Daily CT Images Research Articles

TH‐C‐ValB‐06: Thoracic Proton Treatment Planning Strategies Based On the 4D CT Information

Purpose: Particularly in the case of thoracic radiation therapy, there are substantial inter‐ and intra‐fractional variations in shape, volume and position of treatment targets and the intervening and surrounding normal tissues. The purpose of this work is to develop proton treatment planning strategies for mobile tumors with and without mobile intervening structures based on 4D CT, and to assess the planning strategies using 4D CT data and daily in‐room CT information. Method and Materials: Five treatment planning strategies were evaluated based on (1) free breathing CT with small smearing, (2) free breathing CT with large smearing, (3) average CT with small smearing, (4) average CT with CT numbers inside the tumor volume replaced by higher CT numbers, and (5) maximum intensity projection CT. For a lung patient with large, 1.6 cm, tumor motion and immobile surrounding tissues, treatment plans were designed using strategies 1, 2 and 4. Each treatment plan was recalculated on five daily CT images using bony structure alignment. For an esophagus patient with 3.5 cm tumor motion and large cardiac, liver and spleen motion, treatment plans were designed using strategies 1, 2, 3 and 5. Each treatment plan was recalculated in all 10 phases of the 4D CT. Results: For the lung patient, the tumor coverage evaluated using the five daily CT's is superior when using strategies 2 and 4 as compared to strategy 1. However, the lung sparing is superior using strategy 4 compared to strategy 2. For the esophagus patient, the treatment plans using strategy 2 and 5 covered the targets for each 4D CT phase while the plan using strategy 1 and 3 caused significant under‐dosing. Conclusion: With the strategies developed using 4D CT data, good tumor coverage was achieved for the thoracic patients with large tumor and surrounding tissue motion using proton therapy.

WE-E-ValA-02: Dosimetric Comparison of the No Action Level Alignment Protocol with Daily Alignment Techniques for Prostate Cancer

Purpose: To compare the effectiveness of two off‐line “No‐Action‐Level” (NAL) correction protocols with daily image‐guided alignments using bony registration (simulating electronic portal image alignment), ultrasound, and CT for direct prostate target localization. Method and Materials: Ten prostate patients received 3 CT scans per week using an integrated CTLINAC system immediately prior to radiotherapy (243 CT scans total). A clinical treatment plan was designed on the planning CTimage using current clinical margins and copied onto the daily CTimages. Two NAL protocols, based on CT measurements of the internal prostate shift relative to bony anatomy, were simulated for correcting the predicted internal systematic prostate shifts after 1 week or after 2 weeks of treatment. The NAL protocols were compared to three daily alignment methods, which simulated pelvic bone alignment, ultrasound alignment, and CT alignment. The dosimetric impact on target coverage for each scenario was reported. Reducing the planning margins to 3mm was also evaluated. Results: Daily CT scans are more accurate than daily ultrasound measurements for determining the prostate systematic positional shift, particularly in the anterior/posterior direction. The average minimum prostate dose was greatest with CT alignment (75.8Gy, p<=0.028), then with the two NAL protocols (both 74.Gy, p<=0.017), followed by ultrasound alignment (73.2Gy) and bone alignment (70.2Gy). For plans with 3mm margins, the average minimum dose was greatest with CT alignment (75.1Gy, p<=0.007), then with the two adaptive alignments (71.4Gy and 70.8Gy respectively, p<=0.022), followed by ultrasound alignment (68.4Gy) and bone alignment (63.9Gy). Conclusions: An off‐line NAL correction protocol for reducing systematic internal target shifts proved to be effective when performed after only one treatment week. The target dosimetric coverage from the NAL protocol was as good as daily ultrasound alignments but not as great as daily CT alignments. Using a 3mm planning margin exacerbated the differences in target coverage.

