Daily Image-guided Radiation Therapy Research Articles

Target point correction optimized based on the dose distribution of each fraction in daily IGRT

Purpose: To use daily re-calculated dose distributions for optimization of target point corrections (TPCs) in image guided radiation therapy (IGRT). This aims to adapt fractioned intensity modulated radiation therapy (IMRT) to changes in the dose distribution induced by anatomical changes. Methods: Daily control images from an in-room on-rail spiral CT-Scanner of three head-and-neck cancer patients were analyzed. The dose distribution was re-calculated on each control CT after an initial TPC, found by a rigid image registration method. The clinical target volumes (CTVs) were transformed from the planning CT to the rigidly aligned control CTs using a deformable image registration method. If at least 95% of each transformed CTV was covered by the initially planned D95 value, the TPC was considered acceptable. Otherwise the TPC was iteratively altered to maximize the dose coverage of the CTVs. Results: In 14 (out of 59) fractions the criterion was already fulfilled after the initial TPC. In 10 fractions the TPC can be optimized to fulfill the coverage criterion. In 31 fractions the coverage can be increased but the criterion is not fulfilled. In another 4 fractions the coverage cannot be increased by the TPC optimization. Conclusions: The dose coverage criterion allows selection of patients who would benefit from replanning. Using the criterion to include daily re-calculated dose distributions in the TPC reduces the replanning rate in the analysed three patients from 76% to 59% compared to the rigid image registration TPC.

Analysis of Online and Offline Head and Neck Image-guided Radiation Therapy

PurposeThe introduction of a daily image-guided radiation therapy (IGRT) program is an important step. It has implications for the radiation therapy team in terms of accuracy, workflow, and decision making. This study assesses how successful a radiation therapy department has been in using this technology and the accuracy of individual decision making when comparing online and offline image match data. MethodsTwenty intensity-modulated radiation therapy head and neck patients had their IGRT data assessed retrospectively. Online analysis was completed based on a 0-mm action threshold. Offline analysis was then conducted on the same data. Any discrepancies in decision making were then assessed. ResultsResults indicated that the treating radiation therapy team was able to image match consistently when benchmarked against their colleagues in the offline environment. Analysis of online versus offline corrections in each of the three orthogonal directions showed strong agreement. Further analysis revealed no statistically significant differences for systematic errors, whereas a statistically significant but small difference was present for random error. ConclusionIn this age of sophisticated equipment, daily IGRT is a valuable modality. However, the introduction of daily online IGRT inclusive of a 0-mm action threshold for head and neck IGRT requires careful consideration and evidence that such accuracy can be achieved. Ultimately, it is still the radiation therapist who must make the decision, which places great importance on the competency of the treating radiation therapy team.

Assessing the Efficiency and Consistency of Daily Image-guided Radiation Therapy in a Modern Radiotherapy Centre

BackgroundPatients at Radiation Oncology Queensland Toowoomba are treated using the assistance of daily image-guided radiation therapy (IGRT). Each patient's daily setup is exposed to a number of variables. This study investigates the effect that these variables have on the total time taken to analyse field placement and the total time taken for treatment, as well accessing setup error across a variety of treatment types. MethodsThis is a retrospective study of 80 patients across a variety of treatment sites where daily IGRT was undertaken using kilovoltage and megavoltage orthogonal images. Variables investigated include the treatment type, the imaging modality used, and the setup error of each session. Statistical analysis was then performed on the data. ResultsPatients being treated in the thoracic region had the greatest random setup error. The mean matching times were also longer for chest patients (197 seconds), whereas there were minimal differences in times regarding modality. Treatment times were longest for head and neck variables (399–405 seconds). ConclusionsPretreatment daily IGRT is beneficial to all patients and can be performed efficiently. Pelvic variables were the strongest performer, with fiducial markers providing the most consistent and rapid match times. Chest variables were the worst performer specifically regarding random setup error and match times indicating future work required on chest stabilization.

