Interfractional Variations Research Articles

Estimation of inter-fractional variations in interstitial multi-catheter breast brachytherapy using a hybrid treatment delivery system

PurposeIrradiation of the tumor bed using interstitial multi-catheter brachytherapy is one of the treatment options for breast cancer patients. In order to ensure the planned dose delivery an advanced quality intervention method using an electromagnetic tracking (EMT) system is presented. The system is used to assess inter-fractional variations within the framework of a patient study. Methods and materialsUntil now 41 patients were included in the study for the evaluation and overall 355 EMT measurements were performed. The catheter traces are measured automatically and sequentially using an afterloader prototype (Flexitron, Elekta, Veenendaal, The Netherlands) equipped with an EMT sensor. The implant geometry is tracked directly after implantation, after CT imaging and after each irradiation fraction. The acquired data is rigidly registered to the catheter traces defined in the treatment plan and the dwell positions (DP) are reconstructed. DPs defined in treatment planning serve as reference. Breathing motion was corrected and recorded using three reference 6DoF sensors placed on the patients’ skin. The Euclidean distance between the planned and reconstructed DPs provides information about possible inter-fractional deviations. Further, the influence of various factors on the occurrence of large deviations was investigated, like the patients’ age, the length of the catheter, the breast volume, etc. ResultsOver all patient measurements a median Euclidean distance of 2.19 mm was determined between the reconstructed DPs and the reference DPs. The median deviation combining all datasets was minimal (1.67 mm) at the measurement directly after CT imaging. The deviations between the different fractions have a median distance of 2.31 mm which could be improved to 2.05 mm by adapting the treatment plan according to the follow-up CT. No correlation between the distance to the skin, ribs, mammilla or the breast volume and the occurrences of large deviations was found. The largest deviations were determined in the upper inner quadrant of the breast. ConclusionThe afterloader prototype could be well integrated into the clinical routine and is beneficial for ensuring the quality of the brachytherapy. Overall, a small median DP deviation, lower than the used step size of 2.5 mm, was detected.

Interfractional Variation of Treatment Delivery for High-Dose-Rate Interstitial Brachytherapy for Primary or Recurrent Gynecologic Malignancies: A Single-Institution Experience

For women who require high-dose-rate interstitial brachytherapy (HDR-ISBT) for primary or recurrent gynecologic malignancy, there can be significant interfractional variation from patient motion, post-implant swelling and organ filling. The purpose of this study was to evaluate the interfractional variation of dose distribution and displacement of catheters during HDR-ISBT in a single implant, consecutive day regimen. A total of 15 patients with primary or recurrent gynecologic malignancies who underwent HDR-ISBT were reviewed. Interstitial catheters and fiducials were placed under fluoroscopy and ultrasound guidance to confirm desired placement for clinical target volume (CTV) coverage and reproducibility. Patients underwent 4-5 treatments over 2-3 consecutive days. CT imaging was acquired on day of implant and days 2 and/or 3. Displacement of fiducials, their center of mass (CM) and self-consistency was calculated for each fraction. Additionally, the position of catheter tips and CTV’s relative to CM of fiducials were measured. The effects on dosimetric parameters such as dose covering 90% of high-risk CTV (D90), equivalent uniform BED (EUBED) and generalized biologically equivalent uniform dose (gBEUD) were compared for each fraction. 50 fiducials were tracked over 70 fractions. For a majority of patients (10 out of 15), the fiducial markers CM moved less than 0.2 cm. The positions of 278 catheters over the same fractions were measured relative to the fiducials CM and found to have an average interfractional movement of -0.06 +/- 0.31 cm. The CTV to needles interfractional shift was -0.09 +/- 0.36 cm. The average D90 for all fractions was 92.5 +/- 6.5% of the prescription dose (PD). The average gEUD across all patients was 1.57 +/- 0.19 and 18.44+/- 15.08 Gy/PD (a = -5 and 2, respectively). The average EUBED across all patients was 1.98+/- 0.16 and 2.43+/- 0.23 Gy/PD (α = 0.3 and 0.1, respectively). Volume integrated radiobiological metrics, gBEUD and EUBED showed very consistent effects for all implants. Center of mass for 3 or more markers seems a clinically acceptable reference for ensuring catheter tip and CTV positional interfractional reproducibility. Given the dose inhomogeneities accepted with HDR-ISBT, D90 is less sensitive than volume integrated metrics for describing the dose delivered. Combining the effects of a reproducibly accurate implant with unique identification of the importance of ‘hot’ sub-volumes is a promising method for improving patient outcomes with HDR-ISBT.

