Hamideh Alasti

Patient dose from kilovoltage cone beam computed tomography imaging in radiation therapy

Kilovoltage cone-beam computerized tomography (kV-CBCT) systems integrated into the gantry of linear accelerators can be used to acquire high-resolution volumetric images of the patient in the treatment position. Using on-line software and hardware, patient position can be determined accurately with a high degree of precision and, subsequently, set-up parameters can be adjusted to deliver the intended treatment. While the patient dose due to a single volumetric imaging acquisition is small compared to the therapy dose, repeated and daily image guidance procedures can lead to substantial dose to normal tissue. The dosimetric properties of a clinical CBCT system have been studied on an Elekta linear accelerator (Synergy RP, XVI system) and additional measurements performed on a laboratory system with identical geometry. Dose measurements were performed with an ion chamber and MOSFET detectors at the center, periphery, and surface of 30 and 16-cm-diam cylindrical shaped water phantoms, as a function of x-ray energy and longitudinal field-of-view (FOV) settings of 5,10,15, and 26 cm. The measurements were performed for full 360 degrees CBCT acquisition as well as for half-rotation scans for 120 kVp beams using the 30-cm-diam phantom. The dose at the center and surface of the body phantom were determined to be 1.6 and 2.3 cGy for a typical imaging protocol, using full rotation scan, with a technique setting of 120 kVp and 660 mAs. The results of our measurements have been presented in terms of a dose conversion factor fCBCT, expressed in cGy/R. These factors depend on beam quality and phantom size as well as on scan geometry and can be utilized to estimate dose for any arbitrary mAs setting and reference exposure rate of the x-ray tube at standard distance. The results demonstrate the opportunity to manipulate the scanning parameters to reduce the dose to the patient by employing lower energy (kVp) beams, smaller FOV, or by using half-rotation scan.

Positioning errors and prostate motion during conformal prostate radiotherapy using on-line isocentre set-up verification and implanted prostate markers.

PURPOSE To evaluate treatment errors from set-up and inter-fraction prostatic motion with port films and implanted prostate fiducial markers during conformal radiotherapy for localized prostate cancer. METHODS Errors from isocentre positioning and inter-fraction prostate motion were investigated in 13 men treated with escalated dose conformal radiotherapy for localized prostate cancer. To limit the effect of inter-fraction prostate motion, patients were planned and treated with an empty rectum and a comfortably full bladder, and were instructed regarding dietary management, fluid intake and laxative use. Field placement was determined and corrected with daily on-line portal imaging. A lateral portal film was taken three times weekly over the course of therapy. From these films, random and systematic placement errors were measured by matching corresponding bony landmarks to the simulator film. Superior-inferior and anterior-posterior prostate motion was measured from the displacement of three gold pins implanted into the prostate before planning. A planning target volume (PTV) was derived to account for the measured prostate motion and field placement errors. RESULTS From 272 port films the random and systematic isocentre positioning error was 2.2 mm (range 0.2-7.3 mm) and 1.4 mm (range 0.2-3.3 mm), respectively. Prostate motion was largest at the base compared to the apex. Base: anterior, standard deviation (SD) 2.9 mm; superior, SD 2.1 mm. Apex: anterior, SD 2.1 mm; superior, SD 2.1 mm. The margin of PTV required to give a 99% probability of the gland remaining within the 95% isodose line during the course of therapy is superior 5.8 mm, and inferior 5.6 mm. In the anterior and posterior direction, this margin is 7.2 mm at the base, 6.5 mm at the mid-gland and 6.0 mm at the apex. CONCLUSIONS Systematic set-up errors were small using real-time isocentre placement corrections. Patient instruction to help control variation in bladder and rectal distension during therapy may explain the observed small SD for prostate motion in this group of patients. Inter-fraction prostate motion remained the largest source of treatment error, and observed motion was greatest at the gland base. In the absence of real-time pre-treatment imaging of prostate position, sequential portal films of implanted prostatic markers should improve quality assurance by confirming organ position within the treatment field over the course of therapy.

