Gerard Bengua

Radiation Pneumonitis After Hypofractionated Radiotherapy: Evaluation of the LQ(L) Model and Different Dose Parameters

PURPOSEnTo evaluate the linear quadratic (LQ) model for hypofractionated radiotherapy within the context of predicting radiation pneumonitis (RP) and to investigate the effect if a linear (L) model in the high region (LQL model) is used.nnnMETHODS AND MATERIALSnThe radiation doses used for 128 patients treated with hypofractionated radiotherapy were converted to the equivalent doses given in fractions of 2 Gy for a range of alpha/beta ratios (1 Gy to infinity) according to the LQ(L) model. For the LQL model, different cut-off values between the LQ model and the linear component were used. The Lyman model parameters were fitted to the events of RP grade 2 or higher to derive the normal tissue complication probability (NTCP). The lung dose was calculated as the mean lung dose and the percentage of lung volume (V) receiving doses higher than a threshold dose of xGy (V(x)).nnnRESULTSnThe best NTCP fit was found if the mean lung dose, or V(x), was calculated with an alpha/beta ratio of 3 Gy. The NTCP fit of other alpha/beta ratios and the LQL model were worse but within the 95% confidence interval of the NTCP fit of the LQ model with an alpha/beta ratio of 3 Gy. The V(50) NTCP fit was better than the NTCP fit of lower threshold doses.nnnCONCLUSIONSnFor high fraction doses, the LQ model with an alpha/beta ratio of 3 Gy was the best method for converting the physical lung dose to predict RP.

Evaluation of the Effectiveness of the Stereotactic Body Frame in Reducing Respiratory Intrafractional Organ Motion Using the Real-Time Tumor-Tracking Radiotherapy System

PURPOSEnTo evaluate the effectiveness of the stereotactic body frame (SBF), with or without a diaphragm press or a breathing cycle monitoring device (Abches), in controlling the range of lung tumor motion, by tracking the real-time position of fiducial markers.nnnMETHODS AND MATERIALSnThe trajectories of gold markers in the lung were tracked with the real-time tumor-tracking radiotherapy system. The SBF was used for patient immobilization and the diaphragm press and Abches were used to actively control breathing and for self-controlled respiration, respectively. Tracking was performed in five setups, with and without immobilization and respiration control. The results were evaluated using the effective range, which was defined as the range that includes 95% of all the recorded marker positions in each setup.nnnRESULTSnThe SBF, with or without a diaphragm press or Abches, did not yield effective ranges of marker motion which were significantly different from setups that did not use these materials. The differences in the effective marker ranges in the upper lobes for all the patient setups were less than 1mm. Larger effective ranges were obtained for the markers in the middle or lower lobes.nnnCONCLUSIONnThe effectiveness of controlling respiratory-induced organ motion by using the SBF+diaphragm press or SBF + Abches patient setups were highly dependent on the individual patient reaction to the use of these materials and the location of the markers. They may be considered for lung tumors in the lower lobes, but are not necessary for tumors in the upper lobes.

A feasibility study of novel plastic scintillation dosimetry with pulse-counting mode

The purpose of this study was to develop a novel scintillation dosimeter for in vivo dosimetry in Ir-192 brachytherapy via the pulse-counting mode. The new dosimeter was made from a plastic scintillator shaped into a hemisphere of diameter 1 mm and connected to the tip of a plastic optical fiber. The relationship between pulse counts and absorbed dose was derived based on the assumption that scintillation photons from the incident gamma ray are proportional to the absorbed dose. An equation for the conversion of pulse counts to water-equivalent dose was deduced wherein the pulse height spectrum from scintillation photons was assumed to be exponential. To confirm its accuracy, the dose rate distribution in a water phantom was measured by the present dosimeter and this was compared with Monte Carlo simulations, resulting in a discrepancy of less than 1.97%. It was found that the dosimeter has a wide dynamic range of linearity up to an order of magnitude of almost 10(3), including corrections for loss of counts due to pile-up.

Optimization of fluoroscopy parameters using pattern matching prediction in the real-time tumor-tracking radiotherapy system

In the real-time tumor-tracking radiotherapy system, fluoroscopy is used to determine the real-time position of internal fiducial markers. The pattern recognition score (PRS) ranging from 0 to 100 is computed by a template pattern matching technique in order to determine the marker position on the fluoroscopic image. The PRS depends on the quality of the fluoroscopic image. However, the fluoroscopy parameters such as tube voltage, current and exposure duration are selected manually and empirically in the clinical situation. This may result in an unnecessary imaging dose from the fluoroscopy or loss of the marker because of too much or insufficient x-ray exposure. In this study, a novel optimization method is proposed in order to minimize the fluoroscopic dose while keeping the image quality usable for marker tracking. The PRS can be predicted in a region where the marker appears to move in the fluoroscopic image by the proposed method. The predicted PRS can be utilized to judge whether the marker can be tracked with accuracy. In this paper, experiments were performed to show the feasibility of the PRS prediction method under various conditions. The predicted PRS showed good agreement with the measured PRS. The root mean square error between the predicted PRS and the measured PRS was within 1.44. An experiment using a motion controller and an anthropomorphic chest phantom was also performed in order to imitate a clinical fluoroscopy situation. The result shows that the proposed prediction method is expected to be applicable in a real clinical situation.