Dosimetric Impact of Inter-Fraction Bladder Variation Volume on Colorectal Cancers

Dosimetric Impact of Inter-Fraction Bladder Variation Volume on Colorectal Cancers

Early Clinical Experience with Varian Halcyon V2 Linear Accelerator: Dual -Isocenter IMRT Planning and Delivery with Portal Dosimetry for Gynecological Cancer Treatments

A linear accelerator was recently released with new upgraded features. One of new features is to allow treatment length > 28cm using multiple- isocenters (maximum of 36cm in superior and inferior directions). As one of the first users of Halcyon V2, we report our clinical experience with Halcyon V2 for a dual - isocenter IMRT planning and delivery for gynecological cancer patients and examine the feasibility of in-vivo portal dosimetry. Eight extended field IMRT plans for gynecological cancer patients were generated with a treatment planning system using two isocenters to treat pelvis and pelvic/or para-aortic nodes region. Auto feathering technique was employed to split fields between isocenters during IMRT optimization. The prescription dose was 45Gy in 25 fractions (fxs) with simultaneous integrated boost (SIB) dose of 55Gy or 57.5 Gy in 25 fxs to involved nodes. All treatment plans, pre-treatment patient specific QA and treatment delivery records including daily in-vivo portal dosimetry were retrospectively reviewed. For in-vivo daily portal dosimetry analysis, each fraction was compared to the reference baseline (1st fraction) using gamma analysis criteria of 4%/4mm with 90% of total pixels in the portal image planar dose. All 8 extended field IMRT plans met the planning criteria and delivered as planned (a total of 200 fractions). Conformity Index (CI) for the primary target was achieved with the range of 0.99-1.14. For organs at risks, most were well within the dose volume criteria. Treatment delivery took from 5.0 to 6.5 minutes. All 8 cases passed pre-treatment QA using gamma analysis criteria of 3%/2mm with 95 % of total pixels. Interfractional in-vivo dose variation exceeded gamma analysis threshold for 8 fractions out of total 200 (4%). Those 8 fractions were found to have relatively large difference in small bowel filling or motion and SSD change at the isocenter compared to the baseline. Halcyon V2 is effective to create complex extended field IMRT plans using dual isocenters with efficient delivery. Also Halcyon in-vivo dosimetry is feasible for daily treatment monitoring for possible (or significant) changes of organ motion, internal or external anatomy and weight which could further lead to adaptive radiation therapy.

In-Room MRI for Adaptive Radiotherapy for Cervical Cancer Using an Integrated MR-Guided Radiation Therapy System

The superior soft-tissue visualization enabled with online MRI provides an opportunity for high-precision image guidance, response assessment and novel adaptive radiotherapy (ART) processes. At our institution, we developed a novel platform integrating a 1.5 T MR scanner-on-rails with a 6 MV linear accelerator capable of daily in-room MRI. The aim of this work was to develop a hybrid CBCT-MRI image-guided radiotherapy (IGRT) workflow and describe our preliminary experience with implementing in-room MRI for offline ART for intact-cervix cancer. Process mapping, hazard analysis and mock procedures were used to design a hybrid IGRT workflow consisting of CBCT-guided treatment delivery, immediately followed by post-delivery in-room MRI. Patients were transferred to the MR in the treated position via an air-cushion hover-board and then coils were placed for imaging. Treatment planning using VMAT was performed as per EMBRACE II (45 Gy/25 fractions). The primary CT was acquired with full-bladder, and simulation MRI with both full and empty-bladder were included to define the internal target volume (ITV). Daily CBCT guidance used the primary CT as the reference and in-room MRI consisted of a fast T2w axial scan covering the pelvis and primary target. All images were imported to the planning system, co-registered to the plan via the treatment position alignment and reviewed offline by a multi-disciplinary team for geometric assessment of organ motion and target coverage. Two patients with intact-cervix cancer were treated to date with the CBCT-MRI workflow. In-room MRI were acquired for 17/25 and 8/25 fractions. Facility down-time for servicing was the primary reason for missed MRI. The mean time between CBCT and MRI was 16 mins (range = 10–27 mins). The mean time between treatment delivery completion and MRI was 10 mins (6–18 mins). During offline review, large inter-fraction variations in bladder, rectum and target positioning were noted for the first patient. Systematic motion of the uterine fundus beyond the PTV was noted on CBCT, but complete visualization of the GTV, vagina and uterus required MRI. An adaptive replan with modified ITV was created for the final 7 fractions for this patient, which effectively maintained target coverage by the PTV. Comparing the CBCT and MRI, large intra-fraction variations in the bladder and primary target were observed. The mean change in bladder volume was 60 cc (1–161 cc) for the first patient and 29 cc (1–83 cc) for the second. We have successfully demonstrated a hybrid IGRT workflow with in-room MRI for offline ART for cervical cancer. Monitoring daily variations with MRI empowers a comprehensive adaptive paradigm that will become our new standard-of-care for intact-cervix cancer treated with VMAT. Further efforts are underway to optimize IGRT workflows with pre-delivery MRI, as well as to develop state-of-the-art dose accumulation and online ART processes.

