Lateral loss and dose discrepancies of multileaf collimator segments in intensity modulated radiation therapy

Abstract

In the step-and-shoot technique delivery of intensity modulated radiation therapy (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 a range of field sizes: 2 x 2 to 12 x 10 cm2 for 6 and 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. A set of fields is delivered with the accelerator operated in the IMRT mode. Using an ion chamber, the output factors at 1 cm and 3 cm laterally from a field edge are measured at different depths in a solid water phantom. Our results show that with insufficient lateral distance in at least one direction, the absorbed dose never reaches the equilibrium values, and can be significantly lower for very small field sizes. For example, the output factor of the 2 x 2 cm2 field relative to 10 x 10 cm2 at d(max0 is 0.832 and 0.790 for 6 MV and 15 MV x rays, respectively. Multiple output factor curves are obtained for different field lengths and different buildup conditions. Thus under nonequilibrium conditions, output factors are critically dependent on the field size and the conventional method of determining the equivalent square does not apply. Comparison of output factors acquired in the commissioning of the accelerator with those measured in the present study under conditions of nonequilibrium shows large discrepancies between the two sets of measurements. Thus monitor units generated by a treatment planning system using beam data commissioned with symmetric fields may be underestimated by > 5%, depending on the size and shape of the segments. To facilitate manual MU calculation as an independent check in step-and-shoot IMRT, the concept of effective equivalent square (EES) is introduced. Using EES, output factors can be calculated using existing beam data for fields with asymmetric collimator settings and under conditions of lateral disequilibrium.

Stereotactic Body Radiosurgery for Spinal Metastatic Disease: An Evidence-Based Review

“…Beam shaping with the Trilogy is achieved with the 120-leaf Millennium multileaf collimator (MLC). The Millennium MLC has been previously described and its advantages with respect to beam penumbra are established [22, 23]. Finally, the Trilogy features dual beam energies (6 and 18 MV photons) providing greater flexibility in the type of treatment plans that can be used.…”

Determining the optimal dosimetric leaf gap setting for rounded leaf‐end multileaf collimator systems by simple test fields

“…Individual patient quality assurance (QA) is required for MLC‐involved intensity‐modulated plans (i.e., intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT)) to verify agreement between the planned and delivered doses, which is critical for patient care (1) . Over two decades, MLC properties, their uncertainties, and their potential clinical effects have been extensively investigated 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 . With recently developed narrow leaf MLC systems and their applications in hypofractionated treatment, it is necessary to more accurately determine the properties and associated uncertainties.…”

Combining segment generation with direct step‐and‐shoot optimization in intensity‐modulated radiation therapy

“…Simple plans have shorter delivery times than complex plans, which, in addition to increasing the throughput of patients, avoids the risk of reduction in radiobiological effects due to prolonged delivery times [16,27]. A simple plan is less sensitive to geometrical uncertainties and can be delivered with higher accuracy than a plan with many small and irregular segments [8,23].…”