Proton therapy technology evolution in the clinic: impact on radiation protection.

The use of proton therapy as a treatment modality is becoming more widespread in conventional radiation therapy practice. Commercialisation and introduction of compact systems has led to embedding of proton therapy facilities in existing hospital environments. In addition, technologically, proton therapy is currently undergoing an important evolution, moving from passive scattering delivery techniques to active pencil beam scanning, adopting image guidance techniques from conventional radiotherapy and introducing various range verification techniques in the clinic. An overview is given of today's technological evolution of proton therapy in clinical environments, and its impact on aspects of radiation protection.

A spectroscopic study of the chromatic properties of GafChromicEBT3 films.

PURPOSE: This work provides an interpretation of the chromatic properties of GafChromicEBT3 films based on the chemical nature of the polydiacetylene (PDA) molecules formed upon interaction with ionizing radiation. The EBT3 films become optically less transparent with increasing radiation dose as a result of the radiation-induced polymerization of diacetylene monomers. In contrast to empirical quantification of the chromatic properties, less attention has been given to the underlying molecular mechanism that induces the strong decrease in transparency. METHODS: Unlaminated GafChromicEBT3 films were irradiated with a 6 MV photon beam to dose levels up to 20 Gy. The optical absorption properties of the films were investigated using visible (vis) spectroscopy. The presence of PDA molecules in the active layer of the EBT3 films was investigated using Raman spectroscopy, which probes the vibrational modes of the molecules in the layer. The vibrational modes assigned to PDA's were used in a theoretical vis-absorption model to fit our experimental vis-absorption spectra. From the fit parameters, one can assess the relative contribution of different PDA conformations and the length distribution of PDA's in the film. RESULTS: Vis-spectroscopy shows that the optical density increases with dose in the full region of the visible spectrum. The Raman spectrum is dominated by two vibrational modes, most notably by the ν(C≡C) and the ν(C=C) stretching modes of the PDA backbone. By fitting the vis-absorption model to experimental spectra, it is found that the active layer contains two distinct PDA conformations with different absorption properties and reaction kinetics. Furthermore, the mean PDA conjugation length is found to be 2-3 orders of magnitude smaller than the crystals PDA's are embedded in. CONCLUSIONS: Vis- and Raman spectroscopy provided more insight into the molecular nature of the radiochromic properties of EBT3 films through the identification of the excited states of PDA and the presence of two PDA conformations. The improved knowledge on the molecular composition of EBT3's active layer provides a framework for future fundamental modeling of the dose-response.

SU-E-J-140: Initial Clinical Assessment of a Gimbaled Linac Tumor Tracking System in a Patient Simulation Study.

PURPOSE: A simulation study was conducted on patients to evaluate the workflow and quantify the performance of the BrainLab/MHI Vero dynamic tumor tracking system in clinical circumstances. METHODS: The gimbals hold the linac-MLC assembly which enables tracking of moving tumors. Two kV imaging systems are attached at ±45° from the 6MV beam allowing simultaneous X-rays. A simulation study was conducted on 5 lung-liver patients. The procedure involved quantification of tumor motion based on localization of Visicoil gold markers implanted in the tumor. Except for switching on the treatment beam, the entire tumor tracking workflow was executed involving patient positioning, synchronized acquisition of skin marker motion and X-ray images, fiducial marker detection, external-internal correlation model calculation, skin marker surrogate guided tracking and monitoring imaging. Tracking error was calculated from gimbals log-files and the acquired monitoring X-rays. Imaging dose was measured with TLD on phantoms and on the patients. RESULTS: Imaging for correlation model building resulted in 17.6mGy skin dose. Taking the treatment duration of a 3×20Gy lung SBRT treatment, depending on the treatment fields orientation an additional maximal exposure of 28.8mGy was estimated for acquiring 1Hz X-ray monitoring during tracking. A mean absolute tracking error of 1.1 mm was measured, with a 90% percentile of 2.1 mm. The average time to set up the patient entering the room to the first MV beam-on was 9min. From the acquisition of the modeling images sequence up to beam-on took 3min. CONCLUSIONS: A clinical version of the Vero tumor tracking system has been installed, including automatic detection of fiducial markers implanted in the tumor. An initial assessment has shown that the tracking system is functional and its performance adequate to move forward to final commissioning and initiation of patient treatments. This collaborative work was supported by the Flemish government through the Hercules foundation and the â€Å“Fonds voor Wetenschappelijk Onderzoek - Vlaanderen†grants G.0486.06 and G.0412.08, and corporate funding from BrainLab AG. There are no other conflicts of interest.

WE-G-213CD-03: A Dual Complementary Verification Method for Dynamic Tumor Tracking on Vero SBRT.