Portal imaging for evaluation of daily on-line setup errors and off-line organ motion during conformal irradiation of carcinoma of the prostate

PURPOSE To use portal imaging to measure daily on-line setup error and off-line prostatic motion in patients treated with conformal radiotherapy to determine an optimum planning target volume (PTV) margin incorporating both setup error and organ motion. RESULTS A total of 2549 portal images from 33 patients were acquired over the course of the study. Of these patients, 23 were analyzed for setup errors while the remaining 10 were analyzed for prostatic motion. Setup errors were characterized by standard deviations of 1.8 mm in the anterior-posterior (AP) direction and 1.4 mm in the superior-inferior (SI) direction. Displacements due to prostatic motion, with standard deviations of 5.8 mm AP and 3.3 mm SI, were found to be more significant than setup errors. CONCLUSIONS Taking into account both setup errors and target organ motion, optimum PTV margins to ensure 95% coverage are 10.0 mm AP and 5.9 mm SI. The portal imaging protocol established in this study allows radiation therapists to accept or adjust a treatment setup based upon daily on-line image matching results. The successful localization of radiopaque fiducial markers on a significant number of portal images acquired in the study gives hope that more accurate on-line targeting verification may soon be possible through the visualization of the prostate itself as opposed to the surrounding bony structures of the pelvis.

A Facility for Magnetic Resonance-Guided Radiation Therapy

Magnetic resonance (MR) imaging is routinely employed in the design of radiotherapy (RT) treatment plans for many disease sites. It is evident that tighter integration of MR imaging into the RT process would increase confidence in dose placement and facilitate the integration of new MR imaging information (including anatomical and functional imaging) into the therapy process. To this end, a dedicated MR-guided RT (MRgRT) facility has been created that integrates a state-of-the-art linear accelerator delivery system, high-dose rate brachytherapy afterloader, and superconducting MR scanner to allow MR-based online treatment guidance, adaptive replanning, and response monitoring while maintaining the clinical functionality of the existing delivery systems. This system is housed within a dedicated MRgRT suite and operates in a coordinated fashion to assure safe and efficient MRgRT treatments.

A novel four-dimensional radiotherapy method for lung cancer: imaging, treatment planning and delivery.

We present treatment planning methods based on four-dimensional computed tomography (4D-CT) to incorporate tumour motion using (1) a static field and (2) a dynamic field. Static 4D fields are determined to include the target in all breathing phases, whereas dynamic 4D fields are determined to follow the shape of the tumour assessed from 4D-CT images with a dynamic weighting factor. The weighting factor selection depends on the reliability of patient breathing and limitations of the delivery system. The static 4D method is compared with our standard protocol for gross tumour volume (GTV) coverage, mean lung dose and V20. It was found that the GTV delineated on helical CT without incorporating breathing motion does not adequately represent the target compared to the GTV delineated from 4D-CT. Dosimetric analysis indicates that the static 4D-CT based technique results in a reduction of the mean lung dose compared with the standard protocol. Measurements on a moving phantom and simulations indicated that 4D radiotherapy (4D-RT) synchronized with respiration-induced motion further reduces mean lung dose and V20, and may allow safe application of dose escalation and CRT/IMRT. The motions of the chest cavity, tumour and thoracic structures of 24 lung cancer patients are also analysed.

A randomized comparison of interfraction and intrafraction prostate motion with and without abdominal compression

BACKGROUND AND PURPOSE To quantify inter- and intrafraction prostate motion in a standard VacLok (VL) immobilization device or in the BodyFix (BF) system incorporating a compression element which may reduce abdominal movement. MATERIALS AND METHODS Thirty-two patients were randomly assigned to VL or BF. Interfraction prostate motion >3 mm was corrected pre-treatment. EPIs were taken daily at the start and end of the first and last treatment beams. Interfraction and intrafraction prostate motion were measured for centre of mass (COM) and individual markers. RESULTS There were no significant differences in interfraction (p0.002) or intrafraction (p0.16) prostate motion with or without abdominal compression. Median intrafraction motion was slightly smaller than interfraction motion in the AP (7.0 mm vs. 7.6 mm) and SI direction (3.2 mm vs. 4.7 mm). The final image captured the maximal intrafraction displacement in only 40% of fractions. Our PTV incorporated >95% of total prostate motion. CONCLUSIONS Intrafraction motion became the major source of error during radiotherapy after online correction of interfraction prostate motion. The addition of 120 mbar abdominal compression to custom pelvic immobilization influenced neither interfraction nor intrafraction prostate motion.

Comparative evaluation of image quality from three CT simulation scanners.

