Background Radiation Therapy involves the delivery of high energy X-rays to tumours in order to destroy cancerous tissue. It is used either as the primary therapy for treating cancer or is combined with a combination of surgery, chemotherapy, or hormone therapy. In the past, radiation oncologists used to plan their treatments to include approximately two centimetre margins of healthy tissue in order to ensure that sufficient radiation would be delivered to eradicate the actual tumour. This was required in order to take into account any organ motion and change in patient position from one treatment session to the next. These large margins often exaggerated the toxicity of the treatment and also limited the radiation dose that could be safely delivered to the patient. Around ten years ago, due to the advent of higher precision technology, and in particular the introduction of Intensity Modulated Radiation Therapy (IMRT), it was possible for clinicians to reduce margins to around one centimetre.
Image Guided Radiation Therapy Today, through the use of advanced Image Guided Radiation Therapy (IGRT), the tumour can be imaged immediately prior to treatment, thus enabling the radiation oncologist to determine its location precisely. There are a range of image guidance methods that have been developed for this purpose. The most popular approach that has been introduced in recent years relies on radiographic imaging and utilises Cone Beam CT (CBCT) imaging to produce a 3D volumetric image of the patient in the treatment position. This advance has allowed the margin of radiation to be applied to healthy tissue to be reduced to around five millimetres and, in certain cases, to as little as one or two millimetres. If the tumour has moved even a small amount since the previous treatment session, the radiation delivery can be adjusted such that the tumour is targeted precisely. This improvement in accuracy is now allowing radiation oncologists to change dramatically the way in which certain types of cancers can be treated. Traditionally, radiation oncologists have been prescribing a course of low dose treatments administered over a several week period in order to allow any irradiated healthy cells, adjacent to the tumour, time to recover. However, during this extended period, the cancerous cells are also able to repair themselves. With the advent of IGRT and the ability to increase dose and reduce error margins, there is a growing trend to replace the traditional “fractionated” regime of daily low-dose treatments that span several weeks with fewer but larger “hypo-fractionated” doses delivered over a significantly shorter period. The survival rate for patients who undergo surgery for early-stage lung tumours is 70 to 80 percent. Historically, for the same patients to be treated with conventional radiotherapy, the survival rate has been between around 30 to 40 percent. With the application of hypo-fractionated treatments, often referred to as Stereotactic Radiation Therapy (SRT), the percentage survival rate is approaching that of surgery.
| Over the next five to ten years, there may well be a fundamental change in the standard of care for certain patients with early-stage lung cancer in which radiotherapy might replace or perhaps be on par with surgery as a primary choice for treatment. The trend for such hypo-fractionated SRT treatment deliveries is not limited to early-stage lung cancer and is being looked at in a range of other disease sites.
Conventional IGRT is dependent on a radiographic image that is acquired prior to treatment delivery. Whilst, for most patients, there is minimal movement from pre treatment setup until the end of delivery, this is not always the case. In particular, during the higher dose treatments, the patient needs to lie on a hard uncomfortable couch for a long period which increases the likelihood of any movement. Furthermore, the associated risk of motion is of higher consequence when treatment is delivered in only a small number of fractions, increasing the risk of underexposure of the tumour and collateral damage to surrounding tissue. When treating targets affected by respiratory motion, such as in the vicinity of the lung, the tumour can move, in some cases, as much as several centimetres throughout the course of a breathing cycle. Real Time Tracking for Motion Management
Vision RT has developed the AlignRT®, GateCT® and GateRT® products in order to address the challenges of patient positioning and setup, patient motion during radiation therapy, and to facilitate the safe implementation of high end radiation delivery. These track the patient’s 3D surface in real time, calculate precisely any patient movement in all six degrees of freedom, and monitor any respiratory motion. Vision RT's products augment the capabilities of traditional IGRT methods for certain setup procedures and so that following pre-treatment target localisation, the patient can be tracked continuously through their real time surveillance fucntionality. Any motion that exceeds clinically acceptable tolerances may be detected automatically and the beam may be held* thereby averting a potential accident, or may be gated according to the patient’s breathing cycle. Tim Fox Ph.D., Director of Medical Physics at Emory University comments: “We are very excited about using AlignRT® for real time patient tracking during radiation delivery. In particular, with the increasing tendency towards higher dose and fewer fractions, the need for monitoring motion continuously throughout treatment becomes paramount”. *See third party interface statement for list of compatible devices. Glossary
CBCT: Cone Beam CT.
IGRT: Image Guided Radiation Therapy.
IMRT: Intensity Modulated Radiation Therapy.
SRT: Stereotatactic Radiation Therapy. This is hypo-fractionated radiotherapy employing a small number of high dose treatments. It is often referred to as Stereotactic Body Radiation Therapy ('SBRT') when used to target extra-cranial lesions in the body.
SRS: Stereotactic Radio-Surgery. This is very high dose single fraction treatment, usually used to target lesions in the brain. |