État des lieux de la radiothérapie adaptative en 2019 : de la mise en place à l’utilisation clinique

Intensity modulated radiotherapy combined with image guided radiotherapy has led to increase the precision of external beam radiotherapy. However, intra or inter-fraction anatomical variations are frequent during the treatment course and can cause under-dosing of the target volume and/or over-dosing of the organs at risk. Several adaptive radiotherapy (ART) strategies can be defined to compensate these anatomical variations. The purpose of this article is to provide an overview of available ART strategies: offline, online, hybrid (library of treatment plans) or in real-time, while considering the arrival of MR-Linac devices in radiotherapy departments. The tools required to these ART strategies such as auto-segmentation, deformable image registration, calculation of the daily dose or dose accumulation, are also described. Implementing an ART strategy requires a rigorous quality assurance process, at each stage and on the entire workflow, as well as prior organization and training from of all the trades. A strong multidisciplinary involvement is finally required in order to ensure ART treatments.

Indications of Online Adaptive Replanning Based On Organ Deformation

PurposeAlthough vital to account for interfractional variations during radiation therapy, online adaptive replanning (OLAR) is time-consuming and labor-intensive compared with the repositioning method. Repositioning is enough for minimal interfractional deformations. Therefore, determining indications for OLAR is desirable. We introduce a method to rapidly determine the need for OLAR by analyzing the Jacobian determinant histogram (JDH) obtained from deformable image registration between reference (planning) and daily images. Methods and MaterialsThe proposed method was developed and tested based on daily computed tomography (CT) scans acquired during image guided radiation therapy for prostate cancer using an in-room CT scanner. Deformable image registration between daily and reference CT scans was performed. JDHs were extracted from the prostate and a uniform surrounding 10-mm expansion. A classification tree was trained to determine JDH metrics to predict the need for OLAR for a daily CT set. Sixty daily CT scans from 12 randomly selected prostate cases were used as the training data set, with dosimetric plans for both OLAR and repositioning used to determine their class. The resulting classification tree was tested using an independent data set of 45 daily CT scans from 9 other patients with 5 CT scans each. ResultsOf a total of 27 JDH metrics tested, 5 were identified predicted whether OLAR was substantially superior to repositioning for a given fraction. A decision tree was constructed using the obtained metrics from the training set. This tree correctly identified all cases in the test set where benefits of OLAR were obvious. ConclusionsA decision tree based on JDH metrics to quickly determine the necessity of online replanning based on the image of the day without segmentation was determined using a machine learning process. The process can be automated and completed within a minute, allowing users to quickly decide which fractions require OLAR.

On the use of Bayesian statistics in the application of adaptive setup protocols in radiotherapy.

To propose adaptive setup protocols using Bayesian statistics that facilitate, based on a prediction of coverage probability, making a decision on which patients should follow daily imaging prior to treatment delivery. The suitability of the treatment margins was assessed combining interfraction variability measurements of the first days of treatment with previous data gathered from our patient population. From this information, we decided if a patient needs an online imaging protocol to perform daily isocenter correction before each treatment fraction. We applied our method to five different datasets. Protocol parameters were selected from each dataset based on coverage probability, the expected imaging workload of the treatment unit, and the accuracy of patient classification. Time trends were assessed and included in the proposed protocols. To validate the accuracy of the protocols, they were applied to a validation dataset of prostate cancer patients. Adaptive setup protocols lead expected population coverage >97% in all datasets analyzed when time trends were considered. The reduction in imaging workload ranged from 40% in lung treatments to 28.5% in prostate treatments. Results of the protocol on the validation dataset were very similar to those previously predicted. The adaptive setup protocols based on Bayesian statistics presented in this study enable the optimization of imaging workload in the treatment unit ensuring that appropriate dose coverage remains unchanged.