Image Approval Rates for Daily Image Guided Radiation Therapy (IGRT) in a Large, Academic Group Setting: Variation by Physician Familiarity With the Patient

Image-guided radiation therapy (IGRT) is used to improve the precision and accuracy of radiation therapy. It requires expertise from radiation therapists, who make positional adjustments based on daily imaging and customized match instructions, and from physicians who evaluate these images for quality assurance. Practice variations currently exist on physician image review. At the University of Pennsylvania, a single physician is assigned to evaluate all IGRT films for the department, rotating on a daily basis.

Setup Uncertainty as Determined by Daily Image Guided Radiation Therapy (IGRT) for Head-and-Neck Cancer (HNC): Evaluation of Safety and Implications on Defining Planning Target Volume (PTV) Margins

Prior to the implementation of daily IGRT for head and neck radiation, PTV expansions of 1 cm or greater around the clinical target volume (CTV) were commonly used. With IGRT, PTV margins are now routinely on the order of 3-5 mm; however, little data exist on the magnitude of daily setup error. This study aims to: (1) validate the safety of IGRT for HNC, (2) better define the magnitude of daily setup uncertainty, and (3) offer guidance in tailoring PTV expansions in settings where IGRT is unavailable.

A novel setup approach for helical tomotherapy in head and neck cancer: A case report

Head and neck cancers that are associated with high-risk factors have a poor prognosis. In such patients, post-operative radiochemotherapy is mandatory. The present study describes the case of a 71-year-old high-risk head and neck cancer patient who was not able to tolerate the supine position for the radiotherapy setup. A lateral immobilization with a head mask and a vacuum cushion was performed. The patient underwent daily computed tomography (CT)-guided radiation therapy [image-guided radiation therapy (IGRT)]. At nine months post-radiotherapy, the patient had no xerostomia and no swallowing dysfunction. However, the patient suffered a local recurrence and succumbed due to bleeding of the tumor a number of weeks after the recurrence. A recalculation of the actual delivered dose, taking the daily delivered dose into account, was performed. The recurrence occurred within the high-dose region. In selected cases of patients with head and neck cancers who are unable to tolerate the supine position, lateral positioning and high precision treatment is possible using daily IGRT.

Radiation Therapist Perspectives on Daily Image Guided Radiation Therapy (IGRT) for Palliative Patients

Radiation Therapist Perspectives on Daily Image Guided Radiation Therapy (IGRT) for Palliative Patients

WE-G-141-03: Diagnostic Quality CT with Soft Tissue Alignment for IGRT Can Safely Reduce Planning Margins For Prostate Cancer: Implication For SBRT

Purpose: To evaluate whether the use of daily diagnostic quality CT for image‐guided radiation therapy (IGRT) can safely reduce the planning margin using soft tissue alignment for prostate cancer treatment, which can be applied to SBRT. Methods: Nine patients with prostate cancer, who underwent IMRT and daily IGRT using the CT‐on‐rails system, were selected for this study. For all patients, a planning margin of 8mm/5mm posterior was used in the clinically approved plans. For this study, three additional plans were generated with planning margins of 6mm/4mm posterior, 4mm/2mm posterior, and 2mm uniform, resulting a total of four plans for each patient. On the first seven and last seven daily CTs, the prostate, bladder, and rectum were delineated by the same radiation oncologist. Subsequently, for each plan, the corresponding dose distribution on these daily CTs was calculated. The endpoint dose of the prostate was defined as the percentage of the volume receiving prescription dose of 78Gy (V78). V65 and V70 were used for the bladder and rectum, respectively. Results: For 8.7% of the first seven and 13.5% of the last seven fractions (n=126), the daily prostate volumes were 10% higher than those at simulation. Correspondingly, to achieve V78> 95%, the minimum margins of 4mm/2mm posterior and 6mm/4mm posterior were required for the first seven and last seven fractions, respectively. With margin reduced from 8mm/5mm posterior to 6mm/4mm posterior and 4mm/2mm posterior, the average bladder V65 decreased from 17.0±10.6% to 15.2±10.9% (p<0.01) and 9.9±7.4% (p<0.01), respectively. The average rectum V70 decreased from 15.2±8.9% to 13.6±7.9% (p<0.01) and 10.5±6.5% (p<0.01), respectively. Conclusion: A planning margin of 4mm/2mm posterior is sufficient for prostate SBRT with daily IGRT using soft tissue alignment with diagnostic quality CT. Drastic change in daily prostate volumes may increase the planning margin to 6mm/4mm posterior, requiring daily offline dose monitoring. Siemens Medical Solution

