CC Direction Research Articles

Tumour motion analysis from planning to end of treatment course for a large cohort of peripheral lung SBRT targets

Background The aim is to quantify and analyse tumour motion during a course of treatment for lung SBRT patients. Material and methods Peak-to-peak motion of 483 tumours in 441 patients treated with peripheral lung SBRT at a single institution over a two year period was measured on planning CT and at all treatment fractions. Planning 4D-CT scans were analysed using our clinical workflow involving deformable propagation of the delineated target to all phases. Similarly, acquisition of the 4D-CBCT data followed the clinical workflow based on XVI 5.0 available on Elekta linacs. Differences and correlations of the peak-to-peak motion on the planning CT and at treatment were analysed. Results On the planning CT, a total of 81.4% of the tumours had a peak-to-peak motion <10 mm, and 96.1% had <20 mm. The largest motion was observed in the CC direction, with largest amplitude for tumours located in the caudal posterior part of the lung. The difference in amplitude in CC between planning CT and first fraction had a mean and standard deviation of 0.3 mm and 3.5 mm, respectively, and the largest differences were observed in the caudal posterior part of the lung. Patients with a difference in tumour motion amplitude exceeding two standard deviations (>7 mm) at the first fraction were evaluated individually, and they all had poor 4DCT image quality. The difference between the first and second/third fractions had a mean and standard deviation of 0.4 mm/0.5 mm and 2.0 mm/1.9 mm. Conclusion Tumour motion at first treatment was similar to motion at planning, and motion at subsequent treatments was very similar to motion at first treatment. Large tumour motions are located towards the caudal posterior tumour locations. Patients with poor 4D-CT image quality should be closely followed at the first treatment to verify the motion.

Accuracy of an MR-only workflow for prostate radiotherapy using semi-automatically burned-in fiducial markers

BackgroundThe benefit of MR-only workflow compared to current CT-based workflow for prostate radiotherapy is reduction of systematic errors in the radiotherapy chain by 2–3 mm. Nowadays, MRI is used for target delineation while CT is needed for position verification. In MR-only workflows, MRI based synthetic CT (sCT) replaces CT. Intraprostatic fiducial markers (FMs) are used as a surrogate for the position of the prostate improving targeting. However, FMs are not visible on sCT. Therefore, a semi-automatic method for burning-in FMs on sCT was developed. Accuracy of MR-only workflow using semi-automatically burned-in FMs was assessed and compared to CT/MR workflow.MethodsThirty-one prostate cancer patients receiving radiotherapy, underwent an additional MR sequence (mDIXON) to create an sCT for MR-only workflow simulation. Three sources of accuracy in the CT/MR- and MR-only workflow were investigated. To compare image registrations for target delineation, the inter-observer error (IOE) of FM-based CT-to-MR image registrations and soft-tissue-based MR-to-MR image registrations were determined on twenty patients. Secondly, the inter-observer variation of the resulting FM positions was determined on twenty patients. Thirdly, on 26 patients CBCTs were retrospectively registered on sCT with burned-in FMs and compared to CT-CBCT registrations.ResultsImage registration for target delineation shows a three times smaller IOE for MR-only workflow compared to CT/MR workflow. All observers agreed in correctly identifying all FMs for 18 out of 20 patients (90%). The IOE in CC direction of the center of mass (COM) position of the markers was within the CT slice thickness (2.5 mm), the IOE in AP and RL direction were below 1.0 mm and 1.5 mm, respectively. Registrations for IGRT position verification in MR-only workflow compared to CT/MR workflow were equivalent in RL-, CC- and AP-direction, except for a significant difference for random error in rotation.ConclusionsMR-only workflow using sCT with burned-in FMs is an improvement compared to the current CT/MR workflow, with a three times smaller inter observer error in CT-MR registration and comparable CBCT registration results between CT and sCT reference scans.Trial registry Medical Research Involving Human Subjects Act (WMO) does apply to this study and was approved by the Medical Ethics review Committee of the Academic Medical Center. Registration number: NL65414.018.18. Date of registration: 21–08-2018.