A New Brain Positron Emission Tomography Scanner With Semiconductor Detectors for Target Volume Delineation and Radiotherapy Treatment Planning in Patients With Nasopharyngeal Carcinoma

PURPOSEnWe compared two treatment planning methods for stereotactic boost for treating nasopharyngeal carcinoma (NPC): the use of conventional whole-body bismuth germanate (BGO) scintillator positron emission tomography (PET(CONV)WB) versus the new brain (BR) PET system using semiconductor detectors (PET(NEW)BR).nnnMETHODS AND MATERIALSnTwelve patients with NPC were enrolled in this study. [(18)F]Fluorodeoxyglucose-PET images were acquired using both the PET(NEW)BR and the PET(CONV)WB system on the same day. Computed tomography (CT) and two PET data sets were transferred to a treatment planning system, and the PET(CONV)WB and PET(NEW)BR images were coregistered with the same set of CT images. Window width and level values for all PET images were fixed at 3000 and 300, respectively. The gross tumor volume (GTV) was visually delineated on PET images by using either PET(CONV)WB (GTV(CONV)) images or PET(NEW)BR (GTV(NEW)) images. Assuming a stereotactic radiotherapy boost of 7 ports, the prescribed dose delivered to 95% of the planning target volume (PTV) was set to 2000 cGy in 4 fractions.nnnRESULTSnThe average absolute volume (±standard deviation [SD]) of GTV(NEW) was 15.7 ml (±9.9) ml, and that of GTV(CONV) was 34.0 (±20.5) ml. The average GTV(NEW) was significantly smaller than that of GTV(CONV) (p = 0.0006). There was no statistically significant difference between the maximum dose (p = 0.0585) and the mean dose (p = 0.2748) of PTV. The radiotherapy treatment plan based on the new gross tumor volume (PLAN(NEW)) significantly reduced maximum doses to the cerebrum and cerebellum (p = 0.0418) and to brain stem (p = 0.0041).nnnCONCLUSIONnResults of the present study suggest that the new brain PET system using semiconductor detectors can provide more accurate tumor delineation than the conventional whole-body BGO PET system and may be an important tool for functional and molecular radiotherapy treatment planning.

A feasibility study of a molecular-based patient setup verification method using a parallel-plane PET system

A feasibility study of a novel PET-based molecular image guided radiation therapy (m-IGRT) system was conducted by comparing PET-based digitally reconstructed planar image (PDRI) registration with radiographic registration. We selected a pair of opposing parallel-plane PET systems for the practical implementation of this system. Planar images along the in-plane and cross-plane directions were reconstructed from the parallel-plane PET data. The in-plane and cross-plane FWHM of the profile of 2 mm diameter sources was approximately 1.8 and 8.1 mm, respectively. Therefore, only the reconstructed in-plane image from the parallel-plane PET data was used in the PDRI registration. In the image registration, five different sizes of (18)F cylindrical sources (diameter: 8, 12, 16, 24, 32 mm) were used to determine setup errors. The data acquisition times were 1, 3 and 5 min. Image registration was performed by five observers to determine the setup errors from PDRI registration and radiographic registration. The majority of the mean registration errors obtained from the PDRI registration were not significantly different from those obtained from the radiographic registration. Acquisition time did not appear to result in significant differences in the mean registration error. The mean registration error for the PDRI registration was found to be 0.93 ± 0.33 mm. This is not statistically different from the radiographic registration which had a mean registration error of 0.92 ± 0.27 mm. Our results suggest that m-IGRT image registration using PET-based reconstructed planar images along the in-plane direction is feasible for clinical use if PDRI registration is performed at two orthogonal gantry angles.

