Intra-fractional Changes Research Articles

Measuring interfraction and intrafraction lung function changes during radiation therapy using four-dimensional cone beam CT ventilation imaging.

Adaptive ventilation guided radiation therapy could minimize the irradiation of healthy lung based on repeat lung ventilation imaging (VI) during treatment. However the efficacy of adaptive ventilation guidance requires that interfraction (e.g., week-to-week), ventilation changes are not washed out by intrafraction (e.g., pre- and postfraction) changes, for example, due to patient breathing variability. The authors hypothesize that patients undergoing lung cancer radiation therapy exhibit larger interfraction ventilation changes compared to intrafraction function changes. To test this, the authors perform the first comparison of interfraction and intrafraction lung VI pairs using four-dimensional cone beam CT ventilation imaging (4D-CBCT VI), a novel technique for functional lung imaging. The authors analyzed a total of 215 4D-CBCT scans acquired for 19 locally advanced non-small cell lung cancer (LA-NSCLC) patients over 4-6 weeks of radiation therapy. This set of 215 scans was sorted into 56 interfraction pairs (including first day scans and each of treatment weeks 2, 4, and 6) and 78 intrafraction pairs (including pre/postfraction scans on the same-day), with some scans appearing in both sets. VIs were obtained from the Jacobian determinant of the transform between the 4D-CBCT end-exhale and end-inhale images after deformable image registration. All VIs were deformably registered to their corresponding planning CT and normalized to account for differences in breathing effort, thus facilitating image comparison in terms of (i) voxelwise Spearman correlations, (ii) mean image differences, and (iii) gamma pass rates for all interfraction and intrafraction VI pairs. For the side of the lung ipsilateral to the tumor, we applied two-sided t-tests to determine whether interfraction VI pairs were more different than intrafraction VI pairs. The (mean ± standard deviation) Spearman correlation for interfraction VI pairs was r̄(Inter)=0.52±0.25, which was significantly lower than for intrafraction pairs (r̄(Intra)=0.67±0.20, p = 0.0002). Conversely, mean absolute ventilation differences were larger for interfraction pairs than for intrafraction pairs, with |ΔV̄(Inter)|=0.42±0.65 and |ΔV̄(Intra)|=0.32±0.53, respectively (p < 10(-15)). Applying a gamma analysis with ventilation/distance tolerance of 25%/10 mm, we observed mean pass rate of (69% ± 20%) for interfraction VIs, which was significantly lower compared to intrafraction pairs (80% ± 15%, with p ∼ 0.0003). Compared to the first day scans, all patients experienced at least one subsequent change in median ipsilateral ventilation ≥10%. Patients experienced both positive and negative ventilation changes throughout treatment, with the maximum change occurring at different weeks for different patients. The authors' data support the hypothesis that interfraction ventilation changes are larger than intrafraction ventilation changes for LA-NSCLC patients over a course of conventional lung cancer radiation therapy. Longitudinal ventilation changes are observed to be highly patient-dependent, supporting a possible role for adaptive ventilation guidance based on repeat 4D-CBCT VIs. We anticipate that future improvement of 4D-CBCT image reconstruction algorithms will improve the capability of 4D-CBCT VI to resolve interfraction ventilation changes.

SU‐E‐CAMPUS‐T‐01: Analysis of the Precision of Patient Set‐Up, and Fidelity of the Delivered Dose Distribution in Proton Therapy of Ocular Tumors

