Ehab M. Attalla

Megavoltage cone beam computed tomography: Commissioning and evaluation of patient dose.

The improvement in conformal radiotherapy techniques enables us to achieve steep dose gradients around the target which allows the delivery of higher doses to a tumor volume while maintaining the sparing of surrounding normal tissue. One of the reasons for this improvement was the implementation of intensity-modulated radio therapy (IMRT) by using linear accelerators fitted with multi-leaf collimator (MLC), Tomo therapy and Rapid arc. In this situation, verification of patient set-up and evaluation of internal organ motion just prior to radiation delivery become important. To this end, several volumetric image-guided techniques have been developed for patient localization, such as Siemens OPTIVUE/MVCB and MVision megavoltage cone beam CT (MV-CBCT) system. Quality assurance for MV-CBCT is important to insure that the performance of the Electronic portal image device (EPID) and MV-CBCT is suitable for the required treatment accuracy. In this work, the commissioning and clinical implementation of the OPTIVUE/MVCB system was presented. The geometry and gain calibration procedures for the system were described. The image quality characteristics of the OPTIVUE/MVCB system were measured and assessed qualitatively and quantitatively, including the image noise and uniformity, low-contrast resolution, and spatial resolution. The image reconstruction and registration software were evaluated. Dose at isocenter from CBCT and the EPID were evaluated using ionization chamber and thermo-luminescent dosimeters; then compared with that calculated by the treatment planning system (TPS- XiO 4.4). The results showed that there are no offsets greater than 1 mm in the flat panel alignment in the lateral and longitudinal direction over 18 months of the study. The image quality tests showed that the image noise and uniformity were within the acceptable range, and that a 2 cm large object with 1% electron density contrast can be detected with the OPTIVUE/MVCB system with 5 monitor units (MU) protocol. The registration software was accurate within 2 mm in the anterior-posterior, left-right, and superior-inferior directions. The additional dose to the patient from MV-CBCT study set with 5 MU at the isocenter of the treatment plan was 5 cGy. For Electronic portal image device (EPID) verification using two orthogonal images with 2 MU per image the additional dose to the patient was 3.8 cGy. These measured dose values were matched with that calculated by the TPS-XiO, where the calculated doses were 5.2 cGy and 3.9 cGy for MVCT and EPID respectively.

Comparison of Electronic Portal Imaging and Cone Beam Computed Tomography for Position Verification in Children

AIM To compare the accuracy of radiotherapy set-up using an electronic portal imaging device (EPID) versus megavoltage cone beam computed tomography (MV-CBCT) in paediatric patients. MATERIALS AND METHODS In total, 204 pairs of EPID and MV-CBCT were carried out for 72 patients in the first 3 treatment days and weekly thereafter. RESULTS For the whole group, the mean systematic EPID set-up errors were 1.8 (±1.7), 1.6 (±1.3), 1.4 (±1.5) mm and 2.3 (±1.7), 1.6 (±1.3), 2.4 (±1.6) mm for MV-CBCT in the longitudinal, lateral and vertical directions, respectively, whereas the mean EPID random errors were 2.0 (±1.7), 1.4 (±1.5), 1.2 (±1.6) and 1.9 (±1.5), 1.5 (±1.3), 2.1 (±1.7) mm for MV-CBCT in the longitudinal, lateral and vertical directions, respectively. For systematic errors of head and neck patients, there was a statistically significant difference in the lateral and vertical directions (P=0.027, 0.003), whereas in the non-head and neck patients there was a statistically significant difference in the lateral direction only (P=0.031). In head and neck patients, the mean random errors were significantly different in the vertical and lateral directions, whereas in non-head and neck patients, they were significantly different in the vertical direction only. The larger values alternate between the two modalities. The systematic and random errors (detected by EPID and MV-CBCT) were significantly correlated in almost all direction in all tumour sites. CONCLUSIONS The comparison between set-up error in EPID and MV-CBCT was not in favour of any of the two modalities. However, the two modalities were strongly correlated but fairly agreed and the differences between the shifts reported were small and hardly influenced the recommended planning target volume margin.

