Daily Megavoltage CT Research Articles

Assessment of rectal distention in radiotherapy of prostate cancer using daily megavoltage CT image guidance

Purpose Assessment of rectal distention in a group of patients who are not receiving daily rectum emptying procedures during a course of prostate cancer radiotherapy to investigate which patients could benefit from daily rectum emptying. Methods and materials Eighteen patients underwent daily megavoltage CT (MVCT) scanning with positioning based on bony anatomy. Emptying the rectum was only performed before planning CT and not during the actual treatment. The rectal average cross-sectional area (CSA) was determined on the MVCTs. The relative CSA (CSA rel) was defined as CSA on MVCT / CSA on planning CT. Additional prostate soft tissue matching was performed to verify the influence of rectal distention on prostate motion. Results Two distinct subgroups could be defined a posteriori. One group had a limited and stable rectal distention with a CSA (mean ± SD) of 6.6 ± 2.1 cm 2, in contrast with a second group with large and variable rectal filling with a CSA of 9.5 ± 3.7 cm 2 ( p < 0.01). Mean anterior-posterior prostate displacement was 0.4 ± 2.4 mm in the stable group versus -2.4 ± 6.1 mm in the unstable group ( p < 0.01). A mean CSA rel of 1.35 of the first 3 days as cut-off value allowed for a correct a priori classification of 90% and 85% of the patients from groups 1 and 2, respectively. Conclusion Based on a few measurements of the CSA by daily MVCT imaging at the first days of treatment, rectum emptying may be omitted in part of the patients.

Deformable registration of abdominal kilovoltage treatment planning CT and tomotherapy daily megavoltage CT for treatment adaptation

In adaptive radiation therapy the treatment planning kilovoltage CT (kVCT) images need to be registered with daily CT images. Daily megavoltage CT (MVCT) images are generally noisier than the kVCT images. In addition, in the abdomen, low image contrast, differences in bladder filling, differences in bowel, and rectum filling degrade image usefulness and make deformable image registration very difficult. The authors have developed a procedure to overcome these difficulties for better deformable registration between the abdominal kVCT and MVCT images. The procedure includes multiple image preprocessing steps and a two deformable registration steps. The image preprocessing steps include MVCT noise reduction, bowel gas pockets detection and painting, contrast enhancement, and intensity manipulation for critical organs. The first registration step is carried out in the local region of the critical organs (bladder, prostate, and rectum). It requires structure contours of these critical organs on both kVCT and MVCT to obtain good registration accuracy on these critical organs. The second registration step uses the first step results and registers the entire image with less intensive computational requirement. The two-step approach improves the overall computation speed and works together with these image preprocessing steps to achieve better registration accuracy than a regular single step registration. The authors evaluated the procedure on multiple image datasets from prostate cancer patients and gynecological cancer patients. Compared to rigid alignment, the proposed method improves volume matching by over 60% for the critical organs and reduces the prostate landmark registration errors by 50%.

A Comparison of daily megavoltage CT and ultrasound image guided radiation therapy for prostate cancer