Superiority of a Real-time Planning Technique Over Image Guided Radiation Therapy for the Treatment of Primary Prostate Cancers

Image-Guided Radiation Therapy (IGRT) increases the precision and accuracy of external beam radiation therapy and provides a safer delivery of higher, more curative levels of radiation dose. However, when significant changes to internal anatomy occur, IGRT alone will not be able to compensate for the dose degradation that may occur from using the original IMRT plan. A real-time planning (RTP) or adaptive planning technique would enable the continued use of tight margins with the assurance of full target coverage and efficient normal tissue sparing. In its current stage, the adaptive planning technique requires a physician to manually redraw the involved structures before each treatment. These day-to-day contouring variations could potentially be significant and thus overestimate the superiority of the newly created adaptive IMRT plan. A total of 60 CT scans were used during the course of IMRT treatment of 6 prostate cancer patients. An in-room diagnostic CT-on-Rails was used for the purposes of performing daily IGRT in conjunction with an adaptive IMRT planning technique. While the data was analyzed for the standard purposes of IGRT, a brand new optimized IMRT plan was created based on a new set of structures drawn by the on-call physician. This resulting plan was then compared to the initial IMRT plan or “recalculated plan” which was copied onto the new data set and recalculated with the daily IGRT implemented shifts. The RTP plan consistently showed at least equal effectiveness in target coverage and normal-tissue sparing as the initial IMRT recalculated plan. In 25% of the cases, greater rectal sparing was seen in the RTP plan with the largest single case reducing V50 by 40% and V60 by 50%. The most frequent anomalies seen in this study, however, were target coverage of the prostate and PTV. While the RTP plan maintained minimum prescription dose coverage of 97% to the prostate, the recalculated IMRT plan's minimum dose routinely fell below 95%, and in 20% of the cases, it dropped below 90%. In terms of PTV minimum coverage, the recalculated IMRT plan showed consistent dose degradation with 90% of the cases falling below 90% of the prescribed dose. Daily changes of the internal anatomy warrant the need for RTP. To our knowledge, we are one of the first to implement this technique in our daily clinical practice on a consistent basis. RTP possesses the ability to treat other cancer sites besides the prostate and its full potential maybe realized once better and faster auto-segmentation programs are available to minimize manual contouring variability.

Role of image guided radiation therapy in obese patients with gynecologic malignancies

PurposeWe investigated the effect of body mass index on setup errors by analyzing daily shifts required in treating patients undergoing image guided radiation therapy (IGRT) for gynecologic malignancies. Methods and MaterialsForty successive patients treated with daily kV-based IGRT for gynecologic malignancies between April 2009 and June 2012 were identified. Directional setup corrections were analyzed according to patient body mass index. Random and systematic setup errors were calculated. Image acquisition dose was estimated by performing ionization chamber measurements in a phantom. ResultsObese patients had larger random setup errors, particularly in the right-left (R-L) direction, with a setup error of 7.6 mm, versus 3.9 mm for nonobese patients. The range of individual patient random errors in the R-L direction was 1.5 to 7.6 mm among nonobese patients versus 2.0 to 17.0 mm among obese patients (P = .03, F-test). For obese patients, daily IGRT prevented treating outside the planning target volume in 33% of fractions, versus 16% in the nonobese group (P = .001). The mean total image acquisition dose from daily kV-IGRT was approximately 3 cGy, versus 150 cGy if daily megavoltage portal imaging were used to correct for erratic setup errors. ConclusionsDaily kV-based IGRT in obese patients allows for correction of erratic setup error and minimizes excess dose from portal imaging.

Poster - Thur Eve - 28: Optimization of a prostate cancer IGRT protocol.