Impact of a fiducial marker based ART strategy on margins in postoperative IMRT of gynecological tumors

PurposeTo investigate the potential of an offline Adaptive Radiotherapy (ART) strategy, based on the interfractional vagina motion (IVM) measured using fiducial markers (FM) during an initial number of fractions, on the CTV to PTV margins in post-operative gynecological patients. Materials and methodsIn 18 patients, treated post-operatively for gynecological tumors, the systematic residual IVM was quantified after simulating an offline ART procedure, utilizing the average IVM measured with FM for a varying initial numbers of fractions to find the optimal moment to adapt the treatment plan and a threshold for selecting patients for replanning. Clinical margins for a zero, 2 and 5 mm threshold based strategy were calculated to assess the possible margin reduction. ResultsApplying an ART strategy based on the average IVM of the initial 5 fractions reduces the systematic IVM significantly (P < 0.025), allowing a reduction of the clinical margin of 3 mm (20%) in the CC direction and 2 mm (13%) in the AP direction. A 2 mm threshold for selecting patients for replanning shows no difference in the reduction of the clinical margin, but reduces the workload with 12%. ConclusionAn ART strategy based on adapting on the average IVM during the initial 5 fractions of treatment provides an opportunity to reduce the CTV to PTV margins in postoperative gynecological tumors. To keep the workload in balance with the best achievable margin reduction, a threshold for selecting patients for plan adaptation is recommended.

Comparison of carina-based versus bony anatomy-based registration for setup verification in esophageal cancer radiotherapy

BackgroundTo investigate the feasibility and geometric accuracy of carina-based registration for CBCT-guided setup verification in esophageal cancer IGRT, compared with current practice bony anatomy-based registration.MethodsIncluded were 24 esophageal cancer patients with 65 implanted fiducial markers, visible on planning CTs and follow-up CBCTs. All available CBCT scans (n = 236) were rigidly registered to the planning CT with respect to the bony anatomy and the carina. Target coverage was visually inspected and marker position variation was quantified relative to both registration approaches; the variation of systematic (Σ) and random errors (σ) was estimated.ResultsAutomatic carina-based registration was feasible in 94.9% of the CBCT scans, with an adequate target coverage in 91.1% compared to 100% after bony anatomy-based registration. Overall, Σ (σ) in the LR/CC/AP direction was 2.9(2.4)/4.1(2.4)/2.2(1.8) mm using the bony anatomy registration compared to 3.3(3.0)/3.6(2.6)/3.9(3.1) mm for the carina. Mid-thoracic placed markers showed a non-significant but smaller Σ in CC and AP direction when using the carina-based registration.ConclusionsCompared with a bony anatomy-based registration, carina-based registration for esophageal cancer IGRT results in inadequate target coverage in 8.9% of cases. Furthermore, large Σ and σ, requiring larger anisotropic margins, were seen after carina-based registration. Only for tumors entirely confined to the mid-thoracic region the carina-based registration might be slightly favorable.

Cardiac and respiration induced motion of mediastinal lymph node targets in lung cancer patients throughout the radiotherapy treatment course

Background and purposeInvolved mediastinal lymph nodes (LNs) are often included in the radiotherapy target for lung cancer patients. Their motion may differ from the primary tumor motion, possibly undermining the loco-regional control. This study determines the detailed differential target motion throughout the treatment course. Material and methodsTen lung cancer patients with 2–4 fiducial markers implanted in LN targets received IMRT with a daily pre-treatment cone-beam CT (CBCT) scan. Offline, the 3D trajectory of the markers was determined from their projected trajectory in the CBCT projections. Frequency analysis was performed to separate the intrafraction motion into a respiratory and cardiac component. The mean setup error of the markers and the motion range were used to calculate margins required for LN targets when setup is based on soft-tissue match. ResultsRespiration motion was largest in the CC direction and more prominent for more caudal LNs. Cardiac motion was often (73%) largest in the AP direction and tended to be largest for more cranial LNs. Margins for intrafraction motion and daily baseline shifts of LNs were 4.8mm (LR), 6.0mm (CC) and 6.7mm (AP). ConclusionsDetailed mapping showed that LN motion was in general governed by breathing, but some LNs had substantial cardiac induced motion.

