Do collimator angle setup errors cause dose inhomogeneity at the junction zone during craniospinal irradiation? A phantom study.

Purpose: Craniospinal irradiation (CSI) has some geometric uncertainties, especially at the junction zone. In this study we tried to evaluate how possible random setup errors of the collimator angle may contribute to these uncertainties.

Materials And Methods: Cranial and spinal fields were drawn on RW3 solid water phantom in accordance with the divergence matching technique (DMT). Field dimensions were 18x18 cm and 6x30 cm, respectively. We placed light-insulated Kodak X-Omat V films at the junction zone, then we irradiated the films with different collimator angles with both Co-60 and 4 MV conditions, and determined how the junction zone was affected from random setup errors in DMT.

Results: 10.6 degrees collimator angle was proper for 30 cm upper spinal field. For Co-60 machine the dose homogeneity of this angle was +4.5%. For the angles of 8, 9, 11 and 12 degrees the homogeneities were -13%, - 11%, +5% and +10 %, respectively. For 4 MV photon the dose homogeneity of the 10.6 degrees collimator angle was +3%. For the angles of 8, 9, 11 and 12 degrees the homogeneities were -17%, -14%, +6 % and +13%, respectively.

Conclusion: As the CSI has some geometric uncertainties, serious dose inhomogeneities may occur at the junction zone. The collimator angle is of great importance and any random setup errors may not be tolerated.