Today, radiation therapy (RT) is moving toward increased radiation dose to the tumor as a result of 3D conformal RT (3DCRT) and intensity‐modulated RT (IMRT), which have been made possible by advances in volumetric‐based image planning with digital imaging systems such as computed tomography (CT). Treatment planning for such RT requires superior CT image quality. Our goal in this study was to evaluate and to compare the image quality of three unique CT simulation scanners available at our center for both single‐ and multiple‐slice helical scanners. These scanners included a conventional 70‐cm bore single‐slice scanner (Philips Medical Systems), a large 85‐cm bore single‐slice scanner (Philips Medical Systems), and a 70‐cm bore multislice scanner (GE Medical Systems). Image quality was evaluated in terms of image noise, low‐contrast detectability (LCD), limiting spatial resolution (modulation transfer function), and slice thickness accuracy in accordance with guidelines set out by the AAPM. A commercially available Catphan® phantom was used to characterize image quality for both axial and helical modes of scanning. We found that image quality was generally comparable for all scanners. Limiting spatial resolution and slice thickness accuracy were comparable for all three scanners for both scanning modes. The multislice unit was superior in terms of noise content, resulting in improved visualization of small, low‐contrast objects, which is of significant clinical importance, particularly for soft tissue delineation. In addition, the multislice unit optimizes volume coverage speed and longitudinal resolution without compromising image quality, a significant advantage for the radiation oncology environment. PACS numbers: 87.57.Ce, 87.59.Fm, 87.57.Nk

Investigation of the dosimetric effect of respiratory motion using four-dimensional weighted radiotherapy

We have developed a four-dimensional weighted radiotherapy (4DW-RT) technique. This method involves designing the motion of the linear accelerator beam to coincide with the tumour motion determined from 4D-CT imaging while including a weighting factor to account for irregular motion and limitations of the delivery system. Experiments were conducted with a moving phantom to assess limitations of the delivery system when applying this method. Although the multi-leaf collimator motion remains within the tolerance of the linear accelerator, the extent of motion was less than 1 mm larger than the designed one, and there was a net system latency of approximately 0.2 s. The dose distributions were measured and simulated using different weighting factors and motion scenarios. The breathing characteristics (period, extent of motion, drift and standard deviations) of 32 patients were evaluated using the Varian RPM system. Breathing variability was assessed by plotting the average breathing motion as a function of the breathing phase. Simulations were carried out to determine the optimal weighting factor based on typical patient breathing characteristics. These results establish that the 4DW-RT method demonstrates potential for dose escalation without increasing exposure to healthy tissue.

Evaluation of high dose volumetric CT to reduce inter-observer delineation variability and PTV margins for prostate cancer radiotherapy

PURPOSE The aim was to determine whether the enhanced soft tissue contrast provided by high-dose volumetric CT (HDVCT) can reduce inter-observer variability in delineating prostate compared to helical conventional CT (CCT) scans and 3T MRI scans for patients undergoing radical prostate cancer radiotherapy. Secondly, to quantify the potential PTV reduction with decreased inter-observer variability. MATERIALS AND METHODS A 320 slice volumetric CT scanner was used. The wide-detector coverage of 16cm enabled volumetric image acquisition of prostate gland in one rotation. Three imaging studies were performed on ten patients. CCT and HDVCT were performed consecutively at the same coordinate system followed by MRI. Five radiation oncologists delineated the prostate. RESULTS The inter-observer variability is 2.0±0.6, 1.9±0.4 and 1.8±0.4mm for CCT, HDVCT and MR respectively with the maximum at the apex region. Comparing inter-observer difference variability between CCT and HDVCT with MR indicates that observers have larger variations in contouring using CCT than HDVCT especially at apex. Jaccard index of HDVCT is significantly higher than CCT with a mean difference of 0.03 (p=0.011). Both MRI and HDVCT provide the opportunity for a 2mm PTV margin reduction at the apex compared to CCT. CONCLUSION Inter-observer variability in delineation remains an important source of systematic error. HDCTV for treatment planning reduces this error without recourse to MRI and permits a PTV reduction of 2mm at the apex. The margins required to account for residual error with any imaging modality are still greater than are used in typical current practice.

‘Compromise position’ image alignment to accommodate independent motion of multiple clinical target volumes during radiotherapy: A high risk prostate cancer example

Inclusion of multiple independently moving clinical target volumes (CTVs) in the irradiated volume causes an image guidance conundrum. The purpose of this research was to use high risk prostate cancer as a clinical example to evaluate a ‘compromise’ image alignment strategy.