A fast algorithm for gamma evaluation in 3D.

The gamma-evaluation method is a tool by which dose distributions can be compared in a quantitative manner combining dose-difference and distance-to-agreement criteria. Since its introduction, the gamma evaluation has been used in many studies and is on the verge of becoming the preferred dose distribution comparison method, particularly for intensity-modulated radiation therapy (IMRT) verification. One major disadvantage, however, is its long computation time, which especially applies to the comparison of three-dimensional (3D) dose distributions. We present a fast algorithm for a full 3D gamma evaluation at high resolution. Both the reference and evaluated dose distributions are first resampled on the same grid. For each point of the reference dose distribution, the algorithm searches for the best point of agreement according to the gamma method in the evaluated dose distribution, which can be done at a subvoxel resolution. Speed, computer memory efficiency, and high spatial resolution are achieved by searching around each reference point with increasing distance in a sphere, which has a radius of a chosen maximum search distance and is interpolated "on-the-fly" at a chosen sample step size. The smaller the sample step size and the larger the differences between the dose distributions, the longer the gamma evaluation takes. With decreasing sample step size, statistical measures of the 3D gamma distribution converge. Two clinical examples were investigated using 3% of the prescribed dose as dose-difference and 0.3 cm as distance-to-agreement criteria. For 0.2 cm grid spacing, the change in gamma indices was negligible below a sample step size of 0.02 cm. Comparing the full 3D gamma evaluation and slice-by-slice 2D gamma evaluations ("2.5D") for these clinical examples, the gamma indices improved by searching in full 3D space, with the average gamma index decreasing by at least 8%.

Reduction of observer variation using matched CT-PET for lung cancer delineation: a three-dimensional analysis.

PURPOSE: Target delineation using only CT information introduces large geometric uncertainties in radiotherapy for lung cancer. Therefore, a reduction of the delineation variability is needed. The impact of including a matched CT scan with 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) and adaptation of the delineation protocol and software on target delineation in lung cancer was evaluated in an extensive multi-institutional setting and compared with the delineations using CT only. METHODS AND MATERIALS: The study was separated into two phases. For the first phase, 11 radiation oncologists (observers) delineated the gross tumor volume (GTV), including the pathologic lymph nodes of 22 lung cancer patients (Stages I-IIIB) on CT only. For the second phase (1 year later), the same radiation oncologists delineated the GTV of the same 22 patients on a matched CT-FDG-PET scan using an adapted delineation protocol and software (according to the results of the first phase). All delineated volumes were analyzed in detail. The observer variation was computed in three dimensions by measuring the distance between the median GTV surface and each individual GTV. The variation in distance of all radiation oncologists was expressed as a standard deviation. The observer variation was evaluated for anatomic regions (lung, mediastinum, chest wall, atelectasis, and lymph nodes) and interpretation regions (agreement and disagreement; i.e., >80% vs. <80% of the radiation oncologists delineated the same structure, respectively). All radiation oncologist-computer interactions were recorded and analyzed with a tool called "Big Brother." RESULTS: The overall three-dimensional observer variation was reduced from 1.0 cm (SD) for the first phase (CT only) to 0.4 cm (SD) for the second phase (matched CT-FDG-PET). The largest reduction in the observer variation was seen in the atelectasis region (SD 1.9 cm reduced to 0.5 cm). The mean ratio between the common and encompassing volume was 0.17 and 0.29 for the first and second phases, respectively. For the first phase, the common volume was 0 in 4 patients (i.e., no common point for all GTVs). In the second phase, the common volume was always >0. For all anatomic regions, the interpretation differences among the radiation oncologists were reduced. The amount of disagreement was 45% and 18% for the first and second phase, respectively. Furthermore, the mean delineation time (12 vs. 16 min, p<0.001) and mean number of corrections (25 vs. 39, p<0.001) were reduced in the second phase compared with the first phase. CONCLUSION: For high-precision radiotherapy, the delineation of lung target volumes using only CT introduces too great a variability among radiation oncologists. Implementing matched CT-FDG-PET and adapted delineation protocol and software reduced observer variation in lung cancer delineation significantly with respect to CT only. However, the remaining observer variation was still large compared with other geometric uncertainties (setup variation and organ motion).

Automatic prostate localization on cone-beam CT scans for high precision image-guided radiotherapy.