Accurate Analysis of the Change in Volume, Location, and Shape of Metastatic Cervical Lymph Nodes During Radiotherapy

PURPOSEnTo establish a method for the accurate acquisition and analysis of the variations in tumor volume, location, and three-dimensional (3D) shape of tumors during radiotherapy in the era of image-guided radiotherapy.nnnMETHODS AND MATERIALSnFinite element models of lymph nodes were developed based on computed tomography (CT) images taken before the start of treatment and every week during the treatment period. A surface geometry map with a volumetric scale was adopted and used for the analysis. Six metastatic cervical lymph nodes, 3.5 to 55.1 cm(3) before treatment, in 6 patients with head and neck carcinomas were analyzed in this study. Three fiducial markers implanted in mouthpieces were used for the fusion of CT images. Changes in the location of the lymph nodes were measured on the basis of these fiducial markers.nnnRESULTSnThe surface geometry maps showed convex regions in red and concave regions in blue to ensure that the characteristics of the 3D tumor geometries are simply understood visually. After the irradiation of 66 to 70 Gy in 2 Gy daily doses, the patterns of the colors had not changed significantly, and the maps before and during treatment were strongly correlated (average correlation coefficient was 0.808), suggesting that the tumors shrank uniformly, maintaining the original characteristics of the shapes in all 6 patients. The movement of the gravitational center of the lymph nodes during the treatment period was everywhere less than ±5 mm except in 1 patient, in whom the change reached nearly 10 mm.nnnCONCLUSIONSnThe surface geometry map was useful for an accurate evaluation of the changes in volume and 3D shapes of metastatic lymph nodes. The fusion of the initial and follow-up CT images based on fiducial markers enabled an analysis of changes in the location of the targets. Metastatic cervical lymph nodes in patients were suggested to decrease in size without significant changes in the 3D shape during radiotherapy. The movements of the gravitational center of the lymph nodes were almost all less than ±5 mm.

Detection of patient setup errors with a portal image – DRR registration software application

The purpose of this study was to evaluate a custom portal image — digitally reconstructed radiograph (DRR) registration software application. The software works by transforming the portal image into the coordinate space of the DRR image using three control points placed on each image by the user, and displaying the fused image. In order to test statistically that the software actually improves setup error estimation, an intra‐ and interobserver phantom study was performed. Portal images of anthropomorphic thoracic and pelvis phantoms with virtually placed irradiation fields at known setup errors were prepared. A group of five doctors was first asked to estimate the setup errors by examining the portal and DRR image side‐by‐side, not using the software. A second group of four technicians then estimated the same set of images using the registration software. These two groups of human subjects were then compared with an auto‐registration feature of the software, which is based on the mutual information between the portal and DRR images. For the thoracic case, the average distance between the actual setup error and the estimated error was 4.3±3.0u2009mm for doctors using the side‐by‐side method, 2.1±2.4u2009mm for technicians using the registration method, and 0.8±0.4u2009mm for the automatic algorithm. For the pelvis case, the average distance between the actual setup error and estimated error was 2.0±0.5u2009mm for the doctors using the side‐by‐side method, 2.5±0.4u2009mm for technicians using the registration method, and 2.0±1.0u2009mm for the automatic algorithm. The ability of humans to estimate offset values improved statistically using our software for the chest phantom that we tested. Setup error estimation was further improved using our automatic error estimation algorithm. Estimations were not statistically different for the pelvis case. Consistency improved using the software for both the chest and pelvis phantoms. We also tested the automatic algorithm with a database of over 5,000 clinical cases from our hospital. The algorithm performed well for head and breast but performed poorly for pelvis cases, probably due to lack of contrast in the megavoltage portal image. The software incorporates an original algorithm to fuse portal and DRR images, which we describe in detail. The offset optimization algorithm used in the automatic mode of operation is also unique, and may be useful if the contrast of the portal images can be improved. PACS numbers: 87.55.Qr, 87.57.nj

Development of Liquid Lithium Target of 7Li(p,n)7Be Reactions for Accelerator Based BNCT Irradiation System

A feasibility study of using a liquid lithium target was performed. The candidate configurations for the neutron generation target was adapted for the 7Li(p,n)7Be assuming a proton beam which is 30 mm in diameter and with an energy and current of up to 3 MeV and 20 mA, respectively. The flowing film of liquid lithium was 0.5 mm-thick with 50 mm width and 50 mm length. The shapes of the nozzle and concave back wall, which creates a stable flowing film jet, were decided based on water experiments. The flowing film on the concave wall is a windowless type target. A lithium hydrodynamic experiment was carried out to observe the liquid lithium flow behavior and to investigate the stability of the lithium target. The flowing film of liquid lithium was found to be feasible at temperatures below the liquid lithium boiling saturation of 342 degrees Celsius at the surface pressure of 1x10− 3 Pa. Using a test liquid lithium circulating loop, it was found that a stable film flow on a concave back wall can be expected at a velocity of up to 30 m/sec. At present, prototype lithium circulating loop for practical BNCT irradiation system was built and is under examination.