Purpose:To quantify daily set‐up variations in fractionated proton therapy of ocular melanomas, and to assess the effect on the fidelity of delivered distribution to the plan.Methods:In a typical five‐fraction course, daily set‐up is achieved by matching the position of fiducial markers in orthogonal radiographs to the images generated by treatment planning program. A patient maintains the required gaze direction voluntarily, without the aid of fixation devices. Confirmation radiographs are acquired to assess intrafractional changes. For this study, daily radiographs were analyzed to determine the daily iso‐center position and apparent gaze direction, which were then transferred to the planning system to calculate the dose delivered in individual fractions, and accumulated dose for the entire course. Dose‐volume metrics were compared between the planned and accumulated distributions for the tumor and organs at risk, for representative cases that varied by location within the ocular globe.Results:The analysis of the first set of cases (3 posterior, 3 transequatorial and 4 anterior tumors) revealed varying dose deviation patterns, depending on the tumor location. For anterior and posterior tumors, the largest dose increases were observed in the lens and ciliary body, while for the equatorial tumors, macula, optic nerve and disk, were most often affected. The iso‐center position error was below 1.3 mm (95%‐confidence interval), and the standard deviation of daily polar and azimuthal gaze set‐up were 1.5 and 3 degrees, respectively.Conclusion:We quantified interfractional and intrafractional set‐up variation, and estimated their effect on the delivered dose for representative cases. Current safety margins are sufficient to maintain the target coverage, however, the dose delivered to critical structures often deviates from the plan. The ongoing analysis will further explore the patterns of dose deviation, and may help to identify particular treatment scenarios which are at a higher risk for such deviations.

SU-E-T-220: Investigation of Intrafraction Changes to Photon Beam Profiles

Purpose: Treatment Planning Systems model beam profiles to be static and independent of gantry angle. This work investigates how beam profiles change over time, with varying gantry angle. Methods: 200 cGy was delivered to ICProfiler (Sun Nuclear) on 2 Elekta Synergys and 1 Varian 21EX linacs at 8 gantry angles and 1 clockwise arc. Measurements were recorded 8 times per second. To investigate the initial portion of the beams, single-shot and multi-control-point beams were measured. The differences in beam profiles were analyzed as well as the length of time for the beam to stabilize. Symmetry was calculated 5cm from the central axis at 100cm SSD. Field sizes ranged from 30×30 cm{sup 2} to 40×40 cm{sup 2}. Results: The beam profiles changed over time and were dependent on gantry angle. The composite profile for each beam met 1% tolerance described in TG-142. It took up to 14.7, 3.7 and 11.3 cGy for the two Elektas and Varian respectively to satisfy the tolerance. All linacs had the most flat, symmetric, and stable beam profile with the gantry at 0 degrees, where the symmetry was 100.5%±0.55, 101.0%±0.52 and 101.1%±0.44 respectively. The symmetry was 100.7%±1.06, 101.1%±1.05 and 101.2%±0.64 at the gantry angle with themore » biggest discrepancy (90, 180 and 180 degrees respectively). Symmetry was better in the X direction for the Elektas and the Y for the Varian. Fluctuations in beam profiles corresponded to changes in dose rate. Conclusion: Flatness and symmetry changed over time and with gantry angle. Because Treatment Planning Systems do not account for these changes, it may be necessary to limit certain beams from clinical use, for example, beams with low MU. It may also be necessary to add additional QA procedures beyond those suggested in TG-142 to account for these changes.« less

SU-E-J-258: Inter- and Intra-Fraction Setup Stability and Couch Change Tolerance for Image Guided Radiation Therapy