Plan-library supported automated replanning for online-adaptive intensity-modulated proton therapy of cervical cancer

Background: Intensity-modulated proton therapy is sensitive to inter-fraction variations, including density changes along the pencil-beam paths and variations in organ-shape and location. Large day-to-day variations are seen for cervical cancer patients. The purpose of this study was to develop and evaluate a novel method for online selection of a plan from a patient-specific library of prior plans for different anatomies, and adapt it for the daily anatomy.Material and methods: The patient-specific library of prior plans accounting for altered target geometries was generated using a pretreatment established target motion model. Each fraction, the best fitting prior plan was selected. This prior plan was adapted using (1) a restoration of spot-positions (Bragg peaks) by adapting the energies to the new water equivalent path lengths; and (2) a spot addition to fully cover the target of the day, followed by a fast optimization of the spot-weights with the reference point method (RPM) to obtain a Pareto-optimal plan for the daily anatomy. Spot addition and spot-weight optimization could be repeated iteratively. The patient cohort consisted of six patients with in total 23 repeat-CT scans, with a prescribed dose of 45 Gy(RBE) to the primary tumor and the nodal CTV. Using a 1-plan-library (one prior plan based on all motion in the motion model) was compared to choosing from a 2-plan-library (two prior plans based on part of the motion).Results: Applying the prior-plan adaptation method with one iteration of adding spots resulted in clinically acceptable target coverage ( and ) for 37/46 plans using the 1-plan-library and 41/46 plans for the 2-plan-library. When adding spots twice, the 2-plan-library approach could obtain acceptable coverage for all scans, while the 1-plan-library approach showed for 3/46 plans. Similar OAR results were obtained.Conclusion: The automated prior-plan adaptation method can successfully adapt for the large day-to-day variations observed in cervical cancer patients.

Evaluation of spine structure stability at different locations during SBRT.

Modern stereotactic body radiotherapy (SBRT) techniques and systems that use online image guidance offer frameless radiotherapy of spinal tumors and the ability to control intrafraction motion during treatment. These systems allow precise alignment of the patient during the entire treatment session and react immediately to random changes in this alignment. Online tracking data provide information about intrafractional changes, and this information can be useful for designing treatment strategies even if online tracking is not being used. The present study evaluated spine motion during SBRT treatment to assess the risk of verifying patient alignment only prior to starting treatment. This study included 123 patients treated with spine SBRT. We analyzed different locations within the spine using system log files generated during treatment, which contain information about differences in the pretreatment reference spine positions by CT versus positions during SBRT treatment. The mean spine motion and intra/interfraction motion was evaluated. We defined and assessed the spine stability and spine significant shifts (SSHs) during treatment. We analyzed 462 fractions. For the cervical (C) spine, the greatest shifts were in the anterior-posterior (AP) direction (2.48 mm) and in pitch rotation (1.75 deg). The thoracic (Th) spine showed the biggest shift in the AP direction (3.68 mm) and in roll rotation (1.66 deg). For the lumbar-sacral (LS) spine, the biggest shift was found for left-right (LR) translation (3.81 mm) and roll rotation (3.67 deg). No C spine case exceeded 1 mm/1 deg for interfraction variability, but 7 of 54 Th spine cases exceeded 1 mm interfraction variability for translations (maximum value, 2.5 mm in the AP direction). The interfraction variability for translations exceeded 1 mm in 2 of 24 LS spine cases (maximum value, 1.7 mm in the LR direction). Only 13% of cases had no SSHs. The mean times to SSH were 6.5±3.9 min, 8.1±5.9 min, and 8.8±7.1 min for the C, Th, and LS spine, respectively, and the mean recorded SSH values were 1.6±0.66, 1.43±0.33, and 1.46±0.47 mm/deg, respectively. Positional tracking during spine SBRT treatments revealed low mean translational and rotational shifts. Patient immobilization did not improve spine shifts compared with our results for the Th and LS spine without immobilization. For the most precise spine SBRT, we recommend checking the patient's position during treatment.