Geometrical uncertainty margins in 3D conformal radiotherapy in the pediatric age group

PURPOSE To evaluate set-up variation of pediatric patients undergoing 3D conformal radiotherapy (3DCRT) using electronic portal image device (EPID), in an effort to evaluate the adequacy of the planning target volume (PTV) margin employed for the 3DCRT treatment of pediatric patients. MATERIALS AND METHODS Set-up data was collected from 48 pediatric patients treated with 3DCRTfor head and neck (31 patients), abdomino-pelvic (9 patients) and chest (8 patients) sites during the period between September 2008 and February 2009. A total of 358 images obtained by EPID were analyzed. The mean (M) and standard deviation (SD) for systematic and random errors were calculated and the results were analyzed. RESULTS All images were studied in anterior and lateral portals. The systematic errors along longitudinal, lateral and vertical directions in all patients showed an M equal to 1.9, 1.6, and 1.6mm with SD of 1.8, 1.4, and 1.8mm, respectively; (head and neck cases: M equal to 1.5, 1.2, and 1.6mm with SD 1.4, 1.2, and 1.8mm; chest cases: M equal to 2.5, 1.8, and 0.8mm with SD 2.7, 1.7, and 1.2mm, abdomino-pelvic cases: M equal to 2.9, 2.8 and 2.3mm with SD 1.6, 1.2, and 2.3mm). Similarly, the random errors for all patients showed SD of 1.9, 1.6, and 1.8mm, respectively (head and neck cases: SD 1.7, 1.3, and 1.5mm; chest cases: SD 1.2, 1.9, and 2.5mm; abdomino-pelvic cases SD 2.5, 2, and 2.4mm, respectively). Using Van Herks formula the suggested (PTV) margin around the clinical target volume (CTV) of 5.5mm appears to be adequate. CONCLUSION The ranges of set-up errors are site specific and depends on many factors.

Comparison of dosimetric characteristics of Siemens virtual and physical wedges for ONCOR linear accelerator

Dosimetric properties of virtual wedge (VW) and physical wedge (PW) in 6- and 10-MV photon beams from a Siemens ONCOR linear accelerator, including wedge factors, depth doses, dose profiles, peripheral doses, are compared. While there is a great difference in absolute values of wedge factors, VW factors (VWFs) and PW factors (PWFs) have a similar trend as a function of field size. PWFs have stronger depth dependence than VWF due to beam hardening in PW fields. VW dose profiles in the wedge direction, in general, match very well with those of PW, except in the toe area of large wedge angles with large field sizes. Dose profiles in the nonwedge direction show a significant reduction in PW fields due to off-axis beam softening and oblique filtration. PW fields have significantly higher peripheral doses than open and VW fields. VW fields have similar surface doses as the open fields, while PW fields have lower surface doses. Surface doses for both VW and PW increase with field size and slightly with wedge angle. For VW fields with wedge angles 45° and less, the initial gap up to 3 cm is dosimetrically acceptable when compared to dose profiles of PW. VW fields in general use less monitor units than PW fields.

A comparison of three commercial IMRT treatment planning systems for selected pediatric cases

This work aimed at evaluating the performance of three different intensity‐modulated radiotherapy (IMRT) treatment planning systems (TPSs) — KonRad, XiO and Prowess — for selected pediatric cases. For this study, 11 pediatric patients with different types of brain, orbit, head and neck cancer were selected. Clinical step‐and‐shoot IMRT treatment plans were designed for delivery on a Siemens ONCOR accelerator with 82‐leaf multileaf collimators (MLCs). Plans were optimized to achieve the same clinical objectives by applying the same beam energy and the same number and direction of beams. The analysis of performance was based on isodose distributions, dose‐volume histograms (DVHs) for planning target volume (PTV), the relevant organs at risk (OARs), as well as mean dose (Dmean), maximum dose (Dmax), 95% dose (D95), volume of patient receiving 2 and 5 Gy, total number of segments, monitor units per segment (MU/Segment), and the number of MU/cGy. Treatment delivery time and conformation number were two other evaluation parameters that were considered in this study. Collectively, the Prowess and KonRad plans showed a significant reduction in the number of MUs that varied between 1.8% and 61.5% (p−value=0.001) for the different cases, compared to XiO. This was reflected in shorter treatment delivery times. The percentage volumes of each patient receiving 2 Gy and 5 Gy were compared for the three TPSs. The general trend was that KonRad had the highest percentage volume, Prowess showed the lowest (p−value=0.0001). The KonRad achieved better conformality than both of XiO and Prowess. Based on the present results, the three treatment planning systems were efficient in IMRT, yet XiO showed the lowest performance. The three TPSs achieved the treatment goals according to the internationally approved standards.