SU‐FF‐J‐122: Systematic Study of Inter‐Fractional Variations for Anatomic Sites From Head to Feet

Purpose: Inter‐fractional variations in patient set‐up and anatomic changes are usually site specific. This work aims to systematically study these inter‐fractional variations for anatomic sites from head to feet based on the daily MV CT images collected using helical tomotherapy. Methods and Materials: A total of first 51 patients treated at various anatomic sites using a helical tomotherapy system (Hi‐Art, Tomotherapy Inc.) were analyzed. Daily Tomotherapy MV CT acquired prior to each treatment were used to correct for daily setup error online. A total of 6120 translational shifts and rotational corrections were performed for the 51 patients. The daily serial MV CT and the planning CT were also used to determine inter‐fractional anatomic changes off‐line. The data for three representative patient cases, pancreas, uterus, and soft tissue sarcoma, are presented. Results: Inter‐fractional set up errors in skull, brain, and head and neck are significantly smaller than those in chest, abdomen/pelvic, and extremity. The translational shifts are mostly within 3 mm in skull, brain, and head and neck, while they are within 6 mm for other sites. The inter‐fractional anatomic changes were significant. For example, during the course of treatment, the pancreas moved up to ±20 mm, and volumes of the uterus and sarcoma varied up to 30% and 100%, respectively. Conclusions: The inter‐fractional variations in patient setup and in shapes, sizes and positions of both targets and normal structures can be significant and are site specific. The helical Tomotherapy technology has the capability of quantifying and addressing these variations. The data presented in this work dealing with several anatomic sites may be useful in developing adaptive radiotherapy.

SU‐FF‐J‐97: Modeling of Lung Tumor Response to Image‐Guided Radiation Therapy

Purpose: To employ locally weighted regression (LWR) as an empirical technique for modeling tumor response to radiation treatment. Methods and Materials: Daily Megavoltage CT (MVCT) images were acquired prior to the delivery of 660 lung treatment fractions for 20 lung cancer patients, with contoured tumor volumes in each image. This data was used to develop a LWR model of the tumor progression throughout the treatment. A leave one out cross validation (LOOCV) technique was used to evaluate the performance of the model with varied combinations of days of patient data. For each patient, the model was run using the remaining patient data as training data, but with different test observations for each trial. Multiple simulations using random sampling without replacement were carried out to ensure that all possible combinations were tried. The best combination of days was determined by finding the mean predictive performance. Since this technique will eventually be used to make decisions about a patient's treatment, each prediction was accompanied by a measure of its uncertainty. A large uncertainty value indicates that the prediction is unreliable, most likely due to poor coverage of the training data. Results: The LWR model did not always accurately predict the shape of the tumor response curve, but the model was accurate in its predictions made at the end of a patient's treatment. The average prediction error was 13%, and even tumor volume increases during the course of the treatment were correctly predicted. Conclusions: The LWR modeling technique proved fast and fairly accurate when predicting the tumor volume at the end of treatment, and improvement is expected as additional data is added to the memory matrix. Future research will use expert opinion and data analysis techniques to determine which additional patient information will improve the model.

SU‐EE‐A3‐05: Accuracy Testing of Deformable Registration Using Dosimetric End‐Points

Purpose: To develop a dose‐based evaluation method to assess deformable image registration accuracy Method and Materials: An algorithm developed for deformable registration of MVCT to kVCT images was evaluated. The algorithm allows the generation of automatic contours on MVCT images by transferring the kVCT contours using the deformation map. The automatically generated MVCT contours can thus be used to test the deformation algorithm by comparing these contours with manual contours. Instead of a geographic contour comparison, dosimetric endpoints were evaluated after the dose distribution was calculated in the MVCT images. Three dosimetric endpoints (Dmax, Dmean, and Dose to the hottest 2 cc (Dmax (2cc)) were compared for spinal cord contours. The evaluation of geometric end‐points is directly related to the clinical information that needs to be evaluated if daily images are used for adaptive radiation therapy. A total of 93 daily megavoltage CT (MVCT) images from three patients treated for cancers in the head and neck region were evaluated. Results: Averaged over all images the calculated Dmax differed between the automatic and manual contours by 1.1 % with a standard deviation of 3.5 %. The respective values for Dmean and Dmax(2cc) are 0.1 ± 2.5 % and 1.8 ± 2.4 %. Maximum deviations between the dosimetric endpoints were 12 % for Dmax, 8% for Dmean, and 13 % for Dmax(2cc). Conclusions: Using deformable image registration, dosimetric end‐points can be generated from automatic contours in the spinal cord region that differ from manual contours by 1–2 % on average with a standard deviation of 2.5 to 3.5 %. In the spinal cord region the developed deformable image registration appears to provide sufficient accuracy to support clinical decisions. Conflict of interest: Research supported by the vendor that is commercializing the algorithm. Several co‐authors are vendor employees.

Daily Megavoltage CT Imaging in Patients with Prostate Cancer Receiving Postoperative Radiation Therapy: Does the Prostate Bed Move?