Purpose: Immobilization devices combine rigid patient fixation as well as comfort and play a key role providing the stability required for accurate radiation delivery. In the setup step, couch re-positioning needed to align the patient is derived via registration of acquired versus reference image. For subsequent fractions, replicating the initial setup should yield identical alignment errors when compared to the reference. This is not always the case and further couch re-positioning can be needed. An important quality assurance measure is to set couch tolerances beyond which additional investigations are needed. The purpose of this work was to study the inter-fraction couch changes needed to re-align the patient and the intra-fraction stability of the alignment as a guide to establish the couch tolerances. Methods: Data from twelve patients treated on the Accuray CyberKnife (CK) system for fractionated intracranial radiotherapy and immobilized with Aquaplast RT, U-frame, F-Head-Support (Qfix, PA, USA) was used. Each fraction involved image acquisitions and registration with the reference to re-align the patient. The absolute couch position corresponding to the approved setup alignment was recorded per fraction. Intra-fraction set-up corrections were recorded throughout the treatment. Results: The average approved setup alignment was 0.03±0.28mm, 0.15±0.22mm, 0.06±0.31mm in the L/R, A/P, S/I directions respectively and 0.00±0.35degrees, 0.03±0.32degrees, 0.08±0.45degrees for roll, pitch and yaw respectively. The inter-fraction reproducibility of the couch position was 6.65mm, 10.55mm, and 4.77mm in the L/R, A/P and S/I directions respectively and 0.82degrees, 0.71degrees for roll and pitch respectively. Intra-fraction monitoring showed small average errors of 0.21±0.21mm, 0.00±0.08mm, 0.23±0.22mm in the L/R, A/P, S/I directions respectively and 0.03±0.12degrees, 0.04±0.25degrees, and 0.13±0.15degrees in the roll, pitch and yaw respectively. Conclusion: The inter-fraction reproducibility should serve as a guide to couch tolerances, specific to a site and immobilization. More patients need to be included to make general conclusions.

PO-0890: Intra-fractional changes and the robustness of adaptive plan selection in radiotherapy for bladder cancer

PO-0890: Intra-fractional changes and the robustness of adaptive plan selection in radiotherapy for bladder cancer

Frameless high dose rate stereotactic lung radiotherapy: Intrafraction tumor position and delivery time

Intrafraction change in tumor position (Δ) was evaluated for stereotactic lung radiotherapy delivered with flattening filter free volumetric modulated arc therapy. In 140 fractions from 32 patients mean Δ (±SD) was −0.7±1.4mm (vertical), −0.7±1.3mm (longitudinal) and +0.2±1.2mm (lateral) with mean vector 2.1±1.2mm. Mean delivery time was 4.4±3.4min (mean beam-on 1.9±0.4min).

Intrafraction Bladder Motion in Radiation Therapy Estimated From Pretreatment and Posttreatment Volumetric Imaging

The use of image guidance protocols using soft tissue anatomy identification before treatment can reduce interfractional variation. This makes intrafraction clinical target volume (CTV) to planning target volume (PTV) changes more important, including those resulting from intrafraction bladder filling and motion. The purpose of this study was to investigate the required intrafraction margins for soft tissue image guidance from pretreatment and posttreatment volumetric imaging. Fifty patients with muscle-invasive bladder cancer (T2-T4) underwent an adaptive radiation therapy protocol using daily pretreatment cone beam computed tomography (CBCT) with weekly posttreatment CBCT. A total of 235 pairs of pretreatment and posttreatment CBCT images were retrospectively contoured by a single radiation oncologist (CBCT-CTV). The maximum bladder displacement was measured according to the patient's bony pelvis movement during treatment, intrafraction bladder filling, and bladder centroid motion. The mean time between pretreatment and posttreatment CBCT was 13 minutes, 52 seconds (range, 7 min 52 sec to 30 min 56 sec). Taking into account patient motion, bladder centroid motion, and bladder filling, the required margins to cover intrafraction changes from pretreatment to posttreatment in the superior, inferior, right, left, anterior, and posterior were 1.25 cm (range, 1.19-1.50 cm), 0.67 cm (range, 0.58-1.12 cm), 0.74 cm (range, 0.59-0.94 cm), 0.73 cm (range, 0.51-1.00 cm), 1.20 cm (range, 0.85-1.32 cm), and 0.86 cm (range, 0.73-0.99), respectively. Small bladders on pretreatment imaging had relatively the largest increase in pretreatment to posttreatment volume. Intrafraction motion of the bladder based on pretreatment and posttreatment bladder imaging can be significant particularly in the anterior and superior directions. Patient motion, bladder centroid motion, and bladder filling all contribute to changes between pretreatment and posttreatment imaging. Asymmetric expansion of CTV to PTV should be considered. Care is required in using image-guided radiation therapy protocols that reduce CTV to PTV margins based only on daily pretreatment soft tissue position.

