Linear Accelerator

What is radiation therapy?
Radiation therapy uses high energy ionizing radiation (principally X-rays and electrons) to treat cancer and some kinds of benign lesions (such as keloids). An oncologist may use this radiation to cure a patient of his cancer or to relieve symptoms of pain when the cancer has already reached an advanced stage.

What type of equipment is used? The type of equipment most commonly used to produce the high energy X-rays and electrons is a linear accelerator (linac, for short) and it can treat areas in all parts of the body. In general it can deliver a high dose to the tumor while sparing the surrounding normal tissue.
In WIMC the linear accelerator we use is a state of the art machine manufactured by SIEMENS and is known as the ONCOR IMPRESSION PLUS.

How does a linac work?
A linac uses advanced microwave and control technology to produce and accelerate electrons through a waveguide. At the end of their travel in the waveguide these electrons collide with a heavy metal target producing high energy X-rays. The SIEMENS ONCOR in WIMC produces two pre-selectable high energy X-rays of 6MV and 10MV and six electron energies ranging from 6MeV to 21MeV. With these energies available an oncologist can treat all types of cancer whether it is deep within the body or is superficially located. The patient lies on an advanced positioning treatment couch (TXT550) and radiation is delivered from any angle by rotating the gantry of the linac around the patient. Positioning lasers and electronic images are used to ensure that the patient is correctly positioned and the radiation beams are aimed at the target.

What is 3D conformal radiation therapy? 3D conformal radiation therapy (3DCRT) is a method of shaping the high dose region to the shape of the target volume. By doing this, during the course of the treatment more of the surrounding critical organs and tissue will receive a much lower radiation dose thus keeping down the side effects of the treatment. The SIEMENS ONCOR in WIMC is equipped with an 82 dual focused multi-leaf collimator and control systems (OPTIFOCUS) that enable this shaping of the high dose volume to be possible.

The process of 3DCRT for a patient involves several steps:

Step 1:
Simulation CT scans are done with the patient usually in a thermoplastic immobilization device custom made to fit him so that the area scanned will be immobile and in the same position when treatment is delivered. Sometimes MRI or PET-CT scans are also done to better visualize the tumor; these scans are co-registered (fused) with the CT scans in the treatment planning process. The simulation process will take about an hour and the patient may need to observe some pre-scan preparation.

Step 2:
The treatment is planned using the images in conjunction with powerful treatment planning software. The targets and critical structures are outlined on the images by the oncologist and the physicists and once the oncologist has approved the outlines, the planning physicist uses the software to produce a treatment plan based on the oncologist's prescription. The oncologist will review the plan and if he finds that it meets all the clinical needs of the patient's treatment he will approve the plan. Once the plan has been approved the physicist will run a series of QA tests to verify that the plan can be faithfully implemented by the linac. All communication between the planning system and the linac is by plan import through a network. This process of planning and QA may take several days depending on the complexity of plan.

Step 3:
When the patient comes for his first treatment at the appointed time the radiographer will place him in exactly the same position that was used during the CT scanning using the same immobilization devices. Lasers are used to do the pre-treatment positioning; then the radiographer leaves the room and uses the linac's flat panel electronic imager (OPTIVUE) to take images to confirm that the patient has been positioned properly. Treatment is then delivered usually from multiple directions and this usually takes several minutes for each field. The patient will not feel anything and will only hear the noise of the high voltage circuits being energized during radiation delivery and will observe the gantry rotate in between treatments. The radiographers who operate the machine have audio and visual contact with the patient during the treatment through CCTVs and an audio device. The treatment is given daily usually for several weeks (3 to 5 weeks). During this period of treatment the patient sees the oncologist every week for review or can whenever he wishes if he does not feel well.

What is intensity modulated radiation therapy?
WIMC also introduced intensity modulated radiation therapy (IMRT) which is an advanced mode of high precision radiation therapy made possible by the fully computerized delivery controls of the SIEMENS ONCOR. In IMRT the intensity of the beam is modulated by the linac to produce far better conformality of the high dose to the target, producing a sharper dose gradient outside the target so that dose to critical structures is minimal. IMRT is able to produce quite easily concave and convex dose distributions which are not possible in 3DCRT thereby making it possible to better treat tumors close to or wrapping around critical structures. There is therefore an expected decrease in side effects when compared to 3DCRT (or other forms of conventional radiation therapy). However IMRT may not be the ideal choice for all tumors in certain circumstances when sharp dose gradients are not ideal making 3DCRT (or conventional) techniques more appropriate. The oncologist will make the decision on which technique is best suited to the patient's condition.

The processes for IMRT are similar but not identical to 3DCRT:

Step 1:
The simulation process is almost identical to 3DCRT except that different types of immobilization devices may be used.

Step 2:
The outlining of the target and critical structures generally follows that of 3DCRT except that multiple targets may be outlined by the oncologist. Planning uses an entirely different software algorithm known as inverse planning where now the oncologist can both set the dose he wants delivered to the target as well as the maximum dose (known as a dose constraint) he will accept for each critical structure. The inverse planning software then tries to produce a plan that matches the goals set out by the oncologist and through a series of iterations an optimal plan is produced. Because of the complex dose distributions a higher level of QA is done to ensure the correct delivery of the planned treatment by the linac. The process of planning and QA for IMRT can take from 3 days to a week.

Step 3:
This essentially is the same as 3DCRT except that the delivery of the treatment takes longer as the dose is delivered by segments (usually 15 minutes to half an hour depending on the number of segments).

What are conventional and palliative treatments?
These are basically large field 2D techniques used for symptomatic treatment or pain relief. The oncologist will make the decision on which technique is best suited to the patient's condition.

On completion of the treatment – follow up
After completion of the treatment, follow up with CT, MRI or PET-CT is usually done within 3 months. Your oncologist will explain to you such procedures at the end of the treatment.