Correct patient positioning is essential for good aiming in radiotherapy. This can be achieved using back and side pointers, but sometimes these devices are not being used in order to simplify and make the positioning faster. The results of the experimental study show that in thicker patients (over 24 cm) the use of back and side pointers is of great importance, because the variation of the patient thickness and angulations can significantly influence the aiming angles and dose percentages.
Patient thickness can be made uniform by using of following items,
To compensate for missing tissue or a sloping surface, a custom made bolus can be built that conforms to the patient’s skin on one side and yields a flat perpendicular incidence to the beam (see Fig. 7.18). The result is an isodose distribution that is identical to that produced on a flat phantom; however, skin sparing is not maintained. A common material used for this kind of bolus is wax, which is essentially tissue equivalent and when heated is malleable and can be fitted precisely to the patient’s contour.
A compensating filter or compensator achieves the same effect on the dose distribution as a shaped bolus but does not cause a loss of skin sparing. Compensating filters can be made of almost any material, but metals such as lead are the most practical and compact. They are usually placed in a shielding slot on the treatment unit head and can produce a gradient in two dimensions (such compensators are more difficult to make and are best suited for a computer controlled milling machine).
The major advantage of a compensating filter over bolus is the preservation of the skin sparing effect.
Patient thickness is the prime factor because some time the under dosage of tumour could happen due to its bulkiness of the patient. In that, particular cases we should give significant attention to account it on.
1. Patient thickness due to patient thickness will be reduced by,
a) Use of proper immobilization devices
b) Prepare the patient before the treatment conditions
1. a) Use of proper immobilization devices.
1. Effects of patient positioning and of thickness variations on radiotherapy. Breyer B, Mekki F.