An indirect in vivo dosimetry system for ocular proton therapy.

Secondary radiation, particularly neutron radiation, is a cause of concern in proton therapy. However, one can take advantage of its presence by using it to retrieve useful information on the primary proton beam. At the Centre Antoine Lacassagne the secondary radiation in the treatment room has been studied in function of the beam modulation. A strong correlation was found between the secondary ambient dose equivalent per proton dose H*(10)/D and proton dose rate D/MU. A large volume ionisation chamber fixed on the wall at 2.5 m from the nozzle was used with an in-house computer interface to retrieve the value of D/MU derived from the measurement of photon H*(10) integrated over treatment time, using the correlation curve. This system enables the verification of D and D/MU to be made independently of the monitoring of the primary beam and represents a first step towards an alternative in vivo dosimetry in proton therapy.

Study of the secondary neutral radiation in proton therapy: toward an indirect in vivo dosimetry.

PURPOSE: Secondary particles produced in the collision of protons with beam modifiers are of concern in proton therapy. Nevertheless, secondary radiation can provide information on the dosimetric parameters through its dependency on the modulating accessories (range shifter and range modulating wheel). Relatively little data have been reported in the literature for low-energy proton beams. The present study aims at characterizing the neutron and photon secondary radiation at the low-energy proton therapy facility of the Centre Antoine Lacassagne (CAL), and studying their correlation to the dosimetric parameters to explore possible practical uses of secondary radiation in the treatment quality for proton therapy. METHODS: The Monte Carlo code MCNPX was used to simulate the proton therapy facility at CAL. Neutron and photon fluence, Φ, and ambient dose equivalent per proton dose, H∗(10)∕D, were determined across the horizontal main plane spanning the whole treatment room. H∗(10)∕D was also calculated at two positions of the treatment room where dosimetric measurements were performed for validation of the Monte Carlo calculations. Calculations and measurements were extended to 100 clinical spread-out Bragg Peaks (SOBPs) covering the whole range of therapeutic dose rates (D∕MU) employed at CAL. In addition, the values of D and MU were also calculated for each SOBP and the results analyzed to study the relationship between secondary radiation and dosimetric parameters. RESULTS: The largest production of the secondary particles takes place at the modulating devices and the brass collimators located along the optical bench. Along the beam line and off the beam axis to 2.5 m away, H∗(10)∕D values ranged from 5.4 µSv∕Gy to 5.3 mSv∕Gy for neutrons, and were 1 order of magnitude lower for photons. H∗(10)∕D varied greatly with the distance and angle to the beam axis. A variation of a factor of 5 was found for the different range of modulations (SOBPs). The ratios between calculations and measurements were 2.3 and 0.5 for neutrons and photons, respectively, and remained constant for all the range of SOBPs studied, which provided validation for the Monte Carlo calculations. H∗(10)∕D values were found to correlate to the proton dose rate D∕MU with a power fit, both for neutrons and photons. This result was exploited to implement a system to obtain D∕MU values from the measurement of the integrated photon ambient dose equivalent H∗(10) during treatment, which provides a method to control the dosimetric parameters D∕MU and D. CONCLUSIONS: The treatment room at CAL is moderately polluted by secondary particles. The constant ratio between measurements and calculations for all SOBPs showed that simulations correctly predict the dosimetric parameters and the dependence of the production of secondary particles on the modulation. The correlation between H∗(10)∕D and D∕MU is a useful tool for quality control and is currently used at CAL. This system works as an indirect in vivo dosimetry method, which is so far not feasible in proton therapy. This tool requires very simple instrumentation and can be implemented from the measurement of either photons or neutrons.

[Reirradiation of spine and lung tumor with CyberKnife]. / Réirradiation des tumeurs du rachis et du poumon par CyberKnife).

We present the results of two retrospective studies, one regarding reirradiation of spinal tumours and the second, concerning lung tumours. In the first case, primary or secondary tumours were located in or in contact with the vertebrae and spinal cord. The first irradiation has given a full dose to the spinal cord. In the second case, primary or secondary lung tumours have already been treated by irradiation alone or by radiochemotherapy. No grade 3 or 4 early toxicity has been found. Preliminary clinical results are encouraging. The use of CyberKnife represents a major therapeutic advance in the management of irradiated spinal or lung lesions. The possibility of sparing organs at risk and increasing the dose in the tumour target volume are the main advantages.

[Current indications and ongoing clinical trials with CyberKnife stereotactic radiotherapy in France in 2009]. / Indications du CyberKnife et essais cliniques en cours en 2009.

Image-guided frameless fractionated stereotactic radiotherapy can be performed with millimetric accuracy using the CyberKnife (Accuray Inc. Sunnyvale, USA) equipped with an integrated tracking system for intra- and extracranial lesions. Highly conformal hypofractionated irradiation has been used to treat lesions with curative or palliative intent. It is advantageous for radioresistant tumors, re-irradiating lesions, boosting small volumes and treating tumors that move with respiration. It also limits travel costs and improves the quality of life. Over 60,000 patients have been treated worldwide using CyberKnife including 600 patients in the three French cancer centres of Nice, Nancy and Lille. These expert Cyberknife centres follow quality assurance programs and work together with the "Haute Autorité de santé" and the French National Cancer Institute (INCa) to promote clinical developments. The CyberKnife has been used to treat intracranial lesions including (but not limited to) meningiomas, acoustic schwannomas, brain oligometastases, as well as skull base tumors like chordomas, or para- or intraspinal tumors, and extracranial tumors such as lung cancers. Currently, extracranial stereotactic radiotherapy is particularly attractive for tumors moving with respiration and is being evaluated in liver, prostate and re-irradiation including head and neck tumors.