SU‐F‐T‐60: A Quick Dose Calculation Check for Accuboost Breast Treatment

Purpose:Accuboost treatment planning uses dwell times from a nomogram designed with Monte Carlo calculations for round and D‐shaped applicators. A quick dose calculation method has been developed for verification of the HDR Brachytherapy dose as a second check.Methods:Accuboost breast treatment uses several round and D‐shaped applicators to be used non‐invasively with an Ir‐192 source from a HDR Brachytherapy afterloader after the breast is compressed in a mammographic unit for localization. The breast thickness, source activity, the prescription dose and the applicator size are entered into a nomogram spreadsheet which gives the dwell times to be manually entered into the delivery computer. Approximating the HDR Ir‐192 as a point source, and knowing the geometry of the round and D‐applicators, the distances from the source positions to the midpoint of the central plane are calculated. Using the exposure constant of Ir‐192 and medium as human tissue, the dose at a point is calculated as: D(cGy) = 1.254 × A × t/R2, where A is the activity in Ci, t is the dwell time in sec and R is the distance in cm. The dose from each dwell position is added to get the total dose.Results:Each fraction is delivered in two compressions: cranio‐caudally and medial‐laterally. A typical APBI treatment in 10 fractions requires 20 compressions. For a patient treated with D45 applicators and an average of 5.22 cm thickness, this calculation was 1.63 % higher than the prescription. For another patient using D53 applicators in the CC direction and 7 cm SDO applicators in the ML direction, this calculation was 1.31 % lower than the prescription.Conclusion:This is a simple and quick method to double check the dose on the central plane for Accuboost treatment.

Implementation of single-breath-hold cone beam CT guided hypofraction radiotherapy for lung cancer

BackgroundTo analyze the feasibility of active breath control (ABC), the lung tumor reproducibility and the rationale for single-breath-hold cone beam CT (CBCT)-guided hypofraction radiotherapy.MethodsSingle-breath-hold CBCT images were acquired using ABC in a cohort of 83 lung cancer patients (95 tumors) treated with hypofraction radiotherapy. For all alignments between the reference CT and CBCT images (including the pre-correction, post-correction and post-treatment CBCT images), the tumor reproducibility was evaluated via online manual alignment of the tumors, and the vertebral bone uncertainties were evaluated via offline manual alignment of the vertebral bones. The difference between the tumor reproducibility and the vertebral bone uncertainty represents the change in the tumor position relative to the vertebral bone. The relative tumor positions along the coronal, sagittal and transverse axes were measured based on the reference CT image. The correlations between the vertebral bone uncertainty, the relative tumor position, the total treatment time and the tumor reproducibility were evaluated using the Pearson correlations.ResultsPre-correction, the systematic/random errors of tumor reproducibility were 4.5/2.6 (medial-lateral, ML), 5.1/4.8 (cranial-caudal, CC) and 4.0/3.6 mm (anterior-posterior, AP). These errors were significantly decreased to within 3 mm, both post-correction and post-treatment. The corresponding PTV margins were 4.7 (ML), 7.4 (CC) and 5.4 (AP) mm. The changes in the tumor position relative to the vertebral bone displayed systematic/random errors of 2.2/2.0 (ML), 4.1/4.4 (CC) and 3.1/3.3 (AP) mm. The uncertainty of the vertebral bone significantly correlated to the reproducibility of the tumor position (P < 0.05), except in the CC direction post-treatment. However, no significant correlation was detected between the relative tumor position, the total treatment time and the tumor reproducibility (P > 0.05).ConclusionsUsing ABC for single-breath-hold CBCT guidance is an effective method to reduce the PTV margin of hypofraction radiotherapy for lung cancer. Using ABC, the tumor position was significantly altered relative to the vertebral position. The reproducibility of the tumor position was affected by the vertebral bone but not by the relative tumor position or the total treatment time.

