.
For more information please visit the Stereotactic Radiosurgery website and the Johns Hopkins Leksell Gamma Knife webpage.

Brachytherapy
Brachytherapy is a form of radiation therapy in which a radioactive source or radioactive seeds are placed very close to the tumor. This involves exposure of cancer cells to radioactive material rather than through external beam treatment. In brachytherapy the effective distance of the radiation source is small so effects on healthy tissue are reduced. Also this type of treatment requires a shorter exposure time and smaller number of treatments for dose delivery. In some cases, the seeds are permanently implanted and the patient is allowed to leave shortly after the procedure is completed.