Daily Megavoltage CT Imaging in Patients with Prostate Cancer Receiving Postoperative Radiation Therapy: Does the Prostate Bed Move?

Use of Daily Megavoltage CT Imaging to Determine Delivered Dose Variations in Head and Neck Cancer Patients

Use of Daily Megavoltage CT Imaging to Determine Delivered Dose Variations in Head and Neck Cancer Patients

Image-guided adaptive radiation therapy (IGART): Radiobiological and dose escalation considerations for localized carcinoma of the prostate

The goal of this work was to evaluate the efficacy of various image-guided adaptive radiation therapy (IGART) techniques to deliver and escalate dose to the prostate in the presence of geometric uncertainties. Five prostate patients with 15-16 treatment CT studies each were retrospectively analyzed. All patients were planned with an 18 MV, six-field conformal technique with a 10 mm margin size and an initial prescription of 70 Gy in 35 fractions. The adaptive strategy employed in this work for patient-specific dose escalation was to increase the prescription dose in 2 Gy-per-fraction increments until the rectum normal tissue complication probability (NTCP) reached a level equal to that of the nominal plan NTCP (i.e., iso-NTCP dose escalation). The various target localization techniques simulated were: (1) daily laser-guided alignment to skin tattoo marks that represents treatment without image-guidance, (2) alignment to bony landmarks with daily portal images, and (3) alignment to the clinical target volume (CTV) with daily CT images. Techniques (1) and (3) were resimulated with a reduced margin size of 5 mm to investigate further dose escalation. When delivering the original clinical prescription dose of 70 Gy in 35 fractions, the "CTV registration" technique yielded the highest tumor control probability (TCP) most frequently, followed by the "bone registration" and "tattoo registration" techniques. However, the differences in TCP among the three techniques were minor when the margin size was 10 mm (⩽1.1%). Reducing the margin size to 5 mm significantly degraded the TCP values of the "tattoo registration" technique in two of the five patients, where a large difference was found compared to the other techniques (⩽11.8%). The "CTV registration" technique, however, did maintain similar TCP values compared to their 10 mm margin counterpart. In terms of normal tissue sparing, the technique producing the lowest NTCP varied from patient to patient. Reducing the margin size seemed the only sure way to reduce the NTCP significantly, irrespective of the IGART technique employed. In escalating the dose with the iso-NTCP constraint, the largest average gain in dose was observed with the "tattoo registration" technique, followed by the "CTV registration" and "bone registration" techniques. This is attributed to the fact that in three of the five patients, the "tattoo registration" technique yielded the lowest NTCP, hence a greater window of opportunity to escalate the dose was possible with this technique. However, the variation among the five patients was also largest with the "tattoo registration" technique where, in the case of one patient, the required dose actually needed to be below the original prescription dose of 70 Gy to satisfy the iso-NTCP constraint. This was not the case with the "CTV registration" technique where positive and similar dose escalation was allowed on all five patients. Based on these data, an attractive dose escalation strategy may be to implement the "CTV registration" technique (for consistent dosimetric coverage) for daily target localization in combination with a margin reduction (for increased normal tissue sparing).

TU-C-J-6B-09: Automatic Contour Delineation On Subsequent CT Images Using Deformable Registration

Purpose: To implement a deformable registration algorithm to automatically delineate regions of interest (ROIs) on daily CT images by transforming the corresponding ROIs from a reference CT image. Method and Materials: An intensity-based ‘Demons’ deformable registration algorithm was used to find the correspondence between planning CT image and daily CT image. After reference ROIs were delineated manually on the reference CT image, the reference ROIs can be mapped onto each daily CT image. We tested this method on one head-and-neck patient (tonsil tumor). The patient received 3 CT scans per week prior to treatment for a total of 16 CT scans. The deformed ROIs were visually evaluated by a radiation oncologist. The dose-volume histograms (DVHs) of the deformed left and right parotids were calculated and compared with the planned DVHs. In addition, a cumulative dose distribution was calculated by mapping the daily dose distribution from each of the daily CT images back to the planning CT using the deformable registration method. Thus, the DVHs from the cumulative dose distribution can be calculated to represent the final “delivered” dose distribution. The volumetric changes during the elapsed treatment days for targets and critical structures were also investigated. Results: The deformed contours reasonably matched with gross anatomy presented by the CT images. The daily DVHs showed a large variation during the course of treatment due to both setup errors and internal organ deformation. In spite of large variation in DVHs of the parotids in daily CT images, the DVHs of the cumulative dose agreed reasonably well with the planned DVHs for both parotids in this case. Conclusion: We demonstrated that an intensity-based deformable registration algorithm can be used effectively to map the planning ROIs to subsequent CT images acquired during treatment. This allows effortless replanning in an adaptive CT-guided radiotherapy strategy.

