Target volume size effect on comparison of dynamic arc treatment plans computed using flattened and unflattened 6MV beams.

INTRODUCTION/BACKGROUND: In conventional linear accelerators, to obtain flat profiles leading to uniform dose distribution in homogeneous medium, the flattening filter is usually applied on the beam path. In recent years, to obtain higher dose rates, there have been the options of flattening filter free (FFF) beams and it has been noticed that these have many advantages. The aim of this study was to clearly underline the advantages and the drawbacks of flattened filter free (FFF) beams in comparison with the flattening filter (FF) beams for different clinical contexts (planning target volumes locations). METHODS: Two groups (planned with auto-planning VMAT, full and partial arcs) of eight patients each were analyzed: Group I (small planning target volume PTV, with average volume 48.9 ±44.4 cm3), Group II (large PTV, with average volume 532.4 ±368.8 cm3). Both beam modalities 6MV and 6MVFFF were compared in terms of Dmax, D95%, D1cc, D2cc, homogeneity index (HI), number of monitor units (MU), treatment delivery time. RESULTS: Using the 6MVFFF, the treatment delivery time was significantly reduced (p<0.05). For larger PTVs, the number of MU increased by more than twice, and the p-value shown a significant difference (p= 0.008). The value of Dmax increased by 4%. On the contrary, for small volumes, the results were quite similar from 6MVFFF to 6MV except some differences in terms of MU. CONCLUSION: It is recommended to use 6MVFFF beam with small PTV volumes. Dose distributions are almost the same as with 6MV and there is a significant reduction of the treatment delivery time up to 57%. Due to the dose profile shape in FFF mode, the dose is lowered beyond the central axis for the FFF beams, and the additional MU allows the dose to be delivered away from the beam axis.

Light and shadows of a new technique: is photon total-skin irradiation using helical IMRT feasible, less complex and as toxic as the electrons one?

BACKGROUND: Radiotherapy is one of the standard treatments for cutaneous lymphoma and Total Skin Electrons Beam Irradiation (TSEBI) is generally used to treat diffuse cutaneous lymphoma and some cases of localized disease. Helical IMRT (HI) allows to treat complex target with optimal dose distribution and organ at risk sparing, so helical tomotherapy has been proposed as alternative technique to TSEBI but only one preliminary report has been published. METHODS: Three patients treated (from May 2013 to December 2014) with Helical IMRT, with a total dose between 24 and 30 Gy, were retrospectively evaluated. Data about dosimetric features, response and acute toxicity were registered and analyzed. Planned target coverage was compared with daily in vivo measures and dose calculation based on volumetric images used for set up evaluation as well. RESULTS: The patients had a mean measured surface fraction dose ranging from 1.54 Gy up to 2.0 Gy. A planned target dose ranging from 85 to 120% of prescription doses was obtained. All doses to Organs At Risk were within the required constraints. Particular attention was posed on "whole bone marrow" planned V10Gy, V12Gy and V20Gy values, ranging respectively between 23 and 43%, 20.1 and 38% and 9.8 and 24%. A comparison with the theoretical homologous values obtained with TSEBI has shown much lower values with TSEBI. Even if treatment was given in sequence to the skin of the upper and lower hemi-body, all the patients had anaemia, requiring blood transfusions, leukopenia and thrombocytopenia. CONCLUSION: Based on our limited results TSEBI should still be considered the standard method to treat total skin because of its pattern of acute and late toxicities and the dose distribution. In this particular case the better target coverage obtained with HI can be paid in terms of worse toxicity. Helical IMRT can instead be considered optimal in treating large, convex, cutaneous areas where it is difficult to use multiple electrons fields in relation with the clinical results and the limited and reversible toxicities.