Translational and rotational intra- and inter-fractional errors in patient and target position during a short course of frameless stereotactic body radiotherapy

Background. Implementation of cone beam computed tomography (CBCT) in frameless stereotactic body radiotherapy (SBRT) of lung tumours enables setup correction based on tumour position. The aim of this study was to compare setup accuracy with daily soft tissue matching to bony anatomy matching and evaluate intra- and inter-fractional translational and rotational errors in patient and target positions. Material and methods. Fifteen consecutive SBRT patients were included in the study. Vacuum cushions were used for immobilisation. SBRT plans were based on midventilation phase of four-dimensional (4D)-CT or three-dimensional (3D)-CT from PET/CT. Margins of 5 mm in the transversal plane and 10 mm in the cranio-caudal (CC) direction were applied. SBRT was delivered in three fractions within a week. At each fraction, CBCT was performed before and after the treatment. Setup accuracy comparison between soft tissue matching and bony anatomy matching was evaluated on pretreatment CBCTs. From differences in pre- and post-treatment CBCTs, we evaluated the extent of translational and rotational intra-fractional changes in patient position, tumour position and tumour baseline shift. All image registration was rigid with six degrees of freedom. Results. The median 3D difference between patient position based on bony anatomy matching and soft tissue matching was 3.0 mm (0–8.3 mm). The median 3D intra-fractional change in patient position was 1.4 mm (0–12.2 mm) and 2.2 mm (0–13.2 mm) in tumour position. The median 3D intra-fractional baseline shift was 2.2 mm (0–4.7 mm). With correction of translational errors, the remaining systematic and random errors were approximately 1°. Conclusion. Soft tissue tumour matching improved precision of treatment delivery in frameless SBRT of lung tumours compared to image guidance using bone matching. The intra-fractional displacement of the target position was affected by both translational and rotational changes in tumour baseline position relative to the bony anatomy and by changes in patient position.

Changes in Position and Volume of Lung Cancer Target Volumes during Stereotactic Body Radiotherapy (SBRT): Is Image Guidance Necessary?

The purpose of this study was to analyze inter- and intra-fractional changes in tumor volume with respect to both spatial and volumetric parameters among patients treated by SBRT for lung cancer. Twelve patients (13 tumors) were treated by SBRT with abdominal compression in 3-5 fractions over a 2 week period to a median dose of 60 Gy (range, 48 to 60 Gy). Kilovoltage cone-beam CT (CBCT) was obtained prior to the delivery of each fraction as well as intra-fractionally yielding a total of 55 CBCT scans. All CBCT scans were registered with the planning CT for target alignment and shifts were recorded and analyzed. Retrospectively, gross tumor volume (GTV) was contoured on all CBCT images and compared to initial planning volumes; positional differences were evaluated utilizing directional and vector analysis. Shifts greater than 5 mm were applied inter-fractionally in 6.8% (lateral), 29.5% (longitudinal), and 6.8% (vertical) of all delivered treatments. Using a 10 mm threshold, the corresponding percentages were 2.3%, 13.6%, and 2.3%, respectively. Across all fractions, the calculated inter-fractional shift vectors ranged from 0 to 31.2 mm, with 40.9%, 15.9%, and 11.4% of all fractions having shift vectors≥5 mm, ≥10 mm, and ≥20 mm, respectively. Intra-fractional shifts were also evaluated and found negligible in a small portion of patients evaluated. The mean overall reduction in GTV was 21.1% during SBRT. Significant changes in both position and volume occur during SBRT for lung cancer. Shifts (particularly in the superior-inferior axis) may exceed applied margins and compromise target coverage. Due to the extreme hypofractionation associated with SBRT, inter-fractional image guidance is necessary.