Our image-guided radiation therapy (IGRT) protocol for post-prostatectomy patients involves acquiring a kV cone beam computed tomography (CBCT) dataset at each fraction and shifting the treatment couch to align the surgical clips. This IGRT strategy requires significant resources, and delivers non-negligible dose to normal tissues. The objective of this work is to evaluate this IGRT protocol against two alternative strategies in terms of the dose-volume statistics for target and organ at risk regions. Our method involves deforming the planning CT to the CBCT dataset acquired at each fraction, computing dose on the deformed dataset, and inversely transforming the dose back onto the original planning CT dataset. The treatments of six patients were evaluated assuming three IGRT scenarios: no IGRT, daily IGRT using the clinically employed couch shifts, and alternating day IGRT. The doses delivered to the clinical target volumes are within approximately 3.2, 1.3, and 2.1% of the plan for the non-IGRT, daily, and alternating day IGRT protocols, respectively. Doses to relevant portions of the organs at risk deviate from the plan by up to 10.5, 13.1 and 10.7% for non-IGRT, daily IGRT, and alternating day IGRT protocols, respectively. Some cases do not differ significantly between IGRT and non-IGRT protocols in terms of cumulative DVHs, highlighting the difficult task of correcting prostate bed deformations via the treatment couch translations. In general, the alternating day IGRT protocol was found to result in a clinically insignificant deviation in delivered dose while providing a significant reduction in resource use and patient imaging dose.

Successful treatment of a free-moving abdominal mass with radiation therapy guided by cone-beam computed tomography: a case report

IntroductionBecause tumors in the abdomen can change position, targeting these tumors for radiation therapy should be done with caution; use of daily image-guided radiation therapy is advised.Case presentationWe report the case of a 72-year-old Caucasian man with recurrent mantle cell lymphoma who was referred for palliative radiation therapy for an abdominopelvic tumor. Computed tomography was used to generate images for radiation treatment planning. Comparison of those planning images with a positron emission tomography/computed tomography scan ordered during the planning period revealed that the tumor had moved from one side of the abdomen to the other during the three-day interval between scans. To account for this unusual tumor movement, we obtained a second set of planning computed tomography scans and used a Varian cone-beam computed tomography scanner with on-board imaging capability to target the tumor before each daily treatment session, leading to successful treatment and complete resolution of the mass.ConclusionAbdominal masses associated with the mesentery should be considered highly mobile; thus, radiation therapy for such masses should be used with the utmost caution. Modern radiation therapy techniques offer the ability to verify the tumor location in real time and shift the treatment ports accordingly over the course of treatment.

SU‐GG‐J‐65: CBCT Image Guidance for Head and Neck IMRT Radiation Therapy—A Study on the Tumor Coverage and Critical Structure Avoidance

Purpose: Cone beam computed tomography (CBCT) is becoming more prevalently used for image‐guided radiation therapy (IGRT). The ideal CBCT frequency and protocol will limit patient imaging dose, while insuring the accuracy of the radiation therapy (RT). We herein examine the effects of a possible CBCT imaging protocol on the delivered RT. Methods and Materials: The intensity modulated radiation therapy (IMRT) plans for 6 head and neck cancer patients undergoing IGRT with CBCT were retrospectively studied. The CBCT protocol called for daily IGRT for the first treatment week, followed by twice‐weekly imaging for the remaining RT fractions. The statistics of the IGRT patient set‐up shifts throughout the treatment course was used to create the treatment dose for non‐IGRT days using our in‐house TPS PlanUNC. The cumulative dose from the entire treatment course is the sum of the non‐IGRT daily doses and the planned daily doses for IGRT days. The resulting cumulative doses to the clinical target volume (CTV) and critical structures were examined.Results: The delivered treatment plans maintained the CTV treatment planning goals for 6 patients. Incorporating the IGRT shift statistics caused the CTV of one patient to be under dosed. The hotspots for the optic chiasm, cord, and brainstem were all within the treatment goals, and the absolute doses were within 5% of the planned RT. The dose constraints for the larynx and parotids were all maintained in the delivered treatment plans.Conclusions: The proposed CBCT protocol is sufficient for accurate RT delivery to the target volumes and critical structures of head and neck IMRT patients.

