  The lightcone formalism including SU(3) breaking effects for the light
pseudoscalar mesons is studied using soft-collinear effective theory (SCET),
where the conformal symmetries needed for the expansion can be clearly
implemented. The lightcone distribution amplitudes (LCDAs) are well-defined at
each order in the SCET power counting, lambda. Relations between the LCDAs are
reproduced using the SCET formalism. Treating the SU(3) breaking
perturbatively, the leading breaking effects can be described in a simple
manner. As a result, a new relationship among the LCDAs for the light mesons
pi, K, and eta is developed, valid to leading order in the SU(3) breaking.