TU-C-J-6B-08: A Variational Deformable Image Registration Method Capable of Handling Large Organ Deformation

Purpose: The goal of this research is to develop an accurate and reliable deformable image registration algorithm capable of handling large organ deformations for mapping daily CT images to the reference planning CT. Method and Materials: We previously developed a diffusion-based deformable image registration algorithm using Thirion's “demons” algorithm. However, this implementation did not work well when large deformations occurred. In this study, we proposed a variational approach for deformable image registration within a multigrid computational framework. The differences of the two images were measured and minimized, and a regularization term was introduced to rule out discontinuities and other irregular solutions. In our numerical implementation, the well-studied multigrid method was used for solving the nonlinear Euler-Lagrange partial differential equation (PDE). In this multigrid approach, the results from the previous deformation field were used to initialize the next stage of deformation. Different smoothing parameters were also used at different stages. The solution to these PDEs gave the displacement fields which can be used to transform the daily CT images into the planning CT or vice versa. To test this approach, we performed deformable image registration using CT images of prostate cancer patients collected with an in-room CT-on-rails system under an IRB-approved protocol. Results: Daily CT images were selected for cases with severely distorted rectum (due to the presence of large bowl gas) or large variations in bladder filling. The results indicated that the proposed approach was capable of deforming very large shape changes of soft tissue or organ structures. Compared to the “demons” algorithm, the resulting registration was pronounced better visually and quantitatively using both distance and correlation measures. Conclusion: Our deformable registration approach demonstrated the capability of handling large deformations. Further validation studies need to be done to test its robustness.

TU‐A‐T‐6E‐01: Patient QA

The number of patients treated with Intensity Modulated Radiation Therapy (IMRT) has grown dramatically over the past five years. Recent clinical studies have shown that IMRT has the potential to improve outcomes for multiple disease sites and pathologies. However, the clinical successful or failure of each individual institution's IMRT program is dependant on the correct delivery of the 3D dose distributions calculated by the planning system (Planned Dose) to the correct location in the patient (Delivered Dose). There are many sources for errors that can arise during IMRT planning and delivery, but it is ultimately the responsibility of the Medical Physicist to ensure that the Planned Dose “agrees” with the Delivered Dose. Unlike conventional and 3D conformal radiotherapy, IMRT plans must be verified for each individual patient because of the many sources of potential error. The difference between Planned Dose and Delivered Dose (i.e. Error) in IMRT can originate from at least three different sources: 1.) The Treatment Planning Model, 2.) Treatment Delivery Mechanics, and 3.) Time‐Dependant Target/Tissue Positioning. The only method for determining the cumulative effect of all these potential sources of error is to measure the entire delivery sequence on the linear accelerator for each patient. Patient‐Specific IMRT quality assurance (QA) measurements can be classified into three levels. Level I measurements are taken using film, diode arrays, electronic imaging devices, etc. The measurement device(s) are typically placed at depth in water‐equivalent material. Level II measurements are taken with a phantom that moves to simulate intra‐fraction motion (such as respiration motion). These measurements are typically made with film, diodes, and/or ionization chambers. Level III measurements are performed using daily CT imaging acquired on the delivery system (CT‐on‐Rails, Cone‐Beam CT, Megavoltage CT, etc…). Electronic detector data acquired during treatment delivery is then combined with the daily CT images to reconstruct the doses delivered to the patient. This presentation will discuss commercially available dose measurement tools, phantoms, and techniques for performing Level I and II patient‐specific IMRT quality assurance. The current status of research and development in Level III IMRT quality assurance will also be discussed. Issues associate each technique will be discussed and clarified with practical examples. Recommendations will be presented for acceptable tolerances and testing frequencies.Educational Objectives:1. To understand the issues surrounding patient‐specific IMRT quality assurance2. To understand the difference between patient‐specific QA techniques3. To identify potential sources of errors in IMRT quality assurance4. To understand the impact of target localization and patient positioning on IMRT

SU‐EE‐A3‐01: Evaluation of Image‐Guided Radiation Therapy (IGRT) Technology and Their Impact On the Outcome of Hypofractionated Prostate Cancer Treatments: A Radiobiological Analysis