CT-Based 3-Dimensional Dose Planning in the Delivery of Vaginal Brachytherapy: New Instrumentation and Technique

Purpose: CT-based 3-dimensional volumetric dose planning, when used with multichannel brachytherapy applicators, has demonstrated utility in certain settings such as in the post-lumpectomy treatment of breast cancer. A parallel situation exists for patients in need of transvaginal brachytherapy (e.g. post-hysterectomy for endometrial cancer). The purpose of this study is to evaluate the feasibility of this approach, using a new flexible, expandable applicator for vaginal cuff brachytherapy. Materials and Methods: Patients were identified as candidates for transvaginal brachytherapy in accordance with current FIGO guidelines. In 14 cases, a flexible, multi-lumen expandable vaginal applicator (CAPRI applicator) was inserted into the introitus in a low profile configuration, advanced to the vaginal vault and then expanded in-situ with saline until the device was held firmly in the vaginal canal. A pelvic brachytherapy external compression garment was used to help secure the applicator in position. No Foley or rectal catheters were used. The pelvic area was imaged via CT and a 3-dimensional dose plan was created, using an evenly spaced array of up to 13 available source lumens within the applicator. Bladder and rectum were fully contoured. The target volume was nominally a 5mm expansion of the applicator surface, with localized reductions (2e4mm expansions) at regions immediately adjacent to the bladder and rectum. Coverage was 90/100. After simulation, the patient returned on subsequent alternate days for applicator placement and dose delivery. Applicator position was monitored closely, with regard to any potential intra-fraction changes in applicator depth, rotation, fill volume and diameter. Dose was delivered via a remote high-dose-rate afterloader in accordance with relevant dose and fractionation schemes (e.g. 700cGy to depth x 3). Results: CTebased 3-dimensional dose planning with the flexible, expandable multi-lumen applicator provided for an anatomically conformal dose delivery. Monitoring of applicator position confirmed intra-fraction consistency, thereby allowing the same dose plan to be administered to that patient on subsequent fractions. Simulation and dose delivery were well tolerated by the patients and were easily adopted by the nursing, dosimetry and physics staff. Avoidance of the rigid applicator clamp and other indwelling catheters expedited applicator placement and contributed to patient comfort. Conclusions: CTebased 3-dimensional dose planning, when used with a new flexible, expandable multi-lumen vaginal applicator is feasible and allows for highly conformal dose delivery to the targeted tissue. Well received by patients and brachytherapy technical staff, this approach may play an increasing role in the treatment of endometrial cancers and other cancers that benefit from vaginal brachytherapy.

Special report: Workshop on 4D‐treatment planning in actively scanned particle therapy—Recommendations, technical challenges, and future research directions

This article reports on a 4D-treatment planning workshop (4DTPW), held on 7-8 December 2009 at the Paul Scherrer Institut (PSI) in Villigen, Switzerland. The participants were all members of institutions actively involved in particle therapy delivery and research. The purpose of the 4DTPW was to discuss current approaches, challenges, and future research directions in 4D-treatment planning in the context of actively scanned particle radiotherapy. Key aspects were addressed in plenary sessions, in which leaders of the field summarized the state-of-the-art. Each plenary session was followed by an extensive discussion. As a result, this article presents a summary of recommendations for the treatment of mobile targets (intrafractional changes) with actively scanned particles and a list of requirements to elaborate and apply these guidelines clinically.

Interfraction and Intrafraction Changes in Amplitude of Breathing Motion in Stereotactic Liver Radiotherapy

