K Paskalev

Investigation of MR image distortion for radiotherapy treatment planning of prostate cancer

MR imaging based treatment planning for radiotherapy of prostate cancer is limited due to MR imaging system related geometrical distortions, especially for patients with large body sizes. On our 0.23 T open scanner equipped with the gradient distortion correction (GDC) software, the residual image distortions after the GDC were <5 mm within the central 36 cm x 36 cm area for a standard 48 cm field of view (FOV). In order to use MR imaging alone for treatment planning the effect of residual MR distortions on external patient contour determination, especially for the peripheral regions outside the 36 cm x 36 cm area, must be investigated and corrected. In this work, we performed phantom measurements to quantify MR system related residual geometric distortions after the GDC and the effective FOV. Our results show that for patients with larger lateral dimensions (>36 cm), the differences in patient external contours between distortion-free CT images and GDC-corrected MR images were 1-2 cm because of the combination of greater gradient distortion and loss of field homogeneity away from the isocentre and the uncertainties in patient setup during CT and MRI scans. The measured distortion maps were used to perform point-by-point corrections for patients with large dimensions inside the effective FOV. Using the point-by-point method, the geometrical distortion after the GDC were reduced to <3 mm for external contour determination and the effective FOV was expanded from 36 cm to 42 cm.

Target localization for post-prostatectomy patients using CT and ultrasound image guidance.

We conducted a study comparing B‐mode acquisition and targeting (BAT) ultrasound alignments based on CT data in the postoperative setting. CT scans were obtained with a Primatom CT‐on‐rails on nine patients. Two CT scans were obtained each week, while setup error was minimized by BAT ultrasounds. For the first three patients, a direct comparison was performed. For the next six patients, a template based on the shifts from the week 1 CT during treatment was used for subsequent setup. Comparison of isocenter shifts between the BAT ultrasound and CT was made by the difference, absolute difference, and improvement (using CT alignments as the reference technique). A total of 90 image comparisons were made. The average interfraction motion was 3.2 mm in the lateral, 3.0 mm in the longitudinal, and 5.1 mm in the AP direction. The results suggest that the CT‐based ultrasound templates can improve the localization of the prostate bed when the initial displacements are greater than 4 mm. For initial displacements smaller than 4 mm, the technique neither improved nor worsened target localization. However, ultrasound alignments performed without the use of a template deteriorated patient positioning for two out of three patients, demonstrating that the use of a CT template was beneficial even at small initial displacements. PACS numbers: 87.53.‐j, 87.53.Kn, 87.53.Xd

Ultra-thin TLDs for skin dose determination in high energy photon beams.

Estimation of surface dose is very important for patients undergoing radiation therapy. In this work we investigate the dose at the surface of a water phantom and at a depth of 0.007 cm, the practical reference depth for skin as recommended by ICRP and ICRU, with ultra-thin TLDs and Monte Carlo calculations. The calculations and measurements were carried out for fields ranging from 5 x 5 cm2 to 20 x 20 cm2 for 6 MV, 10 MV and 18 MV photon beams. The variation of the surface dose with angle of incidence and field size was investigated. Also, the exit dose was computed and measured for the same fields and angles of incidence. The dose at the ICRU reference depth was computed. Good agreement (+/-5%) was achieved between measurements and calculations. The surface dose at the entrance increased with the angle of incidence and/or the field size. The exit dose decreased with the angle of incidence but it increased with field size. The dose at the surface of the patient is mostly dependent on the beam energy, modality and beam obliquity rather than the field size and field separation. By correlating TLD measurements with Monte Carlo calculations, we were able to predict the dose at the skin surface with good accuracy. Knowing the dose received at the surface of the patient can lead to prediction of skin reactions helping with the design of new treatment techniques and alternative dose fractionation schemes.

Rectal dose variation during the course of image-guided radiation therapy of prostate cancer

BACKGROUND AND PURPOSE To investigate the change in rectal dose during the treatment course for intensity-modulated radiotherapy (IMRT) of prostate cancer with image-guidance. MATERIALS AND METHODS Twenty prostate cancer patients were recruited for this retrospective study. All patients have been treated with IMRT. For each patient, MR and CT images were fused for target and critical structure delineation. IMRT treatment planning was performed on the simulation CT images. Inter-fractional motion during the course of treatment was corrected using a CT-on-rails system. The rectum was outlined on both the original treatment plan and the subsequent daily CT images from the CT-on-rails by the same investigator. Dose distributions on these daily CT images were recalculated with the isocenter shifts relative to the simulation CT images using the leaf sequences/MUs based on the original treatment plan. The rectal doses from the subsequent daily CTs were compared with the original doses planned on the simulation CT using our clinical acceptance criteria. RESULTS Based on 20 patients with 139 daily CT sets, 28% of the subsequent treatment dose distributions did not meet our criterion of V(40) < 35%, and 27% did not meet our criterion of V(65) < 17%. The inter-fractional rectal volume variation is significant for some patients. CONCLUSIONS Due to the large inter-fractional variation of the rectal volume, it is more favorable to plan prostate IMRT based on an empty rectum and deliver treatment to patients with an empty rectum. Over 70% of actual treatments showed better rectal doses than our clinical acceptance criteria. A significant fraction (27%) of the actual treatments would benefit from adaptive image-guided radiotherapy based on daily CT images.

