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Effect of field placement errors on dose at junction between divergence matching and half-beam block techniques in craniospinal irradiation.

PURPOSE
In craniospinal irradiation, both overlapping and gapping between the cranial and upper spinal field may occur due to patient's daily position (setup). This can cause dose inhomogeneities at the craniospinal junction. In this study we tried to find out possible dose changes at the craniospinal junction due to setup mistakes.


MATERIALS AND METHODS
Both divergence matching technique (DMT) and half-beam block technique (HBBT) were used. At first step, a gap between cranial and spinal field borders was used, and at the second step overlapping between the fields for both techniques was applied. We irradiated films located in water equivalents solid phantoms with Co60 teletherapy machine.


RESULTS
Dose homogeneity was 4% when no gap was used. When 2, 5, 8 and 10 mm gap were used, overlap occurred. Dose inhomogeneities were 6%, 17%, 27%,34% and 16%, 24%, 29%, 43%, respectively for DMT. Dose inhomogeneity was 5% with no gap used. With gaps of 2, 5, 8, and 10 mm, overlap occurred. Dose inhomogeneities were 8%, 12%, 36%, 51% and 17%, 29%, 35%, and 47%, respectively for HBBT.


CONCLUSION
With no gap or with 2mm gap between the fields the dose homogeneity is in acceptable dose variation limits (10%) for both techniques (DMT and HBBT). Beyond this distance the setup mistakes are not tolerable.