Patient positioning and immobilization are said to be the most crucial parts of radiation therapy treatment.
For accurate delivery of a prescribed radiation dose to a target volume,while sparing sourrounding normal and critical tissues.
Without proper immobilization,the patient is at risk for improper treatment and unwanted side effects
Immobilization such as molds,casts,headrests,and other devices are constructed to reduce setup error and patient movement during treatment.
Although the primary goal of immobilization system is to limit patient motion and reduce the probability of major positioning errors, there are other benefits as well..
A well constructed immobilizing system can reduce the time for daily patient set up.
It can make the patient feel more secure and less apprehensive.
It can reduce the reliance on patient co-operation and alertness, and it can help to stabilize the relationship between external skin marks and internal structures.
4. Desirable characteristics of immobilization devices
Ease of use.
Ease of making the device.
Comfort for the patient.
Minimal space requirement for storage.
Resistance to bending and streching.
Minimal perturbation of the beam so as not to produce artefacts in image acquisition.
5. HEAD,NECK AND BRAIN IMMOBILIZATION
Initial construction and selection of proper immobilization for head and neck cancer is one of the most important parts of ensuring proper treatment of the patient.
In the past,patients for head&neck cancers were taped to the table across their forehead.a bite block was occasionally used in order to tilt the neck in a reproducible manner.
Current methods for head&neck and brain tumours include a variety of masks,molds and frame systems.In some instances,shoulder straps are used to pull the shoulders out of the treatment field for the neck.
The most common immobilization device used to treat head&neck cancers is an aquaplast.
This device is made from a rigid plastic that becomes pliable when placed in warm water.
When it is wet ,it is placed over the patients body and conforms to the contour of the treatment area.
Advantages: Aquaplast provide great reproducibility in daily treatments,lightweight and easy to use .
Additionally modifications can be made to the mask if the patient experiences swelling or weight loss during the treatment.
Disadvantages: windows cut cannot be reused.
when old becomes brittle and too soft when activated.
Base plate: the plate onto which the head immobilization systems are secured is usually referred to as a base plate.
Its material should be strong ,yet it should minimally attenuate the radiation beam.
Most base plates are acrylic and recently carbon fiber base plates are hugely devolped
Indexing bar: the two pin indexing bar is designed for reproducible positioning and stable fixation of base plates on the treatment couch.
The indexing bar can be placed at the desired indexing indents of the couch and it can be locked down by rotating the levers.
The base plates then can be positioned over the pins of the two pin indexing bar.
Shoulder retraction: in patients with short neck or in patients where opp lateral fields are used to treat the neck, the shoulders need to be retracted caudally to avoid entrance and exit doses.
An elastic strap ,equipped with handles and attached to each side of the treatment couch,can be used for the patient to pull on.
11. IMMOBILIZATION IN BRAIN TUMORS
Immobilization becomes more complex for radiosurgery or gamma knife procedures,because the requirements for positional and numerical accuracy in dose delivery are 1mm and 5% respectively.
Hence highly precised and accurate reproducibility is needed.
Two types of fixation systems are used in radiosurgery
1)GTC frame for fractionated therapy 2)BRW system for single fraction radiosurgery.
1)GTC relocatable head ring is a non invasive device that can be repeatedly fixed to the patients head in precisely the same position.
It has an impression taken of the patients upper dentition and the mold is then mounted to the head ring.
An impression is also taken of the occiput to conform to the shape of the patients head.
A biasing strap holds these two components firmly in place,so that precise repositioning can be achieved in each treatment session
The depth helmet provides quality assurance.it allows to conform the positional accuracy.
A depth probe with mm scale can be passed in to the guide tubes on the helmet,so that the distance from the hole to the patients head can be measured to mm accuracy.
2)In single fraction radiosurgery,invasive procedure is followed,the patient is fitted with a BRW system and then bolted to the treatment table.Bolts are fitted and screwed into the patients skull to minimize any movement during treatment.
15. THORAX AND BREAST IMMOBILIZATION
There are two common immobilization systems used when treating tumors in chest cavity.
1)VACLOK:these are custom,beanbag type pillows that are placed around the patients upper body.
The air is then vaccumed out of the bag for a custom fit and sealed in order to retain its shape.
vacloks are radiotranslucent.,almost air equivalent. they are washable and reusable.
2)BREAST BOARD: is used specifically for the treatment of breast cancer.
It has several adjustable features to allow for the manipulation of patients arms,wrists,head and shoulders.
Breast boards are generally constructed of carbon fiber,allowing the device to be lightweight and durable.
Carbon fiber boards have lower attenuation levels,which permit maximum beam penetration.
The breast can also be immobilized using a thermoplastic breast support system,the aquaplast mold is taken of the breast to aid in placement when lying supine on a breast board.
Vacloks can also be used to improve patient set-ups for the breast field.this type of treatment can involve the simultaneous use of the vaclok system with breast board.
18. PELVIC IMMOBILIZATION
There are two different crucial aspects in the immobilization of pelvic region.
1)to properly maintain patients outer contour during treatment..with respect to isocentre.
2)other is related to variation of internal structures during daily treatments.
Immobilization systems such as vacloks,belly boards,and the hip-fix(using aquaplast) system are commonly used during treatments of pelvic region