AbstractID: 7000 Title: Lateral loss and dose discrepancies in the step-and-shoot
approach of IMRT beamlets
In the step-and-shoot technique delivery of IMRT, each static field consists of a number
of beamlets, some of which may be very small. In this study, we measured the dose
characteristics for the range of field sizes:2x2 to 12x10cm2 for 15 MV x rays. For a given
field length, a number of treatment fields are set up by sequentially increasing the field
width using a multi-leaf collimator. The set of fields are delivered with the accelerator
operated in the IMRT mode. Using an ion chamber, the doses at 1 cm and 3 cm laterally
from a field edge are measured at different depths in a phantom. Reults show that with
insufficient lateral equilibrium in at least one direction, the absorbed dose never reaches
the equilibrium values, and can be significantly lower for very small field sizes. For
2x2cm2 field at dmax, it is about 50% of the dose relative to 10x10cm2 and about 60% for
the 12x2 field. At 10 cm depth, the dose is about 79% of the dmax dose for the 2x2cm2
field, more than 10% higher than the central axis depth dose due to the increased lateral
dose at depths. Thus inclusion of a large number of small segments may be significantly
underdose a shallow tumor with the step-and-shoot approach. Such extent of underdosage
cannot be accurately predicted by any treatment planning system utilizing beam data
measured under full buildup conditions. It may therefore be important to avoid the use of
small segments in the step-and-shoot technique.