Hypofractionated stereotactic radiotherapy for challenging brain metastases using 36 Gy in six fractions.

PURPOSE: Stereotactic radiosurgery and hypofractionated stereotactic radiotherapy are standard treatments for brain metastases when they are small in size (at the most 3cm in diameter) and limited in number, in patients with controlled extracerebral disease and a good performance status. Large inoperable brain metastases usually undergo hypofractionated stereotactic radiotherapy while haemorrhagic brain metastases have often been contraindicated for both stereotactic radiosurgery or hypofractionated stereotactic radiotherapy. The objective of this retrospective study was to assess a six 6Gy-fractions hypofractionated stereotactic radiotherapy scheme in use at our institution for haemorrhagic brain metastases, large brain metastases (size greater than 15cm3) or brain metastases located next to critical structures. MATERIAL AND METHODS: Patients with brain metastases treated with the 6×6Gy scheme since 2012 to 2016 were included. Haemorrhagic brain metastases were defined by usual criteria on CT scan and MRI. Efficacy, acute and late toxicity were evaluated. RESULTS: Sixty-two patients presenting 92 brain metastases were included (32 haemorrhagic brain metastases). Median follow up was 10.1 months. One-year local control rate for haemorrhagic brain metastases, large brain metastases, or brain metastases next to critical structures were 90.7%, 73% and 86.7% respectively. Corresponding overall survival rates were 61.2%, 32% and 37.8%, respectively. Haemorrhagic complications occurred in 5.3% of patients (N=5), including two cases of brain metastases with pretreatment haemorrhagic signal. Tolerance was good with only one grade 3 acute toxicity. CONCLUSION: The 6×6Gy hypofractionated stereotactic radiotherapy scheme seems to yield quite good results in patients with haemorrhagic brain metastases, which must be confirmed in a prospective way.

A new Monte Carlo model of a Cyberknife® system for the precise determination of out-of-field doses.

In a previous work, a PENELOPE Monte Carlo model of a Cyberknife system equipped with fixed collimator was developed and validated for in-field dose evaluation. The aim of this work is to extend it to evaluate peripheral doses and to determine the precision of the treatment planning system (TPS) Multiplan in evaluating the off-axis doses. The Cyberknife® head model was completed with surrounding components based on manufacturer drawings. The contribution of the different head parts on the out-of-field dose was studied. To model the attenuation and the modification of particle energy caused by components not modelled, the photon transport was modified in one of the added components. The model was iteratively adjusted to fit dose profiles measured with EBT3 films and an ionization chamber for several collimator sizes. Finally, dose profiles were calculated using the two Multiplan TPS algorithms and were compared to our simulations. The contributions to out-of-field dose were identified as scattered radiation from the phantom and head leakage and scatter originating at the secondary collimator level. Particle transport in the additional pieces was modified to model this radiation. The maximum differences between simulated and measured doses are of 20.4%. Regarding the detector responses away from axis, EBT3 films and the Farmer chamber give similar response (less than 20% difference). The TPS Monte Carlo algorithm underestimates the doses away from axis more importantly for the smaller field sizes (up to 98%). Besides, RayTracing simplifies peripheral dose to a constant value with no inclusion of particle transport. A Monte Carlo model of a Cyberknife system for the determination of out-of-field doses up to 14 cm off-axis was successfully developed and validated for different depths and field sizes in comparison with measurements. This study also confirms that TPS algorithms do not model peripheral dose properly.

[Digital applicator by 3D printing in contact brachytherapy]. / Applicateur numérique par impression tridimensionnelle en curiethérapie de contact.

Brachytherapy of skin tumours uses custom applicators that are manufactured manually. The integration of 3D printing customization of applicators during hidh dose rate brachytherapy planning could allow a better skin conformation and a better reproducibility of the positioning and treatment. We present the technical implementation of this method for our first two patients. A provisional planning scanner was carried out to create a digital applicator. The creation of the digital applicator used successively several software programs. The first, commercial, was RhinocerosR 3D used via Grasshopper, an integrated open source plug-in. The 3D applicator was then exported to the commercial software Simplify3DR. A g-code format file was generated for the printer. A second scanner was made with a 3D applicator in place to plan the final treatment. The treatment was planned by reverse optimization. The applicator could be designed within 15 days. For patient A, it was noted that 95 % of the clinical target volume received at least 35.4Gy (63Gy EQD2). For patient B, 95 % of the clinical target volume received at least 36Gy (64.8Gy EQD2). The forecast and actual planimetry met the coverage criteria of D95. Contact brachytherapy with 3D bioimpression is feasible, after software training, for complex treatment lesions. This technique could be extended to other indications.