EVALUATION OF THE DOSE DUE TO THE POSITION OF THE PROTECTION BLOCKS USED TO PROTECT THE EYES

• Published in: Brazilian Journal of Radiation Sciences 2014-01, Vol.2 (1)
• Main Authors: Oliveira, Fernanda Lucia, Lima, Fabiana Farias de, Vilela, Eudice Correia
• Format: Article
• Language: English
• Subjects: Dosimetry; Protection blocks; Radiotherapy
• ISSN: 2319-0612; 2319-0612
• DOI: 10.15392/bjrs.v2i1.11

Several techniques have been proposed to perform the radiation treatment of medulloblastoma. However, each method presents limitations, and problems have occurred during irradiation due to the application of non-coplanar fields, such as the joint protection of fields and nearby organs that may be affected during irradiation. Among these organs is the eye, located 9 mm from the cribriform plate. With the improper positioning of the protection blocks, an underdosage in the cribriform plate could bring about a relapse of the tumor due to the migration of tumor cells, whereas an overdosage in the eye may well cause opacities of the crystalline lens or a total loss of vision. This study aims to evaluate the dose levels in four techniques of radiotherapy planning according to the positioning of the protection blocks and the collimator angle. In the first two treatments, the prescribed dose was 1.5 Gy/day, using a lead block with collimator angles of 11.3° and 348.7°. In the final two treatments, the prescribed dose was of 1.8 Gy/day, using the cerrobend block with a single field set-up and collimator angles of 9° and 351°. An ALDERSON-RANDOM anthropomorphic phantom and thermoluminescent dosimeters were used to implement these measures during treatment. Dose levels applied to the eyes were as follows: in the first two treatments, 27% of the prescribed dose; in the third treatment, 41% of the prescribed dose; and in the fourth treatment, 55% of the prescribed dose.  In the first two treatments the dose are still within acceptable limits to prevent future sequel. In the cribriform plate, the doses were much higher: in the first treatment, doses were on average 128.4% of the prescribed dose; in the second and third treatments, doses reached 121.7%; and in the fourth treatment, doses reached 108.7%. These values show that the lead blocks protected better than did the cerrobend blocks, and that the collimator angles of 9° and 351° reduced the dose levels in the cribriform plate; however, these remained above the original prescribed dose.