SU-FF-J-132: Calculation of Cumulative Dose for Daily CT-Guided Prostate Irradiation Using Deformable Image Registration

Purpose: To calculate the cumulative dose actually delivered to prostate cancer patients treated with daily image-guided radiation therapy (IGRT) and to quantify differences between planned and delivered doses to determine the need of better adaptive strategies. Method and Materials: Daily kV CT data, acquired for ten prostate cancer patients treated with daily IGRT repositioning based on soft-tissue registration on a CT on Rails (CTVision, Siemens), were analyzed. The dose actually delivered at each fraction was reconstructed by applying the original plan to the daily CT considering the repositioning shifts performed. Each daily CT was deformably registered to the plan CT using a newly developed deformable image registration tool. The resulting deformation field was used to map the delivered daily dose onto the planning CT. The cumulative dose over all treatment fractions was calculated and compared with the planed dose. Results: Accumulated prostate dose is consistently lower than the planned dose. For example, prostate D100 and D95 were reduced by 0.1–6.6% and 0.3–1.2%, respectively, as compared to their planed values. The variation in cumulative dose is reduced as the treatment progresses. The rectum had the largest and most erratic volume disparity, leading to frequent overdosing (mean dose increasing up to 11%). The bladder delivered dose is more likely to be an under-dose. Conclusion: The cumulative doses actually delivered to the prostate, rectum and bladder are different from their planned values, even with the soft-tissue-registration based daily repositioning, the most accurate IGRT, indicating a better adaptive strategy, such as re-planning, is required to account for interfraction anatomy variation observed in certain treatment fractions/patients.

Prostate bed localization with image-guided approach using on-board imaging: Reporting acute toxicity and implications for radiation therapy planning following prostatectomy

Objectives To report our experience using Image-Guided Radiation Therapy (IGRT) in patients undergoing post-prostatectomy irradiation. Methods Twenty-six patients were treated with radiotherapy following radical prostatectomy using Intensity Modulated Radiation Therapy (IMRT). Prostate bed localization was done using image guidance to align surgical clips relative to the reference isocenter on the planning digitally reconstructed radiographs. Assuming surgical clips to be surrogate for prostate bed, daily shifts in their position were calculated after aligning with the bony anatomy. Shifts were recorded in three dimensions. The acute toxicity was measured during and after completion of treatment. Results The average (standard deviation) prostate bed motion in anterior–posterior, superior–inferior and left–right directions were: 2.7 mm (2.1), 2.4 mm (2.1) and 1.0 mm (1.7), respectively. The majority of patients experienced only grade 1 symptoms, two patients had grade 2 symptoms and none had grade 3 or higher acute toxicity. Conclusions Daily IGRT is recommended for accurate target localization during radiation delivery to improve efficacy of treatment and enhance therapeutic ratio. Larger studies with longer follow-up are necessary to make definitive recommendations regarding magnitude of margin reduction around clinical target volume.

SU‐GG‐J‐110: Initial Assessment of Peripheral Dose for Image Guided Radiation Therapy (IGRT) Using Cone Beam CT

Purpose: With the growing use of intensity modulated radiation therapy (IMRT) combined with daily kilovoltage cone beam CT (CBCT) for image guided radiation therapy (IGRT) patient alignment, concerns are growing over the additional (typically unaccounted) dose outside the irradiated volume associated with the overall procedure. Published data exists for both the in‐field and organ specific doses that arise from IGRT. Additionally the peripheral dose associated with IMRT has been documented. This present study is intended to add to the data available on peripheral dose; particularly comparing IMRT and imaging dose for patient positioning with CBCT using an Elekta Synergy platform. Method and Materials: Measurements of both the in field and peripheral imaging doses were made in an anthropomorphic, solid water phantom where “thin” and “fat” patients were simulated with TLD detectors. These were then compared with peripheral dose measurements for typical prostate and head &amp; neck IMRT treatments with MOSFET detectors. Results: The IGRT dose in the imaged volume correlates well with published data, with 3 – 7 cGy being measured depending on the phantom configuration. The peripheral imaging dose measured was found to be approximately half the amount associated with a typical prostate IMRT treatment at a given point outside the field; with 0.2cGy and 0.4cGy being measured 25cm from the central axis. Thus, a typical patient receiving daily IMRT and IGRT for a fractionated course of radiation therapy for prostate cancer the total peripheral dose would be of the order of 0.2 – 0.5Gy. Conclusion: This study showed that an appreciable dose is delivered outside the field when daily CBCT IGRT is utilized. These doses should be taken into account when a patient image is requested, as all doses, above the prescription, require justification.