Motion compensated digital tomosynthesis

Background and purposeDigital tomosynthesis (DTS) is a limited angle image reconstruction method for cone beam projections that offers patient surveillance capabilities during VMAT based SBRT delivery. Motion compensation (MC) has the potential to mitigate motion artifacts caused by respiratory motion, such as blur. The purpose of this feasibility study was therefore to develop and evaluate motion-compensated DTS (MC-DTS). Material and methodsMC-DTS images were reconstructed by back projection of X-ray projection images acquired over 30° arcs. Back projection lines were deformed according to an a priori motion model derived from the 4D planning CT. MC-DTS was evaluated on a respiratory motion phantom and 3 lung cancer patients. Respiratory artifact reduction was assessed visually and quantified by fitting a cumulative Gaussian function to profiles along the background-GTV transition in the CC direction. ResultsMC reconstruction was fast enough to keep up with image acquisition and considerably reduced motion blur visually. Quantitatively, MC reduced the background-GTV transition distance by 49%. ConclusionMotion compensation considerably improved the image quality of DTS images of lung cancer patients, giving an opportunity for more accurate DTS guidance and intra-fraction monitoring concurrent with VMAT delivery.

Predictive Factors for Brain Metastases and Relapse and Prognostic Factors for Survival in Stage III-IV Non-Small Cell Lung Cancer (NSCLC)

resulted in a mean of 40% excess volume in the target when using only a 3D scan for volume generation. The over treated volume was greater in conv vs SBRT (p Z .002). An average of 13.2% (20.8% for SBRT and 4.9% for conv) of the 4D PTV was excluded from the 3D PTV. The percentage of the 4D PTV missed by 3D PTvs was greater with SBRT plans compared to both min and max conv expansions (p Z .015). In 9/11 SBRT cases, the ITV was missed by the 3D PTV radially, but never in the CC plane. The 4D PTV was missed radially in all 11 tumors, and the CC direction in 4, by the 3D PTV. The geometric centers of GTV and ITV were deviant by 0.13 mm laterally, 0.52 in the AP direction and 0.92 mm in the CC plane. The difference of geometric centers between GTV and ITV in the CC direction was greater in conv vs SBRT volumes (p Z .04). Conclusions: Based on the small series of patients evaluated, utilization of a 4D scan may result in less irradiation of normal tissue with conventional fractionation. ITV also allows for increased accuracy in target delineation for SBRT, which may be missed if only a 3D scan is used. Differences in geometric centers between GTV and ITV suggest that a standard radial expansion is not ideal. Therefore, generation of volumes for treatment of NSCLC should derive from a combination of 3D and 4D CT simulation. Analysis of additional patients is in process. Author Disclosure: K. Patel: None. M.C. Garofalo: None. B. Yi: None. C.G. Morris: None. S.J. Feigenberg: None.

SU‐E‐J‐56: Surface Imaging for Breast Patients

Purpose: Compares surface imaging (SI) to MV films for positioning of 13 patients receiving whole‐breast irradiation. Methods: AlignRT v5.0 was used to capture non‐gated, 3D surface images during filmed fractions (n=55) after positioning with skin marks and correction for rotations. Registrations of SI to a reference surface, generated from 3mm‐thick CT data, were analyzed off‐line for both the “entire” and the “breast” regions. MV films were compared to digitally‐reconstructed radiographs generated from the same CT dataset. The group mean, systematic error (σ), and random error (g) were calculated for positioning with MV and SI both before and after positioning correction. The 95% limits of agreement (LOA) were used for statistical comparisons. Results: Inter‐fraction systematic/random errors calculated from MV filming were 1.6/2.1mm (Anterior‐Posterior), 2.9/3.0mm (Cranio‐Caudal), and 2.4/4.1mm (Left‐Right), resulting in a setup margin of 5.5/9.4/9.0mm in AP/CC/LR dimensions using the van Herk formulation. Registering the “entire” and “breast” surfaces acquired before MV positioning correction resulted in setup margins that were larger by 1.6– 3.3mm in AP/LR but smaller by 0.4–0.6mm in CC direction, thereby indicating comparable inter‐fraction errors. After positioning with films, group means for the “entire” surface decreased by 0.5–2mm and the systematic/random errors decreased by 0.3–0.9mm. In contrast, group mean for “breast” increased in the CC direction. The LOA showed large discrepancies between the two modalities (smallest:‐3.5 to 10.3mm along AP for “entire”, largest:‐11.3 to 9.9mm along CC for “breast”). Conclusion: This study shows that the two modalities provide comparable systematic/random errors for breast positioning. However, MV positioning of bony anatomy led to a consistent cranio‐caudal offset (i.e., larger group mean) from the position indicated by AlignRT probably due to deformation of the breast tissue. These results suggest that the use of MV and SI should be integrated to ensure simultaneous coverage of chestwall and soft tissue targets.