Interfraction and intrafraction changes in amplitude of liver motion were assessed in patients with liver cancer treated with kV cone beam computed tomography (CBCT)-guided stereotactic body radiation therapy (SBRT). A total of 314 CBCTs obtained with the patient in the treatment position immediately before and after each fraction, and 29 planning 4DCTs were evaluated in 29 patients undergoing six-fraction SBRT for unresectable liver cancer, with (n = 15) and without (n = 14) abdominal compression. Offline, the CBCTs were sorted into 10 bins, based on phase of respiration. Liver motion amplitude was measured using liver-to-liver alignment from the end-exhale and end-inhale CBCT and four-dimensional CT reconstructions. Inter- and intrafraction amplitude changes were measured from the difference between the pre-SBRT CBCTs relative to the planning four-dimensional CT, and from the pre-SBRT and post-SBRT CBCTs, respectively. Mean liver motion amplitude for all patients (range) was 1.8 (0.1-7.0), 8.0 (0.1-18.8), and 4.3 (0.1-12.1) mm in the mediolateral (ML), craniocaudal (CC), and anteroposterior (AP) directions, respectively. Mean absolute inter- and intrafraction liver motion amplitude changes were 1.0 (ML), 1.7 (CC), and 1.6 (AP) mm and 1.3 (ML), 1.6 (CC), and 1.9 (AP) mm, respectively. No significant correlations were found between intrafraction amplitude change and intrafraction time (range, 4:56-25:37 min:sec), and between inter- and intrafraction amplitude changes and liver motion amplitude. Intraobserver reproducibility (sigma, n = 29 fractions) was 1.3 (ML), 1.4 (CC), and 1.4 (AP) mm. For the majority of liver SBRT patients, the change in liver motion amplitude was minimal over the treatment course and showed no apparent relationships with the magnitude of liver motion and intrafraction time.

Evaluation of a Software Tool for Verifying Tumor Motion and Breathing Patterns in Patients undergoing Stereotactic Radiotherapy (SRT)

A single 4D-CT scan performed for SRT planning may fail to capture all lung tumor motion due to limited duration of imaging. Cone-beam CT scans verify mean tumor position but cannot detect intra-fractional changes due to changed respiratory pattern. Intrafraction Motion Review (IMR, Varian Medical Systems) is a software tool that permits quantitative assessment of both tumor motion from fluoroscopic images and respiratory waveforms from the Real-time Position Management (RPM) system. We used IMR (1) to determine the frequency with which tumor motion exceeded that seen on 4D-CT, (2) to evaluate the correlation between baseline shifts in the respiratory waveforms and the corresponding tumor position, and (3) study the consistency of the relationship.

Inter- and Intrafraction Variability in Liver Position in Non–Breath-Hold Stereotactic Body Radiotherapy

The inter- and intrafraction variability of liver position was assessed in patients with liver cancer treated with kilovoltage cone-beam computed tomography (CBCT)-guided stereotactic body radiotherapy. A total of 314 CBCT scans obtained in the treatment position immediately before and after each fraction were evaluated from 29 patients undergoing six-fraction, non-breath-hold stereotactic body radiotherapy for unresectable liver cancer. Off-line, the CBCT scans were sorted into 10 bins, according to the phase of respiration. The liver position (relative to the vertebral bodies) was measured using rigid alignment of the exhale CBCT liver with the exhale planning CT liver, following the alignment of the vertebrae. The interfraction liver position change was measured by comparing the pretreatment CBCT scans, and the intrafraction change was measured from the CBCT scans obtained immediately before and after each fraction. The mean amplitude of liver motion for all patients was 1.8 mm (range, 0.1-5.7), 8.0 mm (range, 0.1-18.8), and 4.3 mm (range 0.1-12.1) in the medial-lateral (ML), craniocaudal (CC), and anteroposterior (AP) directions, respectively. The mean absolute ML, CC, and AP interfraction changes in liver position were 2.0 mm (90th percentile, 4.2), 3.5 mm (90th percentile, 7.3), and 2.3 mm (90th percentile, 4.7). The mean absolute intrafraction ML, CC, and AP changes were 1.3 mm (90th percentile, 2.9), 1.6 mm (90th percentile, 3.6), and 1.5 mm (90th percentile, 3.1), respectively. The interfraction changes were significantly larger than the intrafraction changes, with a CC systematic error of 2.9 and 1.1 mm, respectively. The intraobserver reproducibility (sigma, n = 29 fractions) was 1.3 mm in the ML, 1.4 mm in the CC, and 1.6 mm in the AP direction. Interfraction liver position changes relative to the vertebral bodies are an important source of geometric uncertainty, providing a rationale for prefraction soft-tissue image guidance. The intrafraction change in liver position from the beginning to the end of each fraction was small for most patients.