TU‐D‐M100J‐01: The Great Debate: The Future of IGRT Is…Megavolt CT…Kilovoltage CT…Ultrasound‐Based Hybrids…MRI Guidance…3D Deformable Image Registration

Introduction: Image‐guided radiation therapy (IGRT) promises to reduce or eliminate conventional limitations posed by geometric uncertainty, opening the way for dose escalation, margin reduction, innovative treatment techniques, hypofractionation, patient‐specific protocols, reduced normal tissue toxicity, and increased tumor control. This symposium presents a debate regarding the numerous technologies brought to bear in IGRT, including image — and information‐based technologies for therapy guidance. Topics and Speakers: The symposium features five distinguished speakers. Dr. J. Pouliot will present on the topic of megavoltage (MV) CT and cone‐beam CT (CBCT), reviewing the advances and advantages associated with imaging the patient in the treatment position with the therapy beam itself. Dr. J.‐J. Sonke will present the case for kilovoltage (kV) CBCT, discussing the latest advances in CBCT technology, image quality and accuracy, and protocols for offline, adaptive, and online 3D and 4D guidance. In light of the radiation risk posed by such modalities, Dr. W. Tome will present a strategy that hybridizes MV or kV CBCT (obtained at weekly intervals) with 3D ultrasound (obtained for daily guidance), wherein a (weekly) gold‐standard 3D ultrasound image provides close correspondence to CBCT. In this way, conventional uncertainties in ultrasound‐based alignment are minimized, and accurate daily ultrasound guidance is achieved with a ∼80% reduction in cumulative dose to the patient. Dr. J. Lagendijk will offer an innovative approach in which the treatment machine is fully integrated with magnetic resonance imaging (MRI), allowing precise soft‐tissue visualization for online guidance, verification, monitoring, and biological optimization. The technological challenges and advances in such development are described, including potential applications in treatment of the prostate, cervix, liver, and lung. Finally, Dr. K. Brock will argue that the future of IGRT lies not within a given imaging modality, but in the use of multiple structural and functional modalities geometrically resolved by means of deformable modeling. By combining diagnostic quality images with daily IGRT, accurate tumor targeting can be achieved in a manner that accounts not only for daily setup error but also morphological deformation and physiological change over the course of treatment. Format: The debate will consist of three rounds: 1.) a summary / overview of each IGRT approach; 2.) presentation, debate, and rebuttal regarding the role of each approach in the future of IGRT; and 3.) open format question and answer from the panel and audience. Time and technology permitting, a winner will be informally determined by feedback from the audience.

2276: Daily Image Guided Radiation Therapy for Prostate Cancer: An assessment of treatment plan reproducibility

2276: Daily Image Guided Radiation Therapy for Prostate Cancer: An assessment of treatment plan reproducibility

SU‐FF‐I‐94: Intensity‐Based 3D/3D Image Registration for Prostate Localization On CBCT Images

