Geneviève Gaboriaud

Pattern of relapse and outcome of non-metastatic germinoma patients treated with chemotherapy and limited field radiation: the SFOP experience

Over the last two decades, chemotherapy has been introduced in protocols for patients with intracranial germinoma with the objective of reducing the volume and the dose of irradiation without compromising survival rates. The aim of this work is to critically analyze the pattern of relapse in a cohort of patients with nonmetastatic germinoma prospectively treated with chemotherapy followed by focal field radiation. Data of all germinoma patients registered in the French protocol for intracranial germ cell tumors between 1990 and 1999 were reviewed. The pattern of relapse, management, and outcome were analyzed in 10 of 60 patients who developed a recurrence after initial treatment. In 9 patients, the site of recurrence was local or loco-regional, notably in the periventricular area for 8. One patient only had isolated distant leptomeningeal relapse. The review of the sites of relapse suggests that most recurrences could have been avoided with a larger ventricular field of radiation. Treatment at first relapse included chemotherapy (10 patients), high-dose chemotherapy and stem cell transplant (8 patients), and/or radiation therapy (4 patients). Five patients experienced a second relapse. At a median follow-up of 72 months since the first relapse, 8 patients are alive in second or third remission. This review identified an excess of periventricular relapses when the focal field of radiation is used in the combined management of germinoma. These relapses are predominantly marginal or outside radiation fields. Ventricular field radiation appears a logical alternative to decrease the incidence of such relapses. Future trials should aim at better identifying patients who may benefit from local and ventricular radiation, respectively.

Experimental determination and verification of the parameters used in a proton pencil beam algorithm

We present an experimental procedure for the determination and the verification under practical conditions of physical and computational parameters used in our proton pencil beam algorithm. The calculation of the dose delivered by a single pencil beam relies on a measured spread-out Bragg peak, and the description of its radial spread at depth features simple specific parameters accounting individually for the influence of the beam line as a whole, the beam energy modulation, the compensator, and the patient medium. For determining the experimental values of the physical parameters related to proton scattering, we utilized a simple relation between Gaussian radial spreads and the width of lateral penumbras. The contribution from the beam line has been extracted from lateral penumbra measurements in air: a linear variation with the distance collimator-point has been observed. Analytically predicted radial spreads within the patient were in good agreement with experimental values in water under various reference conditions. Results indicated no significant influence of the beam energy modulation. Using measurements in presence of Plexiglas slabs, a simple assumption on the effective source of scattering due to the compensator has been stated, leading to accurate radial spread calculations. Dose measurements in presence of complexly shaped compensators have been used to assess the performances of the algorithm supplied with the adequate physical parameters. One of these compensators has also been used, together with a reference configuration, for investigating a set of computational parameters decreasing the calculation time while maintaining a high level of accuracy. Faster dose computations have been performed for algorithm evaluation in the presence of geometrical and patient compensators, and have shown good agreement with the measured dose distributions.

Assessment of organ absorbed doses and estimation of effective doses from pediatric anthropomorphic phantom measurements for multi-detector row CT with and without automatic exposure control.

This study was designed to measure organ absorbed doses from multi-detector row computed tomography (MDCT) on pediatric anthropomorphic phantoms, calculate the corresponding effective doses, and assess the influence of automatic exposure control (AEC) in terms of organ dose variations. Four anthropomorphic phantoms (phantoms represent the equivalent of a newborn, 1-, 5-, and 10-y-old child) were scanned with a four-channel MDCT coupled with a z-axis-based AEC system. Two CT torso protocols were compared: a first protocol without AEC and constant tube current-time product and a second protocol with AEC using age-adjusted noise indices. Organ absorbed doses were monitored by thermoluminescent dosimeters (LiF: Mg, Cu, P). Effective doses were calculated according to the tissue weighting factors of the International Commission on Radiological Protection (ICRP Publication 103). For fixed mA acquisitions, organ doses normalized to the volume CT dose index in a 16-cm head phantom (CTDIvol16) ranged from 0.6 to 1.5 and effective doses ranged from 8.4 to 13.5 mSv. For the newborn-equivalent phantom, the AEC-modulated scan showed almost no significant dose variation compared to the fixed mA scan. For the 1-, 5- and 10-y equivalent phantoms, the use of AEC induced a significant dose decrease on chest organs (ranging from 61 to 31% for thyroid, 37 to 21% for lung, 34 to 17% for esophagus, and 39 to 10% for breast). However, AEC also induced a significant dose increase (ranging from 28 to 48% for salivary glands, 22 to 51% for bladder, and 24 to 70% for ovaries) related to the high density of skull base and pelvic bones. These dose increases should be considered before using AEC as a dose optimization tool in children.