In order to quantify the differences between ultrasound-imaging and megavoltage-CT (MVCT) daily prostate localization in prostate-cancer radiotherapy and their dosimetric impacts, daily shifts were analyzed for a total of 140 prostate cancer patients; 106 positioned using ultrasound-based imaging [B-mode Acquisition and Targeting (BAT)], and 34 using the MVCT from a TomoTherapy Hi-Art unit. The shifts indicated by the two systems were compared statistically along the right/left (R/L), superior/inferior (S/I), and anterior/posterior (A/P) directions. The systematic and random variations among the daily alignments were calculated. Margins to account for these shifts were estimated. The mean shifts and standard deviations along the R/L, S/I, and A/P directions were -0.11 +/- 3.80, 0.67 +/- 4.67, and 2.71+/- 6.31 mm for BAT localizations and -0.98 +/- 5.13, 0.27 +/- 3.35, and 1.00 +/- 4.22 mm for MVCT localizations, respectively. The systematic and random variations in daily shifts based on MVCT were generally smaller than those based on BAT, especially along the A/P direction. A t-test showed this difference to be statistically significant. The planning target volume margins in the A/P direction estimated to account for daily variations were 8.81 and 14.66 mm based on MVCT and BAT data, respectively. There was no statistically significant difference in the daily prostate movement pattern between the first few fractions and the remaining fractions. Dosimetric comparison of MVCT and BAT prostate alignments was performed for seven fractions from a patient. The degradation from the plan caused by the MVCT alignment is trivial, while that by BAT is substantial. The MVCT technique results in smaller variations in daily shifts than ultrasound imaging, indicating that MVCT is more reliable and precise for prostate localization. Ultrasound-based localization may overestimate the daily prostate motion, particularly in the A/P direction, negatively impacting prostate dose coverage and rectal sparing.

Daily Megavoltage CT Registration on Adaptive Radiotherapy with Helical Tomotherapy

Daily Megavoltage CT Registration on Adaptive Radiotherapy with Helical Tomotherapy

Evaluation of geometric changes of parotid glands during head and neck cancer radiotherapy using daily MVCT and automatic deformable registration

Background and purpose To assess and evaluate geometrical changes in parotid glands using deformable image registration and megavoltage CT (MVCT) images. Methods A deformable registration algorithm was applied to 330 daily MVCT images (10 patients) to create deformed parotid contours. The accuracy and robustness of the algorithm was evaluated through visual review, comparison with manual contours, and precision analysis. Temporal changes in the parotid gland geometry were observed. Results The deformed parotid contours were qualitatively judged to be acceptable. Compared with manual contours, the uncertainties of automatically deformed contours were similar with regard to geometry and dosimetric endpoint. The day-to-day variations (1 standard deviation of errors) in the center-of-mass distance and volume were 1.61 mm and 4.36%, respectively. The volumes tended to decrease with a median total loss of 21.3% (6.7–31.5%) and a median change rate of 0.7%/day (0.4–1.3%/day). Parotids migrated toward the patient center with a median total distance change of −5.26 mm (0.00 to −16.35 mm) and a median change rate of −0.22 mm/day (0.02 to −0.56 mm/day). Conclusion The deformable image registration and daily MVCT images provide an efficient and reliable assessment of parotid changes over the course of a radiation therapy.

Sci-Thurs PM: Delivery-07: Evaluation of prospects to use daily megavoltage CT studies for adaptive radiotherapy

To evaluate gross tumor volume (GTV) changes for non-small cell lung cancer (NSCLC) patients using daily megavoltage CT (MVCT) studies acquired before each treatment fraction on helical tomotherapy, and to relate the potential benefit of adaptive image-guided radiotherapy to changes in GTV. 17 patients were prescribed 30 fractions of radiotherapy on helical tomotherapy for NSCLC at London Regional Cancer Program from December 2005 to March 2007. The GTV was contoured on the daily MVCT studies of each patient. Adapted plans were created using merged MVCT-kVCT image sets to investigate the advantages of replanning for patients with differing GTV regression characteristics. The average GTV change observed over 30 fractions was -38%, ranging from -12 to -87%. No significant correlation was observed between GTV change and patient's physical or tumor features. The pattern of GTV changes of the 17 patients could be broadly divided into 3 groups with distinctive potential for benefit from adaptive planning. GTV changes are difficult to predict quantitatively based on patient or tumor characteristics. If changes do occur, there are points in time during the treatment course when it may be appropriate to adapt the plan to improve sparing of normal tissues. If the GTV decreases by greater than 30% at any point in the first twenty fractions of treatment, adaptive planning is appropriate to further improve the therapeutic ratio.