The Dosimetric Impact of Interfractional Organ-at-Risk Movement During Liver Stereotactic Body Radiation Therapy

PurposeStereotactic body radiation therapy (SBRT) is an effective therapy for treating liver malignancies. However, little is known about interfractional dose variations to adjacent organs at risk (OARs). We examine the effects of interfractional organ movement and setup variation on dose delivered to OARs in patients receiving liver SBRT. Methods and MaterialsThirty patients treated with liver SBRT were analyzed. Daily image guidance with diagnostic quality computed tomography-on-rails imaging was performed before each fraction. In phase 1, these daily images were used to delineate all OARs including the liver, heart, right kidney, esophagus, stomach, duodenum, and large bowel in 10 patients. In phase 2, only OARS in close proximity to the target were contoured in 20 additional patients. Dose distribution on each daily computed tomography was generated, and daily doses to each OAR were recorded and compared with clinical thresholds to determine whether a daily dose excess (DDE) occurred. ResultsIn phase 1, significant interfractional dose differences between planned and delivered dose to OARs were observed, but differences were rarely clinically significant, with just 1 DDE. In phase 2, multiple DDEs were recorded for OARs close to the target, mainly involving the stomach, heart, and esophagus. Tumors in the hilum and liver segments I, IV, and VIII were the most common locations for DDEs. On root cause analysis, 3 etiologies of DDE emerged: craniocaudal shift (69.2%), anatomic changes (28.2%), and anteroposterior shifts (2.6%). ConclusionsOARs close to liver lesions may receive higher doses than expected during SBRT owing to interfractional variations in OARs relative to the target. These differences in planned versus expected dose can lead to toxicity. Efforts to better evaluate OARs with daily image guidance may help reduce risks. Application of adaptive replanning and improved and real-time image guidance could mitigate risks of toxicity, and further study into their applications is warranted.

Accumulation of the delivered treatment dose in volumetric modulated arc therapy with breath-hold for pancreatic cancer patients based on daily cone beam computed tomography images with limited field-of-view.

Inter-fractional changes of the daily anatomy of the patient are a source of error in radiotherapy. This effect is especially present in the abdomen, where there are daily differences in the filling of the stomach or the intestines. This study investigated the impact of inter-fractional changes of pancreatic cancer patients who underwent breath-hold volumetric modulated arc therapy on the applied dose by using deformable image registration. Deformable image registration was used to determine a displacement field which maps the computed tomography (CT) used for treatment planning onto daily breath-hold cone-beam CT images with limited field-of-view (FOV) which were acquired at the beginning of every treatment fraction. Different B-spline registration methods utilizing different metrics and regularizations were used and compared. The resulting deformed planning CT images were used to calculate the dose distributions on the anatomy of the day which can therefore be seen as a good estimation of the actually delivered dose during the treatment fractions. Additionally, the accumulated dose distribution was determined by accumulating all daily dose distributions using inverted deformation fields. The calculated dose distributions were then compared to the planned dose by comparison of dose volume histograms and dosimetric volumetric indices. Best registration results were obtained using a gradient magnitude metric and a regularization of λ=0.05. The accumulated dose distributions showed that the mean dose delivered to the clinical target volume was on median reduced by 0.61Gy after 15 fractions from the planned dose. For the organs at risk, the accumulated dose distributions were still fulfilling all clinical goals for the patient cohort used in this study. It was found that rotating and shearing motions of the tumor led to a greater reduction in the mean target dose which correlated with the proximity of the target to the spine (r=0.54, P=0.038). With the proposed method, it is possible to generate a deformed planning CT image which matches the anatomy of the day based on cone beam computed tomography images with a limited FOV and calculating the dose distribution of the day. The dosimetric impact of the inter-fractional variations was small for most patients but showed larger deviations if the tumor is located in close proximity to the spine.