PURPOSE: Previously, we developed an automatic three-dimensional gray-value registration (GR) method for fast prostate localization that could be used during online or offline image-guided radiotherapy. The method was tested on conventional computed tomography (CT) scans. In this study, the performance of the algorithm to localize the prostate on cone-beam CT (CBCT) scans acquired on the treatment machine was evaluated. METHODS AND MATERIALS: Five to 17 CBCT scans of 32 prostate cancer patients (332 scans in total) were used. For 18 patients (190 CBCT scans), the CBCT scans were acquired with a collimated field of view (FOV) (craniocaudal). This procedure improved the image quality considerably. The prostate (i.e., prostate plus seminal vesicles) in each CBCT scan was registered to the prostate in the planning CT scan by automatic 3D gray-value registration (normal GR) starting from a registration on the bony anatomy. When these failed, registrations were repeated with a fixed rotation point locked at the prostate apex (fixed apex GR). Registrations were visually assessed in 3D by one observer with the help of an expansion (by 3.6 mm) of the delineated prostate contours of the planning CT scan. The percentage of successfully registered cases was determined from the combined normal and fixed apex GR assessment results. The error in gray-value registration for both registration methods was determined from the position of one clearly defined calcification in the prostate gland (9 patients, 71 successful registrations). RESULTS: The percentage of successfully registered CBCT scans that were acquired with a collimated FOV was about 10% higher than for CBCT scans that were acquired with an uncollimated FOV. For CBCT scans that were acquired with a collimated FOV, the percentage of successfully registered cases improved from 65%, when only normal GR was applied, to 83% when the results of normal and fixed apex GR were combined. Gray-value registration mainly failed (or registrations were difficult to assess) because of streaks in the CBCT scans caused by moving gas pockets in the rectum during CBCT image acquisition (i.e., intrafraction motion). The error in gray-value registration along the left-right, craniocaudal, and anteroposterior axes was 1.0, 2.4, and 2.3 mm (1 SD) for normal GR, and 1.0, 2.0, and 1.7 mm (1 SD) for fixed apex GR. The systematic and random components of these SDs contributed approximately equally to these SDs, for both registration methods. CONCLUSIONS: The feasibility of automatic prostate localization on CBCT scans acquired on the treatment machine using an adaptation of the previously developed three-dimensional gray-value registration algorithm, has been validated in this study. Collimating the FOV during CBCT image acquisition improved the CBCT image quality considerably. Artifacts in the CBCT images caused by large moving gas pockets during CBCT image acquisition were the main cause for unsuccessful registration. From this study, we can conclude that CBCT scans are suitable for online and offline position verification of the prostate, as long as the amount of nonstationary gas is limited.

Quantification of shape variation of prostate and seminal vesicles during external beam radiotherapy.

PURPOSE: The prostate is known to translate and rotate under influence of rectal filling changes and many studies have addressed the magnitude of these motions. However, prostate shape variations also have been reported. For image-guided radiotherapy, it is essential to know the relative magnitude of translations, rotations, and shape variation so that the most appropriate correction strategy can be chosen. However, no quantitative analysis of shape variation has been performed. It is, therefore, the purpose of this article to develop a method to determine shape variation of complex organs and apply it to determine shape variation during external beam radiotherapy of a GTV (gross tumor volume) consisting of prostate and seminal vesicles. METHODS AND MATERIALS: For this study, the data of 19 patients with prostate cancer were used. Each patient received a planning computed tomography (CT) scan and 8-12 (11 on average) repeat CT scans that were made during the course of conformal radiotherapy. One observer delineated the GTV in all scans, and volume variations were measured. After matching the GTVs for each patient for translation and rotation, a coverage probability matrix was constructed and the 50% isosurface was taken to determine the average GTV surface. Perpendicular distances between the average GTV and the individual GTVs were calculated for each point of the average GTV, and their variation was expressed in terms of local standard deviation (SD). The local SDs of the shape variation of all 19 patients were mapped onto a reference case by matching and morphing of the individual average GTVs. Repeated delineation of the GTV was done for 6 patients to determine intraobserver variation. Finally, the measured shape variation was corrected for intraobserver variation to estimate the "real" shape variation. RESULTS: No significant variations in GTV volume were observed. The measured shape variation (including delineation variation) was largest at the tip of the vesicles (SD = 2.0 mm), smallest at the left and right side of the prostate (SD = 1.0 mm), and average elsewhere (SD = 1.5 mm). At the left, right, and cranial sides of the prostate, the intraobserver variation was of the same order of magnitude as the measured shape variation; elsewhere it was smaller. However, the accuracy of the estimated SD for intraobserver variation was about half of the accuracy of the estimated SD for the measured shape variation, giving an overall uncertainty of maximum 0.6 mm SD in the estimate of the "real" shape variation. The "real" shape variation was small at the left, right, and cranial side of the prostate (SD <0.5 mm) and between 0.5 mm and 1.6 mm elsewhere. CONCLUSIONS: We developed a method to quantify shape variation of organs with a complex shape and applied it to a GTV consisting of prostate and seminal vesicles. Deformation of prostate and seminal vesicles during the course of radiotherapy is small (relative to organ motion). Therefore, it is a valid approximation in image-guided radiotherapy of prostate cancer, in first order, to correct only for setup errors and organ motion. Prostate and seminal vesicles deformation can be considered as a second-order effect.

Pretreatment probability model for predicting outcome after intraarterial chemoradiation for advanced head and neck carcinoma.