Purpose: To evaluate various image‐guidance technologies and their potential impact on the outcome of hypofractionated prostate cancer radiation therapy to mitigate against geometric uncertainties. Method and Materials: Five prostate cancer patients were analyzed. All patients were planned twice with an 18MV six‐field conformal technique with a 10 and 5 mm margin sizes, with various prescription doses (35 to 70 Gy) of equal late complications (assuming normal tissue α/β = 3 Gy). The various target localization techniques simulated were (1) laser alignment to external tattoo marks, (2) alignment to bony landmarks with daily portal images, (3) alignment to the clinical target volume (CTV) with daily CT imaging, and (4) the repeat of technique (3) with daily monitor unit updates to account for patient shape changes. The impact of uncertainty in the assumed α/β value for the prostate was also assessed. Results: For all treatment schedules simulated, technique (4) achieved the ideal condition most closely (i.e., ΔTCP = TCPtechnique − TCPplan ≈ 0 %), then followed by (3), (2) and (1). As the number of fractions decreased (i.e., increasingly hypofractionated), the ΔTCP generally decreased for all techniques. Because the hypofractionated schedules were designed to keep late complications constant, the NTCP values were also relatively constant for all treatment schedules. Generally, the average NTCP values were lower than the plan for all techniques. However, the most effective way to reduce NTCP was to reduce the margin size from 10 to 5 mm. Overall, the uncertainty in α/β values had a far more influence on the outcome of the hypofractionated treatment than would the geometric uncertainties. Conclusion: This study suggests that, although the impact of geometric uncertainties increases as the number of fractions decrease, the reduction in TCP due to the uncertainties does not significantly offset the expected gain in TCP by hypofractionation.

SU-FF-T-344: Impact of Inter-Fractional Motion of the Anatomy On Prostate Proton Dose

Purpose: The purpose of this study is to determine whether the inter-fractional motion of the anatomy has a more significant impact on proton dose distributions compared to IMRT dose distributions. A secondary objective is to evaluate the impact of CTV-to-PTV margins on plans for protons produced with the double scattering technique. Method and Materials: Repeat CT scans of prostate patients acquired with a CT-on-Rails were used for this study. First, proton and IMRT plans were designed using (1) the CTV-to-PTV margin standard at our institution (normally 8 mm except at the rectum-prostate interface, where it is 5.8 mm) and (2) a small uniform margin (3mm). The proton and IMRT plans were then applied to 8 daily CT images aligned either to skin marks or the center of prostate. The doses for the 8 daily CT images were recalculated using the same beam configurations (aperture, compensators, gantry angles, Monitor Units etc). Results: For proton plans, a 3mm margin appears to be adequate for tumor coverage even when a conventional skin mark alignment technique is used. The proton plans with 3mm margins lead to nearly the same coverage as in the IMRT plans with standard margins. For prostate center of volume-based alignment, this coverage was 98.6% for protons vs. % 98.1 for IMRT and 96.6% vs. 96.3% for skin marks-based alignment. Conclusion: With the double scattering technique, the dose distribution from the proton plan is not very sensitive to the daily variation of the patient anatomy as compared to IMRT plans. The 3 mm CTV-to-PTV margin is acceptable for proton plans but not for IMRT plans for both alignment methods. The preliminary data show that the CTV-to-PTV margin can be reduced to 3mm if we use daily image guided set-up.

SU‐FF‐J‐66: Iso‐NTCP Dose Escalation with Image‐Guided Adaptive Radiation Therapy (IGART) for Localized Carcinoma of the Prostate