Radiopaque marker motion during pre-treatment CBCT as a predictor of intra-fractional prostate movement

The intra-fractional movement of the prostate constitutes a hindrance for the reduction of the planning target volume margin for prostate cancer patients. Monitoring the movement of the prostate during treatment is a promising but in most centres not feasible solution. However, the projection images of the pre-treatment cone-beam computed tomography (CBCT) provide information about the motion of the target immediately preceding the treatment. This motion information can be extracted from any standard CBCT scan which is available in many institutions. In this study we measure the motion of the prostate during the pre-treatment CBCT and investigate whether this motion is correlated with the intra-fractional movement of the prostate. Material and methods. Pre- and post-treatment CBCT scans were made during a number of the fractions (average 11 range 8–12) for 13 prostate cancer patients during the radiation treatment course. The displacement of the post-treatment CBCT scans relative to the pre-treatment position was used to assess the intra-fractional motion. Automated image analysis was used to track the 2D position of radiopaque markers in the projection images of the scans. The most probable 3D trajectory of the markers during the CBCT scan was estimated based on a probability density function which was established for each individual scan. Results. The accuracy of the tracking algorithm was found satisfactory and the motion of the markers during the CBCT scans was successfully extracted from the projection images. This motion was generally small and uncorrelated with the subsequent intra-fractional movement of the prostate. The correlation coefficients were 20.05, 0.07, and 20.05 in the LR, AP, and CC direction, respectively. Conclusion. It is tempting to exploit the pre-treatment CBCT to predict the intra-fractional movement of the prostate but, unfortunately, we have found no correlation between the intra-fractional movement and the motion of the prostate immediately prior to treatment.

Analysis of GTV artifacts in all phases of four dimensional CT for peripheral lung cancer based on three-dimensional CT assisted with active breathing control.

e13524 Background: To analysis artifacts of the gross tumor volume (GTV) and correlated affecting factors in all phases of four dimensional CT (4DCT) for peripheral lung cancer based on three dimensional CT (3DCT) assisted with active breathing control. Methods: After respiratory trainning sufficiently, nineteen patients with peripheral lung cancer underwent 3DCT (CTFB) and 4DCT simulation scans during free breathing and underwent 3DCT simulation scans in end inspiration hold (CTEIH) and end expiration hold (CTEEH) assisted with active breathing control. By comparing GTVEIH and GTVEEH, to choose the reference GTV (GTVref) used to calculate artifacts of GTV in all phases of 4DCT. To calculate delineation error based on re-delineating GTV in one phase of 4DCT , to calculate the relative deviations (Devref) between the size of GTV delineated based on CTFB (GTVFB) and GTVref and between the size of GTV delineated based on all phases of 4DCT , and GTVref respectively, and to determine the cutoff of GTV size according to the relevance of the maximum relative deviation (Devmax) and the delineation error when artifacts occurred. Results: GTVEIH was chosen as the GTVref, the delineation error was 10.2%, for 15 of 19 patients Devmax was larger than delineation error (Z=-2.938, p=0.003). Devmax was larger than delineation error when the mean GTV size was not larger than 6.355 cm3. A significant negative correlation was found between GTVEIH and CVGTV (r=-0.68, p=0.002) and between GTVEIH and Devmax (r=-0.691, p=0.001). The maximum median Devref of GTV size was GTVFB with 17.83% (-16.10~105.88%); The maximum one throughout the 4DCT was GTV30 with 17.20% (-6.64~102.05%) and the minimum one was GTV80 with 4.14% (-18.18~132.53%). The Devref of the size of GTVFB (r=0.520, p=0.022), GTV20 (r=0.545, p=0.016), GTV30 (r=0.617, p=0.005), GTV70 (r=0.542, p=0.017), GTV80 (r=0.576, p=0.01) and GTV90 (r=0.478, p=0.039). Conclusions: The GTVs in each phase of 4DCT for the peripheral lung cancer all had artifacts,the sizes of the artifacts in 30% and 60% phases were larger, the other were relatively smaller. The size of artifacts correlated significantly with the tumor motion in the CC direction.