SU‐GG‐T‐425: Radiobiologically Quantified Treatment Setup Uncertainties Based On Megavoltage (MV) CT Imaging

Purpose: To evaluate and compare the expected effectiveness of the planned and delivered dose distributions in prostate cancer radiotherapy. The average setup uncertainties were determined by using the on‐board megavoltage computed tomography (MVCT) capabilities of the tomotherapy HiArt unit. Method and Materials: Co‐registrations between daily MVCT and planning CT were used. Before the delivery of each treatment an MVCT image was acquired. The therapists registered the MVCT image with the planning kilovoltage computed tomography (kVCT) images. The registration criteria were based on bony anatomy and contoured regions of interest. For a typical prostate cancer patient the dose distributions of the Helical Tomotherapy plan and that of the average shifted delivery registration were used. The dose distributions were compared based on dosimetric criteria and the biologically effective uniform dose (BEUD) together with the complication‐free tumor control probability (P+). Results: The average shifts that were observed are 10.7 mm for the vertical, 4.0 mm for the longitudinal and 0.5 mm for the lateral directions, respectively. At the optimum dose levels of the planned and delivered dose distributions, the P+ values are 84.7% and 84.0%, respectively. The total control probabilities, PB are 93.0% and 92.9%, whereas the total complication probabilities, PI are 8.3% and 8.9%. More specifically, the response probabilities of the different tissues are 95.6% and 95.5% for the GTV, 97.4% and 97.3% for the seminal vesicles, 0.8% and 0.7% for bladder and 7.6% and 8.3% for rectum. Conclusion: It is shown that the intra‐fraction movement of tumors can be reduced by using the MVCT co‐registration. This appears to be a powerful tool, which can result in improved sparing of critical structures, while delivering high doses to the target. The use of P+ — BEUD diagrams to compare similar treatment plans may show that in radiobiological terms they may be quite different.

Evidence of Limited Motion of the Prostate by Carefully Emptying the Rectum as Assessed by Daily MVCT Image Guidance with Helical Tomotherapy

To assess setup and organ motion error by means of analysis of daily megavoltage computed tomography (MVCT) of patients treated with hypofractionated helical tomotherapy (71.4-74.2 Gy in 28 fractions). Data from 21 patients were analyzed. Patients were instructed to empty the rectum carefully before planning CT and every morning before therapy by means of a self-applied rectal enema. The position of the prostate was assessed by means of automatic bone matching (BM) with the planning kilovoltage CT (BM, setup error) followed by a direct visualization (DV) match on the prostate. Deviations between planning and therapy positions referred to BM and BM + DV were registered for the three main axes. In case of a full rectum at MVCT with evident shift of the prostate, treatment was postponed until after additional rectal emptying procedures; in this case, additional MVCT was performed before delivering the treatment. Data for 522 fractions were available; the impact of post-MVCT procedure was investigated for 17 of 21 patients (410 fractions). Prostate motion relative to bony anatomy was limited. Concerning posterior-anterior shifts, only 4.9% and 2.7% of fractions showed deviation of 3 mm or greater of the prostate relative to BM without and with consideration of post-MVCT procedures, respectively. Interobserver variability for BM + DV match was within 0.8 mm (1 SD). Daily MVCT-based correction is feasible. The BM + DV matching was found to be consistent between operators. Rectal emptying using a daily enema is an efficient tool to minimize prostate motion, even for centers that have not yet implemented image-guided radiotherapy.