Quantifying Interfraction and Intrafraction Tumor Motion in Lung Stereotactic Body Radiotherapy Using Respiration-Correlated Cone Beam Computed Tomography

Stereotactic body radiation therapy (SBRT) is an effective treatment for medically inoperable Stage I non-small-cell lung cancer. However, changes in the patient's breathing patterns during the course of SBRT may result in a geographic miss or an overexposure of healthy tissues to radiation. However, the precise extent of these changes in breathing pattern is not well known. We evaluated the inter- and intrafractional changes in tumor motion amplitude (DeltaM) over an SBRT course. Eighteen patients received image-guided SBRT delivered in three fractions; this therapy was done with abdominal compression in four patients. For each fraction, cone beam computed tomography (CBCT) was performed for tumor localization (+/- 3-mm tolerance) and then repeated to confirm geometric accuracy. Additional CBCT images were acquired at the midpoint and end of each SBRT fraction. Respiration-correlated CBCT (rcCBCT) reconstructions allowed retrospective assessment of inter- and intrafractional DeltaM by a comparison of tumor displacements in all four-dimensional CT and rcCBCT scans. The DeltaM was measured in mediolateral, superior-inferior, and anterior-posterior directions. A total of 201 rcCBCT images were analyzed. The mean time from localization of the tumor to the end-fraction CBCT was 35 +/- 7 min. Compared with the motion recorded on four-dimensional CT, the mean DeltaM was 0.4, 1.0, and 0.4 mm, respectively, in the mediolateral, superior-inferior, and anterior-posterior directions. On treatment, the observed DeltaM was, on average, <1 mm; no DeltaM was statistically different with respect to the initial rcCBCT. However, patients in whom abdominal compression was used showed a statistically significant difference (p < 0.05) in the variance of DeltaM with respect to the initial rcCBCT in the superior-inferior direction. The inter- and intrafractional DeltaM that occur during a course of lung SBRT are small. However, abdominal compression causes larger variations in the time spent on the treatment couch and in the inter- and intrafractional DeltaM values.

Intra-fractional uncertainties in image-guided intensity-modulated radiotherapy (IMRT) of prostate cancer

To evaluate intra-fractional uncertainties during intensity-modulated radiotherapy (IMRT) of prostate cancer. During IMRT of 21 consecutive patients, kilovolt (kV) cone-beam computed tomography (CBCT) images were acquired prior to and immediately after treatment: a total of 252 treatment fractions with 504 CBCT studies were basis of this analysis. The prostate position in anterior-posterior (AP) direction was determined using contour matching; patient set-up based on the pelvic bony anatomy was evaluated using automatic image registration. Internal variability of the prostate position was the difference between absolute prostate and patient position errors. Intra-fractional changes of prostate position, patient position, rectal distension in AP direction and bladder volume were analyzed. With a median treatment time of 16 min, intra-fractional drifts of the prostate were >5 mm in 12% of all fractions and a margin of 6 mm was calculated for compensation of this uncertainty. Mobility of the prostate was independent from the bony anatomy with poor correlation between absolute prostate motion and motion of the bony anatomy (R2=0.24). A systematic increase of bladder filling by 41 ccm on average was observed; however, these changes did not influence the prostate position. Small variations of the prostate position occurred independently from intra-fractional changes of the rectal distension; a weak correlation between large internal prostate motion and changes of the rectal volume was observed (R2=0.55). Clinically significant intra-fractional changes of the prostate position were observed and margins of 6 mm were calculated for this intra-fractional uncertainty. Repeated or continuous verification of the prostate position may allow further margin reduction.