Daily target localization for prostate patients based on 3D image correlation

There are several localization techniques that have been used for prostate treatment. Recently, the potential use of a variety of CT-based equipment in the treatment room has been discussed. The goal of our study was to develop an automated procedure for daily treatment table shift calculation based on two CT data sets: simulation CT data and localization CT data. The method suggested in this study is a 3D image cross-correlation of small regions of interest (ROI) within the two data sets. The relative position of the two ROIs with respect to each other is determined by the maximum value of the normalized cross-correlation function, calculated for all possible relative locations of the two ROIs. After the best match is found the shifts are given by the vector connecting the treatment isocentre and the planning isocentre (both determined by the radio opaque fiducial markers on the patients skin). The results have been compared with shifts calculated through manual fusion. The shift differences, averaged over 17 statistically independent shift calculations, are less then 1 mm in the lateral and longitudinal directions, and about 1 mm in the AP direction. The impact of image noise on the performance of the algorithm has been tested. The results show that the algorithm accurately adjusts for target positional changes even with Gaussian noise levels as high as 20% inserted.

Esophageal Motion During Radiotherapy: Quantification and Margin Implications

The purpose was to evaluate interfraction and intrafraction esophageal motion in the right-left (RL) and anterior-posterior (AP) directions using computed tomography (CT) in esophageal cancer patients. Eight patients underwent CT simulation and CT-on-rails imaging before and after radiotherapy. Interfraction displacement was defined as differences between pretreatment and simulation images. Intrafraction displacement was defined as differences between pretreatment and posttreatment images. Images were fused using bone registries, adjusted to the carina. The mean, average of the absolute, and range of esophageal motion were calculated in the RL and AP directions, above and below the carina. Thirty-one CT image sets were obtained. The incidence of esophageal interfraction motion > or =5 mm was 24% and > or =10 mm was 3%; intrafraction motion > or =5 mm was 13% and > or =10 mm was 4%. The average RL motion was 1.8 +/- 5.1 mm, favoring leftward movement, and the average AP motion was 0.6 +/- 4.8 mm, favoring posterior movement. Average absolute motion was 4.2 mm or less in the RL and AP directions. Motion was greatest in the RL direction above the carina. Coverage of 95% of esophageal mobility requires 12 mm left, 8 mm right, 10 mm posterior, and 9 mm anterior margins. In all directions, the average of the absolute interfraction and intrafraction displacement was 4.2 mm or less. These results support a 12 mm left, 8 mm right, 10 mm posterior, and 9 mm anterior margin for internal target volume (ITV) and can guide margins for future intensity modulated radiation therapy (IMRT) trials to account for organ motion and set up error in three-dimensional planning.

Determination of output factors for stereotactic radiosurgery beams

Accurate dosimetry of the narrow beam tends to be difficult to perform due to the absence of lateral electronic equilibrium and the steep dose gradient, as well as the finite size of detectors. Thus, although the high dose rate 6 MV beam on the VARIAN Trilogy accelerator is increasingly utilized for stereotactic radiosurgery (SRS) treatment, there is no general agreement in the SRS beam output factor values among the Trilogy user community. Trilogy SRS beams are confined by cone collimators and the available collimator sizes range from 5 and 10 to 30 mm, in every 2 mm increment. A range of the relative output factors are in clinic use. This variation may impair observations of dose response and optimizations of the prescribed dose. It is necessary to investigate an accurate, easily performable, and detector independent method for the narrow beam output factor measurement. In this study, a scanning beam/scanning chamber method was proposed to overcome the limitation/difficulty of using a relatively large detector in narrow beam output factor measurement. Specifically, for the scanning beam method, multiple narrow beams are used for the dose measurement using a finite size chamber. These multiple scanning beams form an equivalent large uniform field which provides lateral electron equilibrium condition. After the measurement, the contributions from neighboring beams are deconvolved and the value is used for output factor determinations. For a Linac that cannot move a beam laterally, the scanning chamber method can be used to achieve the same result. The output factors determined in such a method were compared to chambers (a 0.015 cc PTW PinPoint ion chamber and a 0.125 cc PTW ion chamber) and film measurement, as well as with Monte Carlo simulation. Film and Monte Carlo results are found to be in excellent agreement with the measurement using the scan beam method. However, the VARIAN recommended output factors measured directly by Wellhöfer CC01 chamber and Scanditronix photon field diode are consistently higher for all the cones. Especially for the 5 mm cone, the difference is more than 10%. Overall, the results suggested that the new method can help overcoming the detector volume averaging effect and the positioning uncertainties, which constitute the major challenge in small radiosurgical beam output factor measurement, and provide reliable output factors.