Actual Dose Variation of Parotid Glands and Spinal Cord for Nasopharyngeal Cancer Patients During Radiotherapy

For intensity-modulated radiotherapy of nasopharyngeal cancer, accurate dose delivery is crucial to the success of treatment. This study aimed to evaluate the significance of daily image-guided patient setup corrections and to quantify the parotid gland volume and dose variations for nasopharyngeal cancer patients using helical tomotherapy megavoltage computed tomography (CT). Five nasopharyngeal cancer patients who underwent helical tomotherapy were selected retrospectively. Each patient had received 70 Gy in 35 fractions. Daily megavoltage CT scans were registered with the planning CT images to correct the patient setup errors. Contours of the spinal cord and parotid glands were drawn on the megavoltage CT images at fixed treatment intervals. The actual doses delivered to the critical structures were calculated using the helical tomotherapy Planned Adaptive application. The maximal dose to the spinal cord showed a significant increase and greater variation without daily setup corrections. The significant decrease in the parotid gland volume led to a greater median dose in the later phase of treatment. The average parotid gland volume had decreased from 20.5 to 13.2 cm3 by the end of treatment. On average, the median dose to the parotid glands was 83 cGy and 145 cGy for the first and the last treatment fractions, respectively. Daily image-guided setup corrections can eliminate significant dose variations to critical structures. Constant monitoring of patient anatomic changes and selective replanning should be used during radiotherapy to avoid critical structure complications.

Do Anatomic Changes During Radiation Impact Normal Tissue and Target Dosimetry? An Analysis of Calculated Cumulative Dose in Head and Neck Patients

To analyze the magnitude of the dosimetric impact of anatomic changes occuring in H&N cancer patients throughout a course of radiotherapy. Eight H&N cancer patients treated definitively on a helical tomotherapy unit constituted the study population. Intensity modulated radiation therapy was planned to be delivered. Planning was performed using the following parameters: 1) Prescription gross target volume (GTV) dose: 64.0 to 74.4 Gy delivered at 2.12–2.00 Gy per fraction; 2) 54–57 Gy to cover ≥95% of low risk PTV; 3) 60 Gy to cover ≥95% of high risk PTV; and 4) <30% of parotid glands receiving 26 Gy. In cases with ipsilateral disease, planning was prioritized to spare the contralateral parotid. Daily MVCT imaging was performed on all cases as a part of routine image guided radiation therapy. At total of 331 daily megavoltage CT scans were evaluated. Daily MVCT scans were processed through a novel deformable image registration (DIR) algorithm to register daily megavoltage CT images to the original planning simulation kilovotage CT image. The deformable registration process was verified by reviewing the adequacy of automatically generated contours on the daily megavoltage CT images. Cumulative dose volume histograms for target volumes and organ at risk volumes were generated and the relative deviation of these DVHs from dosimetry conceived during initial treatment planning was carefully analyzed. The DIR algorithm adequately predicted the location of target and organs at risk volumes in >98% of daily MVCTs reviewed. The average D95 of the high dose target volumes was 68.8 Gy at initial planning versus 67.5 Gy after DIR was applied. In all cases, the difference between planned and actual delivered dose for the target areas was less than 3%. The average parotid dose was higher than initially planned (Fig 1); i.e.113% of planned dose. During the initial planning process in all patients, the dose to at least 1 parotid was kept <25–30 Gy. However, after cumulative DVH evaluation, in 50% of patients, average parotid dose changed from 25–30 Gy (at initial planning) to exceed 30 Gy. Authors have described anatomic changes during a course of radiation therapy1. Using a novel deformable registration algorithm, the impact of these changes on actual delivered doses was evaluated. Anatomic changes throughout the course of treatment had little effect on delivered target doses either to the primary or nodal areas. However, the actual delivered parotid doses were substantially higher than what was planned, exceeding the significant threshold of 25–30 Gy in spared parotids in 50% of cases.1.Barker IJROBP 2004

SU-FF-J-108: Dose Variation of Critical Structures in Daily Treatment of Nasopharyngeal Cancer Patients Using Helical Tomotherapy Megavoltage CT System