Dosimetry and fast neutron energies characterization of photoneutrons produced in some medical linear accelerators

This work focusses on the estimation of induced photoneutrons energy, fluence, and strength using nuclear track detector (NTD) (CR-39). Photoneutron energy was estimated for three different linear accelerators, LINACs as an example for the commonly used accelerators. For high-energy linear accelerators, neutrons are produced as a consequence of photonuclear reactions in the target nuclei, accelerator head, field-flattening filters and beam collimators, and other irradiated objects. NTD (CR-39) is used to evaluate energy and fluence of the fast neutron. Track length is used to estimate fast photoneutrons energy for linear accelerators (Elekta 10 MV, Elekta 15 MV, and Varian 15 MV). Results show that the estimated neutron energies for the three chosen examples of LINACs reveals neutron energies in the range of 1–2 MeV for 10 and 15 MV X-ray beams. The fluence of neutrons at the isocenter (Φtotal) is found to be (4×106 n cm2 Gy−1) for Elekta machine 10 MV. The neutron source strengths Q are calculated. It was found to be 0.2×1012 n Gy−1 X-ray at the isocenter. This work represents simple, low cost, and accurate methods of measuring fast neutrons dose and energies.

The risk of secondary cancer in nasopharyngeal carcinoma paediatric patients due to intensity modulated radiotherapy and mega-voltage cone beam computed tomography

There is a growing interest in the study of radiation‐induced secondary cancer. The aim of this work is (i) to estimate the peripheral doses attributable to intensity modulated radiotherapy (IMRT) and mega‐voltage cone beam computed tomography (MV‐CBCT) for some organs at risk (OARs) which surround the target being treated (Nasopharynx) in paediatric patients. (ii) To estimate the risk of radiation‐induced secondary cancers attributable to patient setup verification imaging dose using MV‐CBCT for Nasopharyngeal Carcinoma (NPC) in paediatric patients and comparing it with that attributable to the therapeutic dose using IMRT.

In-phantom neutron dose distribution for bladder cancer cases treated with high-energy photons

This work presents an estimation of the neutron dose distribution for common bladder cancer cases treated with high-energy photons of 15 MV therapy accelerators. Neutron doses were measured in an Alderson phantom, using TLD 700 and 600 thermoluminescence dosimeters, resembling bladder cancer cases treated with high-energy photons from 15 MV LINAC and having a treatment plan using the four-field pelvic box technique. Thermal neutron dose distribution in the target area and the surrounding tissue was estimated. The sensitivity of all detectors for both gamma and neutrons was estimated and used for correction of the TL reading. TLD detectors were irradiated with a Co60 gamma standard source and thermal neutrons at the irradiation facility of the National Institute for Standards (NIS). The TL to dose conversion factor was estimated in terms of both Co60 neutron equivalent dose and thermal neutron dose. The dose distribution of photo-neutrons throughout each target was estimated and presented in three-dimensional charts and isodose curves. The distribution was found to be non-isotropic through the target. It varied from a minimum of 0.23 mSv/h to a maximum of 2.07 mSv/h at 6 cm off-axis. The mean neutron dose equivalent was found to be 0.63 mSv/h, which agrees with other published literature. The estimated average neutron equivalent to the bladder per administered therapeutic dose was found to be 0.39 mSv Gy−1, which is also in good agreement with published literature. As a consequence of a complete therapeutic treatment of 50 Gy high-energy photons at 15 MV, the total thermal neutron equivalent dose to the abdomen was found to be about 0.012 Sv.