Intrafraction changes of prostate position and geometrical errors studied by continuous electronic portal imaging

Purpose. The use of marker-based on-line image guided radiotherapy for prostate cancer has considerably reduced the treatment margins to sub-cm. In this study we have quantified the residual set-up errors remaining after isocenter correction, studied their development during beam delivery and estimated their impact on margins. Methods and materials. After initial on-line patient set-up based on orthogonal kV x-ray images of implanted fiducial markers, continuous electronic portal imaging was performed during treatment delivery in 10 of 39 treatment sessions for 20 prostate cancer patients. The cranio-caudal (CC) position of the centre-of-mass of the three markers was found using a threshold technique on every single image frame for all patients, typically 12–14 images for 5 treatment beams in every fraction. The CC prostate position was determined relative to its initial position at treatment onset and relative to its planned position within the field aperture. These results allowed determination of the CC intrafraction prostate motion and the intrafraction progression of the geometrical CC error, respectively. Results. At treatment onset the standard deviation (SD) of the set-up error was 1.0mm in the lateral direction and 1.5mm in the cranio-caudal (CC) direction. It did not depend significantly on the duration of the set-up procedure (mean: 3.0min, span 1.2–14.6min). The distribution of CC prostate positions relative to the position at treatment onset broadened from 0 to 1.4mm during the treatment session, while the corresponding CC setup error distribution broadened from 1.5 to 1.9mm. This broadening means that the necessary CC setup margin increased by around 1mm during the treatment fraction. Conclusions. Large differences in the intrafraction CC prostate motion patterns were found, however, intrafraction motion only results in a modest additional CC set-up margin of around 1mm relative to the margins needed for the residual set-up error at treatment start.

Intra-fractional uncertainties in cone-beam CT based image-guided radiotherapy (IGRT) of pulmonary tumors

Purpose Intra-fractional variability of tumor position and breathing motion was evaluated in cone-beam CT (CB-CT) based image-guided radiotherapy (IGRT) of pulmonary tumors. Materials and methods Twenty-four patients (27 lesions: prim. NSCLC n = 6; metastases n = 21) were treated with stereotactic body radiotherapy (SBRT) (one to eight fractions). Prior to every treatment fraction ( n = 66) and immediately after treatment a CB-CT was acquired. Patient motion, absolute drift and drift of the tumor relative to the bony anatomy were measured. Tumor motion was investigated based on the density distribution in the CB-CT. Results Absolute intra-fractional drift (3D vector) of the tumor position was 2.8 mm ± 1.6 mm (mean ± SD), maximum 7.2 mm. Poor correlation between patient motion and absolute tumor drift was observed. Changes of the tumor position due to patient motion and due to drifts independently from the bony anatomy were of similar magnitude with 2.1 mm ± 1.4 mm and 2.3 mm ± 1.6 mm, respectively. No systematic increase or decrease of breathing motion was seen. The intra-fractional change of breathing motion was more than 2 mm and 3 mm in 39% and 16%, respectively. Conclusion Intra-fractional tumor position and breathing motion were stable. In IGRT of pulmonary tumors we suggest an ITV-to-PTV margin of 5 mm to compensate intra-fractional changes.

Speed and amplitude of lung tumor motion precisely detected in four-dimensional setup and in real-time tumor-tracking radiotherapy

To reduce the uncertainty of registration for lung tumors, we have developed a four-dimensional (4D) setup system using a real-time tumor-tracking radiotherapy system. During treatment planning and daily setup in the treatment room, the trajectory of the internal fiducial marker was recorded for 1 to 2 min at the rate of 30 times per second by the real-time tumor-tracking radiotherapy system. To maximize gating efficiency, the patient's position on the treatment couch was adjusted using the 4D setup system with fine on-line remote control of the treatment couch. The trajectory of the marker detected in the 4D setup system was well visualized and used for daily setup. Various degrees of interfractional and intrafractional changes in the absolute amplitude and speed of the internal marker were detected. Readjustments were necessary during each treatment session, prompted by baseline shifting of the tumor position. The 4D setup system was shown to be useful for reducing the uncertainty of tumor motion and for increasing the efficiency of gated irradiation. Considering the interfractional and intrafractional changes in speed and amplitude detected in this study, intercepting radiotherapy is the safe and cost-effective method for 4D radiotherapy using real-time tracking technology.