PURPOSE: to use complementary cine EPID and gimbals log file analysis for in-vivo tracking accuracy monitoring. METHODS: A clinical prototype of dynamic tracking (DT) was installed on the Vero SBRT system. This prototype version allowed tumor tracking by gimballed linac rotations using an internal-external correspondence model. The DT prototype software allowed the detailed logging of all applied gimbals rotations during tracking. The integration of an EPID on the vero system allowed the acquisition of cine EPID images during DT. We quantified the tracking error on cine EPID (E-EPID) by subtracting the target center (fiducial marker detection) and the field centroid. Dynamic gimbals log file information was combined with orthogonal x-ray verification images to calculate the in-vivo tracking error (E-kVLog). The correlation between E-kVLog and E-EPID was calculated for validation of the gimbals log file. Further, we investigated the sensitivity of the log file tracking error by introducing predefined systematic tracking errors. As an application we calculate gimbals log file tracking error for dynamic hidden target tests to investigate gravity effects and decoupled gimbals rotation from gantry rotation. Finally, calculating complementary cine EPID and log file tracking errors evaluated the clinical accuracy of dynamic tracking. RESULTS: A strong correlation was found between log file and cine EPID tracking error distribution during concurrent measurements (R=0.98). We found sensitivity in the gimbals log files to detect a systematic tracking error up to 0.5 mm. Dynamic hidden target tests showed no gravity influence on tracking performance and high degree of decoupled gimbals and gantry rotation during dynamic arc dynamic tracking. A submillimetric agreement between clinical complementary tracking error measurements was found. CONCLUSIONS: Redundancy of the internal gimbals log file with x-ray verification images with complementary independent cine EPID images was implemented to monitor the accuracy of gimballed tumor tracking on Vero SBRT. Research was financially supported by the Flemish government (FWO), Hercules Foundation and BrainLAB AG.

Gating and tracking, 4D in thoracic tumours.

The limited ability to control for a tumour's location compromises the accuracy with which radiation can be delivered to tumour-bearing tissue. The resultant requirement for larger treatment volumes to accommodate target uncertainty restricts the radiation dose because more surrounding normal tissue is exposed. With image-guided radiation therapy (IGRT), these volumes can be optimized and tumouricidal doses may be delivered, achieving maximum tumour control with minimal complications. Moreover, with the ability of high precision dose delivery and real-time knowledge of the target volume location, IGRT has initiated the exploration of new indications in radiotherapy such as hypofractionated radiotherapy (or stereotactic body radiotherapy), deliberate inhomogeneous dose distributions coping with tumour heterogeneity (dose painting by numbers and biologically conformal radiation therapy), and adaptive radiotherapy. In short: "individualized radiotherapy". Tumour motion management, especially for thoracic tumours, is a particular problem in this context both for the delineation of tumours and organs at risk as well as during the actual treatment delivery. The latter will be covered in this paper with some examples based on the experience of the UZ Brussel. With the introduction of the NOVALIS system (BrainLAB, Feldkirchen, Germany) in 2000 and consecutive prototypes of the ExacTrac IGRT system, gradually a hypofractionation treatment protocol was introduced for the treatment of lung tumours and liver metastases evolving from motion-encompassing techniques towards respiratory-gated radiation therapy with audio-visual feedback and most recently dynamic tracking using the VERO system (BrainLAB, Feldkirchen, Germany). This evolution will be used to illustrate the recent developments in this particular field of research.

Comparison of build-up dose between Elekta and Varian linear accelerators for high-energy photon beams using radiochromic film and clinical implications for IMRT head and neck treatments.

Skin toxicity has been reported for IMRT of head and neck cancer. The purpose of this study was to investigate the dose in the build-up region delivered by a 6 MV treatment plan for which important skin toxicity was observed. We also investigated if the different designs of the treatment head of an Elekta and a Varian linear accelerator, especially the lower position of the Varian multi-leaf collimator, give rise to different build-up doses. For regular square open beams, the build-up dose along the central beam axis is higher for the Varian machine than for the Elekta machine, both for 6 MV and 18 MV. At the Elekta machine at 18 MV, the superficial dose of a diamond shaped 10 x 10 cm2 field is 3.6% lower than the superficial dose of a regular 10 x 10 cm2 field. This effect is not seen at 6 MV. At the Varian machine, the superficial dose of the diamond shaped field is respectively 3.5 and 14.2% higher than the superficial dose of the regular 10 x 10 cm2 field for 6 MV and 18 MV. Despite the differences measured in build-up dose for single beams between the Elekta and the Varian linear accelerator, there were no measurable differences in superficial dose when a typical IMRT dose plan of 6 MV for a head and neck tumour is executed at the two machines.