Abstract
PurposeSince the modulation factor (MF) impacts both plan quality and delivery efficiency in tomotherapy Intensity Modulated Radiation Therapy (IMRT) treatment planning, the purpose of this study was to demonstrate a technique in determining an efficient MF from the Multileaf Collimator (MLC) leaf‐open time (LOT) distribution of a tomotherapy treatment delivery plan.MethodsEight clinical plans of varying complexity were optimized with the highest allowed MF on the Accuracy Precision treatment planning system. Using a central limit theorem argument a range of reduced MFs were then determined from the first two moments of the LOT distribution. A step down approach was used to calculate the reduced‐MF plans and plan comparison tools available on the Precision treatment planning system were used to evaluate dose differences with the reference plan.ResultsA reduced‐MF plan that balanced delivery time and dosimetric quality was found from the set of five MFs determined from the LOT distribution of the reference plan. The reduced‐MF plan showed good agreement with the reference plan (target and critical organ dose‐volume region of interest dose differences were within 1% and 2% of prescription dose, respectively).DiscussionPlan evaluation and acceptance criteria can vary depending on individual clinical expectations and dosimetric quality trade‐offs. With the scheme presented in this paper a planner should be able to efficiently generate a high‐quality plan with efficient delivery time without knowing a good MF beforehand.ConclusionA methodology for deriving a reduced MF from the LOT distribution of a high MF treatment plan using the central limit theorem has been presented. A scheme for finding a reduced MF from a set of MFs that results in a plan balanced in both dosimetric quality and treatment delivery efficiency has also been presented.
Highlights
Helical tomotherapy (HT) delivers radiation therapy through synchronization of the binary Multileaf Collimator (MLC) leaf‐pair openings, gantry rotation period, and couch longitudinal speed
VoLO has been shown to produce dosimetrically equivalent plans compared to the original voxel‐based optimization algorithm in a fraction of the computation time.[20]. This approach for determining the modulation factor (MF) is valid for the original Tomotherapy treatment planning system (TPS), the Precision TPS is ideal for this process in that (a) the moments of the leaf‐open time (LOT) distribution are displayed with the distribution in the graphics user interface and (b) two plans can be compared with each other with the Precision plan evaluation feature
There is rapid expansion of the LOT distribution within the first 100–200 iterations followed by a gradual increase
Summary
Helical tomotherapy (HT) delivers radiation therapy through synchronization of the binary Multileaf Collimator (MLC) leaf‐pair openings, gantry rotation period, and couch longitudinal speed. Conformal dose distributions can be achieved through intensity modulation of the HT radiation field. The HT radiation field is divided into 51 projections per gantry rotation (7.06° of gantry rotation per projection). Each projection is further divided into 64 beamlets representing each of the 64 MLC binary leafs (i.e., the leaf being either open or closed). The leaf‐open time (LOT) of each beamlet that intersects a target volume determines the instantaneous radiation dose delivered from it through the projection arc or fraction thereof. Intensity‐modulated radiation therapy (IMRT) is achieved by varying the LOT of each beamlet with inverse‐planning optimization of the treatment plan
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