Purpose: To study daily spinal cord and parotid gland dose variation for nasopharyngeal cancer patients undergoing helical tomotherapy treatments, as a result of patient weight loss during the course of treatments.Method and Materials: Five nasopharygeal cancer patients who received helical tomotherapy treatments were selected. At the end of the treatments, patients lost an average weight of 6.3 kg, or 8.4% relative to the weight at the beginning of the treatments. Daily megavoltage CT (MVCT) scans were performed prior to each treatment fraction, and MVCT images were registered with planning CTimages for patient setup corrections. Contours for spinal cord and parotid glands were drawn on MVCT images throughout the treatment course. Based on daily MVCT images, actual dose delivered to the critical structures were calculated using the Planned Adaptive application in the helical tomotherapy treatment planning system. The maximum dose to the spinal cord and the median doses to the parotid glands were analyzed. Results: The maximum dose to the spinal cord, after setup corrections, did not show significant variation as a result of patient weight loss. However, the median doses to the parotid glands showed significant increase due to facial tissue shrinkage during treatments. The average median dose to the parotid glands for the five patients were 83 cGy for the first treatment fraction, and it was 143 cGy per fraction for the last fraction. The average parotid gland volume decreased from 20.5 cc to 13.2 cc during treatments.Conclusion: Daily patient setup corrections based on MVCT image registrations prevented significant dose variation to the spinal cord due to setup uncertainty. During the course of radiotherapy, as a result of body weight loss, the parotid glands typically receive significantly higher dose compared to the dose dictated in the treatment plan.

SU-FF-J-45: A Comparison of Daily Megavoltage CT and Ultrasound Imaging Guided Radiation Therapy for Prostate Cancer

Purpose: To compare the prostate localization capabilities of ultrasound imaging and megavoltage CT (MVCT) for on-line daily image-guided radiotherapy (IGRT) of the prostate cancer. Method and Materials: Daily shifts were analyzed to account for inter-fractional variations collected from a total of 129 prostate cancer patients, 106 with ultrasound-based imaging (BAT, B-mode Acquisition and Targeting) and 23 with MVCT of a TomoTherapy (HiArt) unit. The shifts indicated by the two systems were compared statistically along the right/left (R/L), superior/inferior (S/I), and anterior/posterior (A/P) directions. The systematic and random variations among the daily alignments were calculated. Margins to account for these shifts were estimated. The daily shifts from the first 25 treatment fractions and from the remaining fractions were compared for both systems to examine whether a difference in the prostate motion pattern exists between the first 25 fractions and the rest. Results: The mean shifts and standard deviations along the R/L, S/I, and A/P directions were −0.11±3.80mm, 0.67±4.67mm, and 2.71±6.31mm for BAT localizations and −0.88±5.36mm, 0.41±3.38mm, and 0.91±3.33mm for MVCT localizations, respectively. It is clear that the systematic and random variations in daily shifts based on MVCT were generally smaller than those based on BAT, especially along the A/P direction. A t-test showed this difference to be statistically significant. The PTV margins estimated to account for daily variations were 6.69 mm and 14.66 mm based on MVCT and BAT data, respectively. There was no statistically significant difference in the daily prostate movement pattern between the first 25 fractions and the remaining. Conclusion: The MVCT technique results in smaller variations in daily shifts than ultrasound imaging, indicating that MVCT is more reliable and precise for prostate localization. Ultrasound-based localization may overestimate the daily prostate motion, particularly in the A/P direction. This work is supported partially by MCW Cancer Center Fotsch Foundation.

Initial experience in treating lung cancer with helical tomotherapy

Helical tomotherapy is a new form of image-guided radiation therapy that combines features of a linear accelerator and a helical computed tomography (CT) scanner. Megavoltage CT (MVCT) data allow the verification and correction of patient setup on the couch by comparison and image registration with the kilovoltage CT multi-slice images used for treatment planning. An 84-year-old male patient with Stage III bulky non-small cell lung cancer was treated on a Hi-ART II tomotherapy unit. Daily MVCT imaging was useful for setup corrections and signaled the need to adapt the delivery plan when the patient’s anatomy changed significantly.