Purpose: To evaluate the efficacy of various image‐guided target localization techniques to assist in the safe dose escalation for prostate cancer radiation therapy in the presence of geometric uncertainties. Method and Materials: Five prostate cancer patients were analyzed, retrospectively. All patients were planned with an 18MV six‐field conformal technique with a 10 mm margin and an initial prescription dose of 70 Gy in 35 fractions. For the dose escalation study, the prescription dose was increased from 50 Gy in 2 Gy increments until the rectum normal tissue complication probability (NTCP) reached the level equal to that of the plan NTCP (i.e., iso‐NTCP). The target localization techniques simulated were (1) laser alignment to external tattoo marks, (2) alignment to bony landmarks with daily portal images, and (3) alignment to the clinical target volume (CTV) with daily CT imaging. Techniques (1) and (3) were re‐simulated with a reduced margin of 5 mm to investigate further dose escalation. Results: The selection of the target localization technique was less critical on the treatment outcome when 10 mm margin was used. Reducing the margin size from 10 to 5 mm effectively decreased the NTCP by ∼60 % for all patients. For iso‐NTCP dose escalation, to our surprise, the average dose and TCP gain for the five patients were largest with the external tattoo technique (6.4 Gy and 7.5 %, respectively) followed by the daily CTV and the bony landmarks. The variability in gain across the five patients, however, was also largest with the tattoo registration technique and smallest with the daily CTV technique. Conclusion: Based on these data, the best dose escalation strategy is to combine margin reduction (for increased normal tissue sparing) with the use of the daily CTV technique to localize the target volume (for consistent dosimetric coverage and escalation).

SU-FF-J-82: Use of CT Based IGRT for Daily Set-Up of Frameless Fractionated Stereotactic Patients

Purpose: To compare orthogonal port films against CT based Image Guided Radiation Therapy(IGRT) for patient isocenter verification and assess the use of CT based IGRT for daily patient set‐up. Method and Materials: Eleven patients with brain metastases were CT'd for each of their fractionated radiosurgerytreatments. The patients were immobilized in the supine position with a stereotactic head cast. Two of the patients had a bite block in their cast to minimize rotational movement. The daily CT was taken on a SOMATON Emotion Duo CT on rails and the treatment was delivered by a SIEMENS PRIMUS linear accelerator(linac). The daily CTimages were then fused to the planning CT using fiducial markers and anatomy as guides. The shifts in the X, Y, and Z directions were computed by the CoherenceDosimetry Workstation and the patient was shifted accordingly to within 1 millimeter accuracy. Results: We assessed the accuracy of the orthogonal port film technique against the CT based IGRT for 8 frameless stereotactic treatments. The implementation of IGRT increased the importance of patient immobilization while decreasing the need for accurate initial patient set‐up since the patient would be imaged immediately prior to treatment and corrections could then be made to the set‐up. Furthermore it was observed that using a bite block inside the head cast significantly improved patient immobilization and decreased the magnitude of IGRT shifts. Conclusion: Orthogonal port film validation of the isocenter is adequate for an accuracy of 5mm for frameless stereotactic treatments.CT based IGRT provides an adequate verification of the isocenter for frameless stereotactic treatments to within 2–3mm accuracy. Furthermore, CT proves to be of clinical benefit to the patient since the mean dose used for each CT was only .7cGy where port films use 3cGy each for the two orthogonal images.

SU-FF-J-55: Cumulative Dose Calculation Using Deformable Image Registration

Purpose: To use deformable image registration to calculate cumulative dose distributions received by a radiotherapy patient in order to evaluate the dosimetric impact of anatomical variations during the course of treatment. Method and Materials: An intensity-based deformable registration algorithm was developed to register the anatomy between the planning CT image and the daily CT images acquired from an in-room CT-on-rails system. After the transformation from daily CT to planning CT was found, daily treatment doses delivered to an organ can be meaningfully accumulated. For one head-and-neck patient, the treatment plan was transferred to 15 daily CT scans using bony registration, and the dose distributions were calculated for the daily anatomy without daily setup errors. Using deformable image registration, these daily dose distributions were mapped back to the planning CT image. The cumulative dose was calculated and compared to the original plan using dose-volume histograms (DVHs). Results: The dose delivered to the patient had minimal changes from the plan. Dose coverage for clinical target volumes at 70Gy, 63Gy, and 56Gy, left and right parotids at 25Gy, and mandible at 70Gy were evaluated. At these dose levels, the DVHs showed less than 1% difference, except for the left parotid, which had a 6% volume difference. The maximum dose in brainstem was 1.4% less than planned, the maximum dose in spinal cord was 2.2% higher than planned. The mean doses in left and right parotid were 4.4% higher and 2.5% lower than planned. Conclusion: In this case, we found little dosimetric differences due to changing anatomy during the course of treatment. Future studies will include daily setup errors. Deformable image registration is proven to be an effective approach for calculating cumulative dose distributions delivered to the patient, which can be used to evaluate the quality of treatment execution.