Automatic exposure control in multichannel CT with tube current modulation to achieve a constant level of image noise: Experimental assessment on pediatric phantoms

Automatic exposure control (AEC) systems have been developed by computed tomography (CT) manufacturers to improve the consistency of image quality among patients and to control the absorbed dose. Since a multichannel helical CT scan may easily increase individual radiation doses, this technical improvement is of special interest in children who are particularly sensitive to ionizing radiation, but little information is currently available regarding the precise performance of these systems on small patients. Our objective was to assess an AEC system on pediatric dose phantoms by studying the impact of phantom transmission and acquisition parameters on tube current modulation, on the resulting absorbed dose and on image quality. We used a four-channel CT scan working with a patient-size and z-axis-based AEC system designed to achieve a constant noise within the reconstructed images by automatically adjusting the tube current during acquisition. The study was performed with six cylindrical poly(methylmethacrylate) (PMMA) phantoms of variable diameters (10-32 cm) and one 5 years of age equivalent pediatric anthropomorphic phantom. After a single scan projection radiograph (SPR), helical acquisitions were performed and images were reconstructed with a standard convolution kernel. Tube current modulation was studied with variable SPR settings (tube angle, mA, kVp) and helical parameters (6-20 HU noise indices, 80-140 kVp tube potential, 0.8-4 s. tube rotation time, 5-20 mm x-ray beam thickness, 0.75-1.5 pitch, 1.25-10 mm image thickness, variable acquisition, and reconstruction fields of view). CT dose indices (CTDIvol) were measured, and the image quality criterion used was the standard deviation of the CT number measured in reconstructed images of PMMA material. Observed tube current levels were compared to the expected values from Brooks and Di Chiros [R.A. Brooks and G.D. Chiro, Med. Phys. 3, 237-240 (1976)] model and calculated values (product of a reference value multiplied by a dose ratio measured with thermoluminescent dosimeters). Our study demonstrates that this AEC system accurately modulates the tube current according to phantom size and transmission to achieve a stable image noise. The system accurately controls the tube current when changing tube rotation time, tube potential, or image thickness, with minimal variations of the resulting noise. Nevertheless, CT users should be aware of possible changes of tube current and resulting dose and quality according to several parameters: the tube angle and tube potential used for SPR, the x-ray beam thickness (tube current decreases and image noise increases when doubling x-ray beam thickness), the pitch value (a pitch decrease leads to a higher dose but also to a higher noise), and the acquisition field of view (FOV) (tube current is lower when using the small acquisition FOV compared to the large one, but the use of small acquisition FOV at 120 kVp leads to a peculiar increase of tube current and CTDIvol).

Dose calculation and verification of intensity modulation generated by dynamic multileaf collimators.

While the development of inverse planning tools for optimizing dose distributions has come to a level of maturity, intensity modulation has not yet been widely implemented in clinical use because of problems related to its practical delivery and a lack of verification tools and quality assurance (QA) procedures. One of the prerequisites is a dose calculation algorithm that achieves good accuracy. The purpose of this work was twofold. A primary-scatter separation dose model has been extended to account for intensity modulation generated by a dynamic multileaf collimator (MLC). Then the calculation procedures have been tested by comparison with carefully carried out experiments. Intensity modulation is being accounted for by means of a 2D (two-dimensional) matrix of correction factors that modifies the spatial fluence distribution, incident to the patient. The dose calculation for the corresponding open field is then affected by those correction factors. They are used in order to weight separately the primary and the scatter component of the dose at a given point. In order to verify that the calculated dose distributions are in good agreement with measurements on our machine, we have designed a set of test intensity distributions and performed measurements with 6 and 20 MV photons on a Varian Clinac 2300C/D linear accelerator equipped with a 40 leaf pair dynamic MLC. Comparison between calculated and measured dose distributions for a number of representative cases shows, in general, good agreement (within 3% of the normalization in low dose gradient regions and within 3 mm distance-to-dose in high dose gradient regions). For absolute dose calculations (monitor unit calculations), comparison between calculation and measurement reveals good agreement (within 2%) for all tested cases (with the condition that the prescription point is not located on a high dose gradient region).

