Tomoterapia

Helical Tomotherapy Unit (Hi-Art 2 System) is a new modality for radiation treatments: it's the first treatment unit dedicated to intensity modulated radiotherapy with a fully integrated image-guided radiotherapy system with the on-board Mega-Voltage CT capability. The tomotherapy system uses a 6 megavoltage accelerator, a 64-leaf binary multileaf collimator and xenon image detector array mounted on a rotating slipring: the radiation is delivered in an helical way, obtained by concurrent gantry rotation and couch/patient travel; together, these components allow continuous intensity modulated rotational delivery of radiation with fan beam entry from 360-degree. Optimisation process in tomotherapy represents a special challenge: "pre-optimisation step" where the main free parameters such as field size, pitch, calculation grid, are determined and beamlets pre-calculated; "optimisation step" where the dose constraints are selected and the plan optimised accordingly by using an iterative last-square minimisation technique; "post-optimisation step" where the dose distribution is finally recalculated to account dosimetric and mechanical MultiLeaf Collimator's characteristics (time leaf latency; tongue-groove effect). Convolution/superposition dose calculation is employed for ultimate dose calculation. A Mega-Voltage CT scan before the treatment (nominal energy of 3.5 MegaVoltage), can be fused with a planning CT scan to determine the correct patient set-up every time that the radiotherapist needs to check it or in agreement with our clinical protocols. The capability to reduce uncertainty in patient set-up and the ability of this technique to produce very large dose gradients and rapid dose falloff outside the target, could allow a number of changes in treatment plan optimization: reduces margins limiting high dose volumes in adjacent structures, increases the biologically effective dose to the target, permits the sparing of organs at risk in the same way as radiosurgery practise, permits hypofractionated treatments. During 2006 in Hospital San Raffaele - Milan, a preliminary comparison between Gamma Knife and Helical Tomotherapy treatments for benign brain disease has been made. Results show that Helical Tomotherapy plans improve dose homogeneity in the target volume, while keeping a good target coverage: the fraction of gross tumor volume receiving more than 100% of the prescribed dose (V 100%) is always larger than 95%. Gamma Knife shows a more pronounced dose unhomogeneity in target coverage, with a better conformity index because of the symmetry of dose distribution. Organs at risk constraints were satisfied both with Gamma Knife and Helical Tomotherapy. Preliminary results show that Helical Tomotherapy, is able to produce high dose gradients comparable with that one obtained with Gamma Knife, thus allowing to treat large brain lesions, not treatable with Gamma Knife. The most important difference seems to be the integral dose to the brain and the low doses distribution. Other studies concerning comparisons between Helical Tomotherapy and Intensity Modulated Radiation Therapy have been made in our centre during last two years: particularly for head and neck district, pancreas cancer, lung and liver metastasis, spinal and paraspinal cord tumors. In every cases Helical Tomotherapy significantly improves the homogeneity of dose distribution within Planning Target Volume and Planning Target Volume coverage together with a larger sparing of organs at risk with respect to conventional Intensity Modulated Radiation Therapy.