AbstractID: 6655 Title: Inverted Field and Its Application to the Verification of Monitor
Unit Calculation in Intensity Modulated Radiation Therapy
An important problem in IMRT is how to verify the MU calculation of the treatment planning
system. Despite intensive efforts in computer based MU verification, it remains a challenge to
validate the dose in a low dose region because of the lack of a uniform QA criterion. In this
work, we introduce the concept of inverted field and apply it to the IMRT dosimetric QA in a
low dose region. We point out that for each intensity-modulated field (primal field), there
exists a unique inverted field. Similar to the inversion of an image, a pixel with low intensity
level in the primal field becomes a pixel with high intensity level in the inverted field, and vice
versa. Therefore, the low dose region of the primal field corresponds to the high dose region of
its inverted field. This complementary relation bridges what appears to be two independent
schemes and allows us to validate the MU of the primal field by using the dosimetric data of
the inverted field. The approach is independent of the leaf sequence algorithms and is
straightforward to implement using simple computer programming. In addition, because the
calculation does not directly invoke a forward dose calculation using the primal field, the
method seems to be more suitable for independent MU check. The utility of the technique for
the dose verification in low dose region is demonstrated using three examples with 6- and 15MV photon energies. The results show that the method provides a useful tool for IMRT QA.