2424: Assessment of Setup Uncertainties for Head-and-Neck Cancer Patients Using Daily Megavoltage CT Imaging

2424: Assessment of Setup Uncertainties for Head-and-Neck Cancer Patients Using Daily Megavoltage CT Imaging

Daily variations in delivered doses in patients treated with radiotherapy for localized prostate cancer

The aim of this work was to study the variations in delivered doses to the prostate, rectum, and bladder during a full course of image-guided external beam radiotherapy. Ten patients with localized prostate cancer were treated with helical tomotherapy to 78 Gy at 2 Gy per fraction in 39 fractions. Daily target localization was performed using intraprostatic fiducials and daily megavoltage pelvic computed tomography (CT) scans, resulting in a total of 390 CT scans. The prostate, rectum, and bladder were manually contoured on each CT by a single physician. Daily dosimetric analysis was performed with dose recalculation. The study endpoints were D95 (dose to 95% of the prostate), rV2 (absolute rectal volume receiving 2 Gy), and bV2 (absolute bladder volume receiving 2 Gy). For the entire cohort, the average D95 (+/-SD) was 2.02 +/- 0.04 Gy (range, 1.79-2.20 Gy). The average rV2 (+/-SD) was 7.0 +/- 8.1 cc (range, 0.1-67.3 cc). The average bV2 (+/-SD) was 8.7 +/- 6.8 cc (range, 0.3-36.8 cc). Unlike doses for the prostate, there was significant daily variation in rectal and bladder doses, mostly because of variations in volume and shape of these organs. Large variations in delivered doses to the rectum and bladder can be documented with daily megavoltage CT scans. Image guidance for the targeting of the prostate, even with intraprostatic fiducials, does not take into account the variation in actual rectal and bladder doses. The clinical impact of techniques that take into account such dosimetric parameters in daily patient set-ups should be investigated.

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.

WE‐E‐224C‐07: Image‐Guided Helical Tomotherapy for Localized Prostate Cancer: Technique and Initial Clinical Observations

Purpose: The purpose of this study was to develop a technique for daily CT based IGRT, and to report clinical observations on treatment planning, imaging and delivery based on the first two years of experience. Method and Materials: Patients with previously untreated stage T1 through T3 biopsy‐proven adenocarcinoma of the prostate were considered eligible for treatment with daily CT guided helical tomotherapy. The prostate was targeted daily using megavoltage CT (MVCT) images that were fused with treatment planning CT images based on anatomical alignments. All patients were treated at 2 Gy per fraction to 76 to 78 Gy (mean 76.7 Gy). Results: Thirty‐three prostate patients were planned, imaged, and treated as part of this study for a total of 1266 CT guided fractions. The prostate, rectum, bladder, femoral heads, and pubis symphysis were visible in one or more slices for all 1266 MVCT image sets. The typical range of measured prostate displacement in this study was 2–10 mm (3.4 mm standard deviation) in the anterior‐posterior direction, 2–8 mm (3.7 mm standard deviation) in the lateral direction, and 1–6 mm (2.4 mm standard deviation) in the superior‐inferior direction. The obese patients in this study had a substantially larger lateral variation (8.2 mm standard deviation) due to mobility of the external skin marks. Conclusions: A technique has been developed, and clinical implemented for daily MVCT based image‐guided radiation therapy. The level of conformal avoidance increased with treatment planning experience, and the level of conformal avoidance can be greater than fix‐gantry based intensity modulation.