Evaluation of variation of interfraction doses to organs at risk during brachytherapy of cervical cancer

BackgroundTwo-dimensional treatment planning using radiographs or simulator films was the standard in planning brachytherapy for patients with cervical cancer. Three-dimensional (3D) treatment planning has improved treatment efficacy. This retrospective study compares conventional and 3D treatment planning of brachytherapy in patients with cervical cancer and interfraction dose variation to bladder and rectum (D2cc). MethodsThe mean doses to bladder and rectum (D2cc) were computed by computed tomography (CT)–based planning during 100 sessions of intracavitary brachytherapy for carcinoma cervix with the same source configuration as generated for conventional planning, and these estimates were compared with the doses at International Commission on Radiation Units and measurements (ICRU) rectal, bladder points and point A. Interfraction variation of doses to bladder and rectum during various sessions was also analysed. ResultThe mean ICRU bladder dose and D2cc of the bladder for all patients was 3.7 Gy and 7.4 Gy, respectively (p < 0.001). The mean ICRU rectal dose from conventional plan was 4.3Gy and with CT planning, 4.45 Gy (p = 0.04). Interfraction dose variations for D2cc of the bladder were min −5.3 Gy and max 4.8 Gy and those of the rectum were min −1.8 Gy and max 1.72Gy. ConclusionDosimetric evaluation of conventional and 3D CT-based treatment planning for the same brachytherapy sessions demonstrated underestimation of ICRU bladder dose points (p < 0.001) and the rectal ICRU point dose and D2cc (p=0.04). The doses to organs at risk did not show a statistically significant variation between the fractions. However, large variation was noted between the interfractional maximum and minimum doses to bladder and rectum.

Assessment and management of interfraction variations of lumpectomy cavities in accelerated partial breast irradiation

Background: The purpose of this study is to quantitatively characterize interfraction variations of lumpectomy cavity (LC) in accelerated partial breast irradiation (APBI) and their dosimetric impacts, and to explore the use of an online adaptive replanning scheme to address these variations. Methods: A total of about 100 diagnostic-quality CT sets acquired using an in-room CT at each fraction during image-guided radiation therapy (IGRT) for ten randomly-selected patients treated with APBI in the supine position were analyzed. The LC, treated breast, lung and heart were delineated on each fraction CT. Organ volume change and deformation were quantified. For each fraction CT, three types of plans were created: adaptive, repositioning, and fully re-optimized plans. The plan qualities were compared. Results: Significant changes in LC shape and volume were observed during APBI. On average, the LC volume decreases by 23% from the planning CT. The average change in LC shape, as measured by the Dice’s coefficient, is 80%. For all patients, the adaptive plans were comparable to the re-optimization plans. For small and moderate LC changes (70%), the three types of plans were comparable, indicating that the current IGRT with the standard margins was sufficient to account for the interfraction variations. For cases with extreme LC change (30%), the adaptive plans offered improved target coverage and/or normal tissue sparing as compared with the repositioning plans. Conclusions: Significant variations in the LC between planning and treatment were found for APBI. The current practice of IGRT with standard planning target volume margins can account for these variations for most cases. Online adaptive replanning was needed for cases with extremely large changes in LC.

EP-2176 Analysis of inter-fraction tumor position variability in lung SBRT

EP-2176 Analysis of inter-fraction tumor position variability in lung SBRT

EP-1992 Dose warping protocol for interfraction variation in Bladder filling in ano-rectal cancer patients

EP-1992 Dose warping protocol for interfraction variation in Bladder filling in ano-rectal cancer patients

Interfraction variation and dosimetric changes during image-guided radiation therapy in prostate cancer patients.

PurposeThe aim of this study was to identify volume changes and dose variations of rectum and bladder during radiation therapy in prostate cancer (PC) patients.Materials and MethodsWe analyzed 20 patients with PC treated with helical tomotherapy. Daily image guidance was performed. We re-contoured the entire bladder and rectum including its contents as well as the organ walls on megavoltage computed tomography once a week. Dose variations were analyzed by means of Dmedian, Dmean, Dmax, V10 to V75, as well as the organs at risk (OAR) volume. Further, we investigated the correlation between volume changes and changes in Dmean of OAR.ResultsDuring treatment, the rectal volume ranged from 62% to 223% of its initial volume, the bladder volume from 22% to 375%. The average Dmean ranged from 87% to 118% for the rectum and 58% to 160% for the bladder. The Pearson correlation coefficients between volume changes and corresponding changes in Dmean were -0.82 for the bladder and 0.52 for the rectum. The comparison of the dose wall histogram (DWH) and the dose volume histogram (DVH) showed that the DVH underestimates the percentage of the rectal and bladder volume exposed to the high dose region.ConclusionRelevant variations in the volume of OAR and corresponding dose variations can be observed. For the bladder, an increase in the volume generally leads to lower doses; for the rectum, the correlation is weaker. Having demonstrated remarkable differences in the dose distribution of the DWH and the DVH, the use of DWHs should be considered.