Purpose: We have developed an intensity‐based 3D/3D image registration program for automatic prostate localization on CBCT images. The main application of this program will be the online and offline analysis of CBCT images for daily image guided radiation therapy. Methods and materials: The program provides with two different ways of defining region of interests (ROI); rectangular ROI and irregular ROI. The rectangular ROI is generally defined around the pelvic bone for bone matching, and the irregular ROI is around prostate and seminal vesicle for prostate matching. The ROIs are defined on the simulation CT images, and the volumes within which are registered with daily CBCT images. Registration is accomplished by maximizing the similarity within the ROIs. Uphill simplex method is utilized for the maximization, and it stops when the number of iteration reaches a preset threshold, or when the simplex shrinks to a preset simplex radius. Then, the users verify the registration result in three different overlay views, and the treatment machine correction shifts are calculated from the verified registration. Results: As a similarity function, mutual information provided very robust and accurate registrations for the pelvic bone registration. In one of our preliminary studies, the prostate registrations with gradient correlation, gradient difference, and pattern intensity showed 1.7±1.0mm average error, estimated from calcification mismatches. The registrations were also compared with the average manual registrations conducted by four clinicians. The isocenter difference in each dimension was around 1.0±0.7mm. The execution time, except the ROI definition and image loading, was around 7 seconds for each pelvic bone and prostate registration on a 1.8 MHz PC. Conclusions: Further thorough experiments are due, but the preliminary results indicated that the newly developed program may be able to provide accurate, robust, and fast online and offline CBCT analysis for image guided radiation therapy.

A Dose Reconstruction Analysis for Determining Optimal Treatment Margins for Image-Guided and Daily Adaptive Radiation Therapy

Purpose/Objective: The utilization of daily Image-Guided Radiation Therapy (IGRT) for the reduction of treatment margins in prostate cancer has been the subject of intense research efforts. Bowel gas, feces, and urine can cause deformation of the prostate and/or seminal vesicles that requires an unknown amount of margin. The purpose of this retrospective study was 1.) Perform dose reconstruction for thirty prostate patients imaged and treated daily with helical tomotherapy and 2.) Calculate the minimum treatment margins required for IGRT treatment delivery. Materials/Methods: A series of retrospective treatment plans were created for 30 prostate patients. The gross tumor volume (GTV) for the first 50 Gy was defined as the entire prostate gland plus the seminal vesicles. A series of planning target volumes (PTVs) were created with gradually decreasing margins: 10-mm/5-mm (5-mm posterior, and 10-mm in all other directions), 8-mm/4-mm, 6-mm/3-mm, 4-mm/2-mm, and 2-mm/0-mm. Archived Megavoltage CT (MVCT) images were used in this dose reconstruction analysis. The MVCT images were initially acquired prior to treatment delivery for CT-guided localization and patient repositioning. Retrospectively, the prostate, seminal vesicles, and rectum were manually contoured on each MVCT image. The delivered doses were then recalculated using the treatment delivery sequence from each of the five treatment plans. The smallest margins required for 100 percent of the prostate or seminal vesicles to be covered by the prescribed dose for 95 percent of the delivered fractions were measured for each patient. Results: The smallest prostate margins for all patients was 2-mm posterior and 4-mm in all other directions (4-mm/2-mm). The prostate was slightly deformed in patients with excessive bowel gas. However, the CT-guided localization and patient repositioning software on the delivery system generally compensated for this deformation. In contrast, the seminal vesicles were substantially more deformable. The smallest seminal vesicle margins were 6-mm/3-mm for 66% of the patients, 8-mm/4-mm for 31% of the patients, and greater than 10-mm/5-mm for one patient. This patient had a rectal diameter that varied from 2 to 8-cm during the course of treatment. During fractions where the rectum was extremely full, the seminal vesicles were severely deformed relative to the initial treatment planning CT. Ideally, a library of treatment plans with a similar range of margins could be utilized prospective at the time of treatment. This simple form of daily adaptive therapy could be easily implemented by prospectively selecting the smallest margins for each fraction. The cost of performing this type of adaptive therapy is the time requirement for creating the additional treatments plans. Tools for automating the creation of the reduced margin plans are currently being developed. Conclusions: A dose reconstruction analysis has been performed for patients treated with CT based IGRT. Treatment margins of 6-mm/3-mm would have been acceptable for 66% of the analyzed patients. Patients with large fluctuations in bowel gas will require larger planning margins, even with IGRT.