Use of deformed intensity distributions for on-line modification of image-guided IMRT to account for interfractional anatomic changes

Recent imaging studies have demonstrated that there can be significant changes in anatomy from day to day and over the course of radiotherapy as a result of daily positioning uncertainties and physiologic and clinical factors. There are a number of strategies to minimize such changes, reduce their impact, or correct for them. Measures to date have included improved immobilization of external and internal anatomy or adjustment of positions based on portal or ultrasound images. Perhaps the most accurate way is to use CT image-guided radiotherapy, for which the possibilities range from simple correction of setup based on daily CT images to on-line near real-time intensity modulated radiotherapy (IMRT) replanning. In addition, there are numerous intermediate possibilities. In this paper, we report the development of one such intermediate method that takes into account anatomic changes by deforming the intensity distributions of each beam based on deformations of anatomy as seen in the beam's-eye-view. The intensity distribution deformations are computed based on anatomy deformations discerned from the changes in the current image relative to a reference image (e.g., the pretreatment CT scan). First, a reference IMRT plan is generated based on the reference CT image. A new CT image is acquired using an in-room CT for every fraction. The anatomic structure contours are obtained for the new image. (For this article, these contours were manually drawn. When image guided IMRT methods are implemented, anatomic structure contours on subsequent images will likely be obtained with automatic or semiautomatic means. This could be achieved by, for example, first deforming the original CT image to match today's image, and then using the same deformation transformation to map original contours to today's image.) The reference intensity distributions for each beam are then deformed so that the projected geometric relationship within the beam's-eye-view between the anatomy (both target and normal tissues) extracted from the reference image and the reference intensity distribution is the same as (or as close as possible to) the corresponding relationship between anatomy derived from today's image and the newly deformed intensity distributions. To verify whether the dose distributions calculated using the deformed intensity distributions are acceptable for treatment as compared to the original intensity distributions, the deformed intensities are transformed into leaf sequences, which are then used to compute intensity and dose distributions expected to be delivered. The corresponding dose-volume histograms and dose-volume and dose-response indices are also computed. These data are compared with the corresponding data derived (a) from the original treatment plan applied to the original image, (b) from the original treatment plan applied to today's image, and (c) from a new full-fledged IMRT plan designed based on today's image. Depending on the degree of anatomic changes, the use of an IMRT plan designed based on the original planning CT for the treatment of the current fraction could lead to significant differences compared to the intended dose distributions. CT-guided setup compared to the setup based on skin marks or bony landmarks may improve dose distributions somewhat. Replanning IMRT based on the current fraction's image yields the best physically deliverable plan (the "gold standard"). For the prostate and head-and-neck examples studied as proof of principle, the results of deforming intensities within each beam based on the anatomy seen in the beam's-eye-view are a good approximation of full-fledged replanning compared with other alternatives. Our preliminary results encourage us to believe that deforming intensities taking into account deformation in the anatomy may be a rapid way to produce new treatment plans on-line in near real-time based on daily CT images. The methods we have developed need to be applied to a group of patients for both prostate and head-and-neck cases to confirm the validity of our approach.

Evaluation of two methods for soft tissue registration in online CT guided prostate cancer radiotherapy