Performance optimization of the Varian aS500 EPID system.

Today, electronic portal imaging devices (EPIDs) are widely used as a replacement to portal films for patient position verification, but the image quality is not always optimal. The general aim of this study was to optimize the acquisition parameters of an amorphous silicon EPID commercially available for clinical use in radiation therapy with the view to avoid saturation of the system. Special attention was paid to selection of the parameter corresponding to the number of rows acquired between accelerator pulses (NRP) for various beam energies and dose rates. The image acquisition system (IAS2) has been studied, and portal image acquisition was found to be strongly dependent on the accelerator pulse frequency. This frequency is set for each “energy — dose rate” combination of the linear accelerator. For all combinations, the image acquisition parameters were systematically changed to determine their influence on the performances of the Varian aS500 EPID system. New parameters such as the maximum number of rows (MNR) and the number of pulses per frame (NPF) were introduced to explain portal image acquisition theory. Theoretical and experimental values of MNR and NPF were compared, and they were in good agreement. Other results showed that NRP had a major influence on detector saturation and dose per image. A rule of thumb was established to determine the optimum NRP value to be used. This practical application was illustrated by a clinical example in which the saturation of the aSi EPID was avoided by NRP optimization. Moreover, an additional study showed that image quality was relatively insensitive to this parameter. PACS numbers: 87.53.Oq; 87.59.Jq

Quantitative magnetic resonance and isotopic imaging: Early evaluation of radiation injury to the brain

PURPOSE Using magnetic resonance (MR) and isotopic imaging to investigate the cerebral alterations after highdose single-fraction irradiation on a pig model. We assessed the nuclear magnetic resonance (NMR) relaxation times as early markers of radiation injury to the healthy brain. METHODS AND MATERIALS A total of 17 animals was studied; 15 irradiated and 2 unirradiated controls. Pigs were irradiated with a 12 MeV electron beam at a rate of 2 Gy/min. Ten animals received 40 Gy at the 90% isodose, five animals received 60 Gy, and two animals were unirradiated. The follow-up intervals ranged from 2 days to 6 months. T1-weighted scans, T2-weighted scans, and scintigrams were performed on all animals to study neurological abnormalities, cerebral blood flow, and blood-brain barrier (BBB) integrity. T1 and T2 relaxation times were measured in selected regions of interest (ROIs) within the irradiated and contralateral hemispheres. A ratio T1 after irradiation/T1 before irradiation, and a ratio T2 after irradiation/T2 before irradiation, were calculated, pooled for each dose group, and followed as a function of time after irradiation. RESULTS Scintigraphy visualized the brain perfusion defect and BBB disruption in all irradiated brains. The ratio T2 after irradiation/T2 before irradiation was proportional to the effective dose received. The T2 ratio kinetics could be analyzed in three phases:an immediate and transient phase, two long-lasting phases, which preceded compression of the irradiated lateral ventricle, and edema and necrosis at later stages of radiation injury, respectively. The magnetic resonance imaging (MRI) observations correlated well with histological analysis. CONCLUSION The results show that quantitative imaging is a sensitive in vivo method for early detection of cerebral radiation injury. The reliability and dose dependence of T2 relaxation time may offer new opportunities to detect and understand brain pathophysiology after high-dose single-fraction irradiation.

Parameter adjustment of a proton pencil beam algorithm using experimental data from complex compensators

We report on an experimental procedure for the adjustment of various parameters used in a proton pencil beam algorithm recently developed by our group. The procedure is aimed at determining the most adequate modeling of the influence from compensator and from the beam line elements in order to improve the dose computation accuracy.

Integration of the KonRad inverse planning tool into the iSis3D treatment planning system

The purpose of this work was to integrate into a modern 3D treatment planning system the software tools that allow to implement radiotherapy with intensity-modulated beams (IMRT), based on the inverse method of treatment design and using a multileaf collimation system operating in the dynamic mode.

Processing Techniques to Improve Microwave Images for Biomedical Applications

This paper deals with the improvement of the quality of images obtained at 2.45 GHz, by means of a planar microwave camera for biomedical applications. The results provided by standard spectral techniques, classically used for diffraction tomography reconstructions, are shown to be significantly improved by different processing techniques. The particular case of human hand imaging is presented for its clinical interest in the early diagnostic of fibrosis after accidental irradiations.