Feasibility report of image guided stereotactic body radiotherapy (IG-SBRT) with tomotherapy for early stage medically inoperable lung cancer using extreme hypofractionation

We report on the technical feasibility, dosimetric aspects, and daily image-guidance capability with megavoltage CT (MVCT) of stereotactic body radiotherapy (SBRT) using helical tomotherapy for medically inoperable T1/2 N0 M0 non-small cell lung cancer. Nine patients underwent treatment planning with 4D-CT in a double vacuum based immobilization system to minimize tumor motion and to define a lesion-specific 4D-motion envelope. Patients received 60 Gy in 5 fractions within 10 days to a PTV defined by a motion envelope plus a 6 mm expansion for microscopic extension and setup error using tomotherapy, with daily pretreatment MVCT image guidance. The primary endpoint was technical feasibility. Secondary endpoints were defining the acute and sub-acute toxicities and tumor response. Forty three of 45 fractions were successfully delivered, with an average delivery time of 22 minutes. MVCT provided excellent tumor visualization for daily image guidance. No significant tumor regression was observed on MVCT in any patient during therapy. Median mean normalized total doses were: tumor 117 Gy10; residual lung 9 Gy3. Maximum fraction-size equivalent dose values were: esophagus 5 Gy39; cord 7 Gy36. No patient experienced ≥ grade 2 pulmonary toxicity. 3 complete, 4 partial and 2 stable responses were observed, with <3 months median follow-up. The mean tumor regression is 72%. SBRT using tomotherapy proved to be feasible, safe and free of major technical limitations or acute toxicities. Daily pretreatment MVCT imaging allows for precise daily tumor targeting with the patient in the actual treatment position, and therefore provides for precise image guidance.

Daily Dose Recalculation Using Megavoltage CT Images During Tomotherapy for Localized Prostate Cancer: Dosimetric Analysis for the Prostate Gland, Rectum and Bladder for the Full Treatment Course

Purpose/Objective: To evaluate the dosimetric consequences of daily anatomical changes on the prostate gland, rectum and bladder. Materials/Methods: Five prostate cancer patients treated to 78 Gy at 2 Gy per fraction with helical tomotherapy constituted the study sample. Prior to each daily fraction, intraprostatic markers detected on a megavoltage CT (MVCT) are registered with the simulation kilovoltage CT (kVCT) for image guidance. The daily MVCTs can also be used to recalculate delivered doses. The recalculation technique uses the intended delivery and the MVCT image to calculate the dose distribution in the anatomy of the day, hence allows a full evaluation of the dosimetric impact of anatomical changes throughout the treatment course. This process was previously tested and verified in phantom studies to be accurate within 0.5%. The MVCT numbers were calibrated in terms of electron densities allowing the use of MVCTs for dose calculations. The entire treatment course (39 fractions per patient) was retrospectively analyzed for all 5 patients. A total of 195 MVCT sets were studied. The rectum and bladder were recontoured on each of the 195 daily MVCTs. The prostate contours were transferred from the original kVCT image set and altered only in position and shape if necessary. For each treatment fraction, DVHs were calculated based on the new contours and recalculated dose distributions. The volume of rectum and bladder that received the prescription dose (in ccs) and the dose to 95% of the prostate volume (D95) were scored. Results: Fig. 1, 2 and 3 show the average (±S.D) and range of daily rectal and bladder volumes receiving full prescription dose and the D95 for each patient. It is clear that the prostate D95 was remarkably constant throughout the course of treatment for all patients indicating adequate coverage of the target regardless of daily variations in the patients′ pelvic anatomy. However, there was remarkable variation in the rectal volumes receiving the prescription fraction dose, varying in some patients (e.g. patient #2) more than in others (e.g. patient #1). There was a trend for higher rectal doses with rectal distention. Distention pushed a larger rectal volume into the high dose region that was occupied by the prostate on the planning kVCT. The prostate D95 also tended to increase with rectal distention. This may be due to the presence of rectal gas and a subsequent reduction in attenuation for beams entering the prostate posteriorly. In the patient with the largest variation in rectal volumes, the degree of rectal distention was unpredictable. Bladder volumes changes were also noticeable, but of lesser magnitude compared to the rectum. Conclusions: Daily anatomical deformations have dosimetric consequences for the rectum, bladder and prostate. Rectal deformation is the most consequential. Daily recalculation of delivered doses using megavoltage images enables a complete dosimetric evaluation of an entire treatment course with external beam radiotherapy.