Method of identifying dynamic multileaf collimator irradiation that is highly sensitive to a systematic MLC calibration error

Abstract

In intensity modulated radiotherapy (IMRT), radiation is delivered in a multiple of multileaf collimator (MLC) subfields. A subfield with a small leaf-to-leaf opening is highly sensitive to a leaf-positional error. We introduce a method of identifying and rejecting IMRT plans that are highly sensitive to a systematic MLC gap error (sensitivity to possible random leaf-positional errors is not addressed here). There are two sources of a systematic MLC gap error: centerline mechanical offset (CMO) and, in the case of a rounded end MLC, radiation field offset (RFO). In IMRT planning system, using an incorrect value of RFO introduces a systematic error ARFO that results in all leaf-to-leaf gaps that are either too large or too small by (2*DeltaRFO), whereas assuming that CMO is zero introduces systematic error DeltaCMO that results in all gaps that are too large by DeltaCMO=CMO. We introduce a concept of the average leaf pair Opening (ALPO) that can be calculated from a dynamic MLC delivery file. We derive an analytic formula for a fractional average fluence error resulting from a systematic gap error of Deltax and show that it is inversely proportional to ALPO; explicitly it is equal to Deltax/(ALPO+ 2 * RFO+ epsilon), in which epsilon is generally of the order of 1 mm and Deltax =2 * Delta RFO + CMO. This analytic relationship is verified with independent numerical calculations.

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.…”

Validation of Pinnacle treatment planning system for use with Novalis delivery unit

“…The difference in the MLC apertures is reflected in the average leaf pair openings (ALPO) ( 39 ) for the similar C‐shape plans: 1.3 cm for the single‐arc realistic Novalis plan vs. 2.3 cm for the hypothetical VMAT‐capable linac. Double‐arc C‐shape plans largely avoid the leaf tips crossing the central axis, resulting in better dosimetric agreement at the isocenter (Table 7).…”

Determination of depth and field size dependence of multileaf collimator transmission in intensity‐modulated radiation therapy beams

“…( 2 , 11 ) The net effect of the two offsets RFO and CMO is sometimes called “dosimetric gap” in the Eclipse treatment planning system (TPS).…”

“…( 5 , 11 – 12 , 19 ) This definition of fluence is not exactly the physical definition (the number of particles per unit area). For this reason, we use a notion of ray density ϕ, which is a mathematical concept used in IMRT optimization and dose calculation.…”

“…In the current TPS fluence model, MLC transmission T is an effective constant for all field sizes and depths. In the present model, the total fluence is essentially a ray density ${\Phi }_{\text{TPS}}$, ( 11 , 12 , 19 ) ${\Phi }_{TPS}=\varphi +T_{TPS}(1-\varphi )\hspace{0.17em}\hspace{0.17em},$ where ϕ is a general ray density. The value of ϕ for the open beam or for static MLC is 1 inside the field and 0 outside the field (sharp edges, no penumbra).…”