Our clinic is conducting a feasibility study for CT-guided hypo-fractionated treatment of prostate cancer. Patients will receive a CT scan on our cone-beam CT-equipped linac immediately prior to treatment, the displacement of the prostate gland with respect to its planning position will be determined, and a correction will be applied accordingly. The purpose of this study is to evaluate two methods of performing the soft tissue registration. To simulate the entire course of the treatment we used serial CT data from 28 prostate cancer patients. In addiction to the planning CT, each patient had a pelvic CT scan (3 mm slice thickness) bi-weekly over the entire course of the treatment for a total of 15 CTs. Using a commercial treatment planning system, each daily CT was fused with the planning CT based on bony anatomy. This step eliminates the setup error component of the inter-fraction geometric variation. The prostate was contoured on each scan by a single physician. Subsequently prostatic image-based registration (IBR) was performed. This was accomplished by translating each daily prostate CT image to match the reference CT prostate contour, allowing sagittal rotation only. The registration was then repeated using only the contour of the daily prostate (CBR). A single investigator performed the two types of registration for all 28 patients. Each registration induces a transformation (Δdx,Δdy,Δdz,) that characterizes the motion of the daily prostate per a rigid body formalism, where Θ is the rotation in the sagittal plane. We assessed the IBR accuracy and stability by comparing each parameter of the 420 transformations between IBR and CBR. The average time to perform the correction was on the order of 3 minutes for the IBR and 1 minute for the CBR. While both techniques have uncertainties, we adopted the CBR as a reference both because there is a greater body of experience in contour definition than in soft tissue registration, and because there is less ambiguity in performing the CBR. The average shift magnitude from the AP perspective was 0.18 ± 0.18cm for CBR and 0.12 ± 0.13cm for IBR. From the lateral perspective, the results for the CBR and IBR are 0.37 ± 0.30cm and 0.25 ± 0.25cm, respectively. The analysis shows a larger shift in the AP direction then in any other direction, both in the IBR and CBR, a result consistent with previous studies. The average rotation angle, allowed only in the sagittal view, was 0.7o ± 2.0o for the CBR and 1.3o ± 2.4o for the IBR. The per-patient average discrepancy (Δdx,Δdy,Δdz) between the CBR and IBR results varies with perspective. While the average difference in the LAT direction is within 1mm for all but 6 patients, the results for the AP and SI directions have considerably more variation between patients. Furthermore, a systematic bias of more than 1mm is observed in the AP direction for 20 of the patients; in particular, the IBR indicates a larger posterior shift of the patient than does the CBR. The composite result over all patients reiterates these observations: (Δdx,Δdy,Δdz) =( 0.04 ± 0.12, 0.16 ± 0.22, 0.0 ± 0.26) cm. The CBR-IBR agreement was within 2mm for 69% of results, between 2 and 4mm for 23%, and greater than 4mm for 8%. The CBR was faster and easier to perform. The IBR showed an overestimation of the sagittal rotation angle and an underestimation of the shift magnitude for both the AP and LAT perspectives when compared to the CBR. The discrepancy between IBR and CBR is a measure of the intraobserver uncertainty of the online CT based correction. Due to the potential for significant discrepancies between the CBR and IBR results, we strongly recommend performing offline CBR verification of an online IBR-based correction. The observed IBR-CBR gap will possibly be narrowed by the use of on board KV conebeam device capable of good quality images in the coronal and sagittal views

Daily megavoltage ct imaging for lung cancer during a course of external beam radiotherapy with a helical tomotherapy device

Our aim was to study the process of daily megavoltage (MV) CT imaging for a pulmonary malignancy. We report our initial experience with a helical tomotherapy device. The initial 20 MVCT images obtained during a course of external beam radiotherapy with the Hi*Art system (Tomotherapy Inc.) were analyzed. The treatment was designed to treat a lesion in the upper right lung to a total of 68 Gy at 2 Gy per fraction. The current study is limited to the first 20 of a total planned 34 fractions. The treatment was designed with the clinical target volume (CTV) defined on exhale and inhale simulation kilovoltage (KV) CT studies. The images were fused with a free breathing CT, since the treatment was to be delivered with free shallow breathing. The fusion was based on the registration of the spinal cord. The maximum extent of the tumor among the three CT scans was used to determine the CTV. A 10 mm margin was used to determine the planning target volume. Daily MVCT images were acquired with the Hi*Art system with the patient in the treatment position. The spinal cord was used to align the daily MVCT images to the simulation KVCT image. Each day, the set-up errors were recorded and corrected by the radiation therapists by adjusting the position of the treatment couch. The treatment was subsequently delivered with the Hi*Art system. Tumor dimensions were later measured on the daily MVCT images in the superior/inferior, anterior/posterior, and lateral dimensions. The tumor volume was calculated using the tumor dimensions as the volume constituting an ellipsoid structure. Figure 1 displays a sample of the treated tumor’s simulation KVCT images, and a sample of the tumor’s MVCT images acquired at the time of the 20th treatment. These corresponding images were obtained after adjusting for set-up errors by aligning on the spinal cord. A clear response is observed with shrinkage of the tumor. The size of the tumor documented on a daily basis clearly decreased over the 20 fractions. Fig. 2 documents the reduction in the tumor volume to approximately half of its original size in 20 fractions. Megavoltage imaging can be useful in two ways in the process of treating of a pulmonary malignancy with radiotherapy. It can be used to adjust patient position by daily aligning the patient’s observed internal anatomy to the expected patient position, thereby eliminating set-up errors. In addition, it can be used to measure previously unobserved tumor response during a radiotherapy course. Tumor shrinkage within the first 20 days of therapy was documented with daily megavoltage imaging.

Daily megavoltage CT image guidance in the treatment of localized prostate cancer

Daily megavoltage CT image guidance in the treatment of localized prostate cancer