BACKGROUND: Concurrent chemoradiation is being used increasingly to treat patients with advanced-stage head and neck carcinoma. In the current study, a clinical nomogram was developed to predict local control and overall survival rates for individual patients who will undergo chemoradiation. METHODS: Ninety-two consecutive patients with UICC TNM Stage III/IV squamous cell carcinoma of the oral cavity, oropharynx, hypopharynx, and supraglottic larynx were treated with selective-targeted chemoradiation (acronym: RADPLAT). All living patients had a minimum follow-up of 2 years. In addition to general factors, the following parameters were analyzed in a multivariable analysis: primary tumor volume, lymph node tumor volume, total tumor volume, lowest involved neck level, comorbidity, pretreatment hemoglobin level, pretreatment weight loss, and unilateral/bilateral intraarterial infusion. Relevant factors for local control and survival were analyzed using the Cox proportional hazards model. RESULTS: At 5 years, the local control and overall survival rates for the whole group were 60% and 38%, respectively. Primary tumor volume (hazard ratio [HR], 1.03; P = 0.01) and unilateral infusion (HR, 5.05; P = 0.004) were found to influence local control significantly. Using tumor volume as a continuous variable, an adjusted risk ratio of 1.026 was found, indicating that each 1-cm(3) increase in volume was associated with a 2.6% decrease in probability of local control. Primary tumor volume (HR, 1.01; P = 0.003), comorbidity (American Society of Anesthesiologists [ASA] physical status 1 vs. > 1; HR, 2.47; P = 0.01), lowest involved neck level (HR, 3.45; P = 0.007), and pretreatment weight loss > 10% (HR, 2.04; P = 0.02) were found to be significant predictors of worse overall survival. Variables from the multivariable analysis were used to develop a nomogram capable of predicting local control and overall survival. CONCLUSIONS: Tumor volume was found to play a significant role in predicting local control and overall survival in patients with advanced-stage head and neck carcinoma who were treated with targeted chemoradiation. The nomograms may be useful for pretreatment selection of patients with advanced-stage head and neck carcinoma.

A model to predict bladder shapes from changes in bladder and rectal filling.

The purpose of this study is to develop a model that quantifies in three dimensions changes in bladder shape due to changes in bladder and/or rectal volume. The new technique enables us to predict changes in bladder shape over a short period of time, based on known urinary inflow. Shortly prior to the treatment, the patient will be scanned using a cone beam CT scanner (x-ray volume imager) that is integrated with the linear accelerator. After (automated) delineation of the bladder, the model will be used to predict the short-term shape changes of the bladder for the time interval between image acquisition and dose delivery. The model was developed using multiple daily CT scans of the pelvic area of 19 patients. For each patient, the rigid bony structure in follow-up scans was matched to that of the planning CT scan, and the outer bladder and rectal wall were delineated. Each bladder wall was subdivided in 2500 domains. A fixed reference point inside the bladder was used to calculate for each bladder structure a "Mercator-like" 2D scalar map (similar to a height map of the globe), containing the distances from this reference point to each domain on the bladder wall. Subsequently, for all bladder shapes of a patient and for all domains on the wall individually, the distance to the reference point was fitted by a linear function of both bladder and rectal volume. The model uses an existing bladder structure to create a new structure via expansion (or contraction), until the expressed volume is reached. To evaluate the predictive power of the model, the jack-knife method was used. The errors in the fitting procedure depended on the part of the bladder and range from 0 to 0.5 cm (0.2 cm on average). It was found that a volume increase of 150 cc can lead to a displacement up to about 2.5 cm of the cranial part of the bladder. With the model, the uncertainty in the position of the bladder wall can be reduced down to a maximum value of about 0.5 cm in case the bladder volume increase is known. Furthermore, it was found that a change in rectal filling causes a shift of the bladder, while its shape is hardly influenced. In conclusion, we developed a model that describes the bladder shape and position as a function of the bladder volume and the rectal filling. The model accurately describes the complex shape of the bladder as it works on each domain of the bladder separately.

Intensity modulated versus non-intensity modulated radiotherapy in the treatment of the left breast and upper internal mammary lymph node chain: a comparative planning study.

BACKGROUND AND PURPOSE: To compare and evaluate intensity modulated (IMRT) and non-intensity modulated radiotherapy techniques in the treatment of the left breast and upper internal mammary lymph node chain. MATERIALS AND METHODS: The breast, upper internal mammary chain (IMC), heart and lungs were delineated on a computed tomography (CT)-scan for 12 patients. Three different treatment plans were created: (1) tangential photon fields with oblique IMC electron-photon fields with manually optimized beam weights and wedges, (2) wide split tangential photon fields with a heart block and computer optimized wedge angles, and (3) IMRT tangential photon fields. For the IMRT technique, an inverse planning program (KonRad) generated the intensity profiles and a clinical three-dimensional treatment planning system (U-MPlan) optimized the segment weights. U-MPlan calculated the dose distribution for all three techniques. The normal tissue complication probabilities (NTCPs) for the organs at risk (ORs) were calculated for comparison. RESULTS: The average root mean square deviation of the differential dose-volume histogram of the breast planning target volume was 4.6, 3.9 and 3.5% and the average mean dose to the IMC was 97.2, 108.0 and 99.6% for the oblique electron, wide split tangent and IMRT techniques, respectively. The average NTCP for the ORs (i.e. heart and lungs) were comparable between the oblique electron and IMRT techniques (or=2%) for the ORs. CONCLUSIONS: The lowest NTCP values were found with the oblique electron and the IMRT techniques. The IMRT technique had the best breast and IMC target coverage.