Comparing computed tomography localization with daily ultrasound during image-guided radiation therapy for the treatment of prostate cancer: a prospective evaluation.

In the present paper, we describe the results of a prospective trial that compared isocenter shifts produced by BAT Ultrasound (Nomos, Sewicky, PA) to those produced by a computed tomography (CT) unit in the treatment room to aid in positioning during image‐guided radiation therapy. The trial included 15 consecutive patients with localized prostate cancer. All patients underwent CT and MR simulation immobilized supine in an Alpha Cradle and were treated with intensity‐modulated radiation therapy. BAT Ultrasound was used daily to correct for interfraction motion by obtaining shift in the x, y, and z directions. Two days per week during therapy, CT scans blinded to the ultrasound shifts were obtained and recorded. We analyzed 218 alignments from the 15 patients and observed a high level of correlation between the CT and ultrasound isocenter shifts (correlation coefficients: 0.877 anterior–posterior, 0.842 lateral, and 0.831 superior–inferior). The systematic differences were less than 1 mm, and the random differences were approximately 2 mm. The average absolute differences, including both systemic and random differences, were less than 2 mm in all directions. The isocenter shifts generated by using a CT unit in the treatment room correlate highly with shifts produced by the BAT Ultrasound system. PACS numbers: 87.53, 87.59.fm, 87.63.Df

Commissioning and quality assurance of a commercial stereotactic treatment-planning system for extracranial IMRT.

A 3D treatment‐planning system (TPS) for stereotactic intensity‐modulated radiotherapy (IMRT) using a micro‐multileaf collimator has been made available by Radionics. In this work, we report our comprehensive quality assurance (QA) procedure for commissioning this TPS. First, the accuracy of stereotaxy established with a body frame was checked to ensure accurate determination of a target position within the planning system. Second, the CT‐to‐electron density conversion curve used in the TPS was fitted to our site‐specific measurement data to ensure the accuracy of dose calculation and measurement verification in a QA phantom. Using the QA phantom, the radiological path lengths were verified against known geometrical depths to ensure the accuracy of the ray‐tracing algorithm. We also checked inter‐ and intraleaf leakage/transmission for adequate jaw settings. Measurements for dose verification were performed in various head/neck and prostate IMRT treatment plans using the patient‐specific optimized fluence maps. Both ion chamber and film were used for point dose and isodose distribution verifications. To ensure that adjacent organs at risk receive dose within the expectation, we used the Monte Carlo method to calculate dose distributions and dose‐volume histograms (DVHs) for these organs at risk. The dosimetric accuracy satisfied the published acceptability criteria. The Monte Carlo calculations confirmed the measured dose distributions for target volumes. For organs located on the beam boundary or outside the beam, some differences in the DVHs were noticed. However, the plans calculated by both methods met our clinical criteria. We conclude that the accuracy of the XKnife™ RT2 treatment‐planning system is adequate for the clinical implementation of stereotactic IMRT. PACS numbers: 87.53.Xd, 87.53.Ly, 87.53.Wz

A method for repositioning of stereotactic brain patients with the aid of real-time CT image guidance.

This note presents a method that recalculates the coordinates of the isocentre for patients undergoing stereotactic radiotherapy to the brain with a relocatable head frame based on a pre-treatment CT scan. The method was evaluated by comparing initial stereotactic coordinates of the isocentre with the recalculated coordinates for eight single-fraction patients. These patients had the Brown-Roberts-Wells (BRW) frame fixed to the outer table of the skull, and therefore the coordinates of any anatomical point should be identical between the initial scan and the pre-treatment scan. The differences between the two sets of coordinates were attributed to errors in the method. The results showed that the systematic errors in the recalculated coordinates were less than 0.05 mm, and they were not statistically significant. The random errors (one standard deviation) were from 0.35 mm (lateral) to 0.58 mm (vertical). The average value of the combined 3D difference was 0.75 mm.