Localization accuracy of robotic radiosurgery in 1-view tracking

PurposeWhen a lung lesion is detected by only one couple of X-ray tube and image detector integrated with CyberKnife®, the fiducial-less tracking is limited to 1-view (34% of lung treatments at Centro Diagnostico Italiano). The aim of the study was mainly to determine the margin needed to take into account the localization uncertainty along the blind view (out-of-plane direction). Methods36 patients treated in 2-view tracking modality (127 fractions in total) were included in the study. The actual tumor positions were determined retrospectively through logfile analysis and were projected onto 2D image planes. In the same plots the planned target positions based on biphasic breath-hold CT scans were represented preserving the metric with respect to the imaging center. The internal margin necessary to cover in out-of-plane direction the 95% of the target position distribution in the 95% of cases was calculated by home-made software in Matlab®. A validation test was preliminarily performed using XLT Phantom (CIRS) both in 2-view and 1-view scenarios. ResultsThe validation test proved the reliability of the method, in spite of some intrinsic limitations. Margins were estimated equal to 5 and 6 mm for targets in upper and lower lobe respectively. Biphasic breath-hold CT led to underestimate the target movement in the hypothetical out-of-plane direction. The inter-fractional variability of spine-target distance was an important source of uncertainty for 1-view treatments. ConclusionThis graphic comparison method preserving metric could be employed in the clinical workflow of 1-view treatments to get patient-related information for customized margin definition.

Investigation of the changes in the prostate, bladder, and rectal wall sizes during external beam radiotherapy.

The change in the prostate size for radiotherapy has not yet been elucidated. The coverage of radiation dose is affected by changes in the prostate size. We evaluated the changes in the prostate, rectum, and bladder wall sizes during IMRT of fraction 2Gy/day using MRI. Twenty-four patients with prostate cancer were enrolled in this study. MRI was performed at three time points. While the initial MRI was performed before the start of radiotherapy (RT), the second MRI was performed at 38Gy (range: 36-40Gy), which represented the halfway point of the RT course. The last MRI was performed on the day of completion of the RT course (76Gy; range: 74-78Gy). We estimated the prostate, rectum, and bladder wall sizes at three time points. We observed no significant difference between the estimated sizes of the prostate during RT in all three phases. In addition, the volume of the rectal wall remained unchanged in all phases. However, the volume of the bladder wall significantly decreased from the initial to the last time points. Furthermore, the standard deviation (SD) obtained by subtracting the final size from the initial one was large (mean, 30.1; SD, 10.1). The volume of the bladder wall decreased during IMRT. The range of subtraction of the volume of the bladder wall was extensive. Thus, the estimation of the bladder wall may be useful to reduce the inter-fraction variation.

Pancreatic cancer treated with SBRT: Effect of anatomical interfraction variations on dose to organs at risk

Background and purposeInterfraction shape and position variations of organs at risk (OARs) may increase uncertainty in dose delivery during stereotactic body radiotherapy (SBRT), potentially leading to overirradiation or concessions in planned tumor dose and/or coverage to prevent clinical constraints violation. The aim of our study was to quantitatively analyze the impact of anatomical interfraction variations on dose to OARs in pancreatic cancer (PC) treated by SBRT using a CyberKnife with integrated CT-on-rails. Materials and methodsThirty-five PC patients treated with SBRT (40 Gy/5 fractions) underwent a CT-scan in treatment position before each of the first three fractions using the CT-on-rails system. OARs (stomach, duodenum, bowel) were manually delineated and concatenated to one structure (Gastro-Intestinal Organ, GIO). To overlay the planned dose distribution, fiducial-based alignment of the fraction CT with the planning CT was performed. Planned DVH parameters of the OAR were compared to the parameters calculated in the fractions CTs. ResultsCompared to the treatment plan, the median V35, D2, D5, D10 and Dmax of the fraction CTs in the GIO was increased by 1.0 (IQR: 0.2–2.6), 4.4% (0.4–10.8), 2.3% (0.2–7.5), 3.3% (−0.4 to 7.1), and 12.0% (5.0–18.9) respectively. Median increase was statistically significant for all parameters in GIO and for V35 in all critical structures at Wilcoxon test. ConclusionsAnatomical interfraction variations increase OAR dose during SBRT for pancreatic cancer daily imaging using integrated CT/CyberKnife may allow to implement strategies to reduce the risk of OAR overirradiation during pancreatic SBRT.