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Saturday
May 08
08:45 - 10:00
Optimal treatment for periorificial high risk non-melanoma skin cancer
Non-melanoma skin cancer incidence is rapidly rising worldwide. When surgery is not feasible (e.g. poor performance patient, significant co-morbidities), or could result in unacceptable functional and / or cosmesis morbidity, radiotherapy can offer an excellent and versatile non-surgical option. Radiotherapy can be delivered as external beam or brachytherapy. In this debate expert speakers from surgical and radiation specialities will provide arguments for the surgery and radiotherapy in the management of NMSC, with emphasis on the need of multidisciplinary cooperation. The debate will be focused on two highly cosmetically sensitive facial locations: lip and nose. The debate will be supported by published results and guidelines in the field.
Debate
00:00 - 00:00
COMPARISON OF TWO ALGORITHMS FOR LEAF MOTION CALCULATION IN ECLIPSE TREATMENT PLANNING SYSTEM
PO-1439

Abstract

COMPARISON OF TWO ALGORITHMS FOR LEAF MOTION CALCULATION IN ECLIPSE TREATMENT PLANNING SYSTEM
Authors: Seoane|, Alejandro(1)*[aseoane@vhebron.net];Sánchez-Artuñedo|, David(1);
(1)Hospital Universitario Vall d'Hebron, Medical Physics Dpt, Barcelona, Spain;
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Purpose or Objective

Eclipse treatment planning system (v15.6, Varian Medical Systems) includes two algorithms that convert fluence into leaf movement for IMRT fields. Varian reports a reduction of MU and less tongue and groove effect using the newer algorithm Smart LMC (SLMC) in comparison to the algorithm Varian Leaf Motion Calculator (VLMC).

The aim of this work is to compare the MU and the leaf sequence of IMRT fields these two algorithms.

Material and Methods

Sixteen IMRT breast plans, with a total of 79 fields, were optimized using PO (v 15.6. 04). All the plans were calculated with 6 MV photons and a dose rate of 600 MU/min. The dosimetric leaf gap configured in Eclipse was 2 mm.

The optimal fluence was converted into leaf motion of a MLC Millenium 120 using SLMC and VLMC. When leaves motion travel exceeds a maximum value fixed by manufacturer, the fields are split into two or more subfields. In our patients, this effect turned out in 123 fields with VLMC and 117 with SLMC.

The following parameters were evaluated for the two algorithms:

  1. Monitor units of the 79 fields.
  2. For every split field and every control point, the leaf gap and tongue-and-groove was calculated according to [1]:

                 - The leaf gap was calculated as the distance between the leaf-ends of every pair of leaves.

                 - Tongue-and-groove of every leaf pair was obtained as the difference between the position of a leaf and the two adjacent leaves. More details can be found in [1].

         Closed leaf pairs were excluded from the analysis.
     3. A complexity index to describe the distribution of leaves speed over control points (MIs), and one to consider not only the leaves speed but also the acceleration (MIa). Further details are described in [2]. As the fluence modulation increases, so do the values of the indices.

Statistical analysis of the parameters derived for VLMC and SLMC were assessed using a Wilcoxon-Mann-Whitney test.

Results
  • Although a mean MU reduction of -6.9% was obtained using SLMC, no statistical difference was obtained.
  • The number of control points obtained with SMLC is fixed, 166 control point per field or 83 per split-field. Conversely, the number of control points in VLMC fields is variable. Because of that, the number of leaf pairs analyzed (N) differs between both algorithms.
  • Leaf gap resulted to be larger for VLMC while tongue and groove turned out to be smaller, being significant for both parameters (table).

  • No statistical significant differences were found in MIs.
  • Graph shows statistical significant differences (p<0.001) of MIa using SMLC (mean=31.3) instead of VLMC (mean=43.2).



Conclusion

Smart LMC algorithm results in a reduction of leaf gap and less change in leaf accelerations, but an increase in tongue-and-groove. A MU reduction was observed for this algorithm, although it was not statistical significant.

References

[1] YAO et al., JACMP. 16(4) (2015)

[2] PARK, Phys.Med.Biol. 59. (2014)