SU‐E‐J‐159: Correlation of Respiration‐Induced Motion of an External Surrogate and Implanted Internal Markers

Purpose: To investigate the correlation between the respiratory motion of implanted internal markers (IM) and external RPM traces for lung cancer patients in dynamic imaging. Methods: Under an IRB‐approved research protocol, 3 patients with VISICOIL marker (10 mm long) implants near/in their lung tumor were scanned with a daily single‐slow gantry rotation arc 4D‐CBCT acquisition. The Varian Real‐time Position Management system (RPM) was used as an external respiratory motion surrogate, with the marker block placed on the patient abdominal surface near the diaphragm. The IM motion was segmented in all 2D projections with an in‐house built Matlab program. The positions in the 3D spatial domain of the markers were extracted to (LR: left‐right, CC: cranial‐caudal, AP: anterior‐posterior). A Pearson‐type auto‐correlation function of the RPM AP trace and cross‐ correlation functions between RPM AP motion and the IM motions in three orthogonal directions was used to analyze the time series. Results: Our data shows that there is a strong positive correlation between the RPM AP motion and the IM motion in the CC direction. The correlation between the external motion and the IM motions in LR and AP directions varied from fraction to fraction. In addition, phase shift is observed in LR and AP directions. Different implanted markers in the same patient presented the very similar correlations, especially in the CC direction. This result suggests that multiple implanted markers can be used to provide supplementary information of the tumor motion. When the patient coughs during their respiratory cycle, internal motion and external motion may become uncorrelated. Conclusions: The RPM respiratory trace provides a good surrogate of the internal respiratory motion in the CC direction when the patient maintains a normal breathing pattern.Supported by NIH grant P01 CA 116602.

SU-E-T-502: Investigation of Interfraction Setup Error from Using Non-Extended Standard Thermoplastic Mask for Head and Neck IMRT Patients

Purpose: Setup accuracy of our head and neck IMRT patients based on the non-extended standard thermoplastic masks was examined using Varian onboard imaging system. Methods: Retrospective analysis of thirty head and neck IMRT patients treated during April 2009–July 2010 was performed. All 2DkV and 3D CBCT images were acquired weekly during the same session and reviewed by oncologists using an offline review 8.6 program on Eclipse TPS. Couch shifts predicted by software between 2DkV and CBCT images were recorded. PTV margin using Van Herkˈs margin formula was calculated . Results: Four-hundred-forty-four images of 2DkV and CBCT alignment were analyzed. Positioning errors within 3 mm. were shown in 84.07% of 2DkV radiographs and 85.84% of CBCT images. Average displacement found in AP, SI and LR axes, were 0.3±2.0 mm, 0±1.7 mm., 0.5 ±1.5 mm. for 2DkV, and 0.3±2.3, 0.7±2.1, 0.4 ±2.1 mm. for CBCT data set, respectively. Systematic and random variations from both methods were seen in the range of 0.5–1.8 mm. PTV margins determined from 2DkV pair images, in AP, CC and LR direction were presented at 4.60, 3.80 and 2.41 mm. when compared to 5.4, 4.32 and 4.35 mm from 3D CBCT. Adaptive treatment planning on six patients were as well undertaken owing to the great benefit of CBCT to detect the patientˈs contour changes,which can be seen in the range of 1.20–3.12 cm Conclusions: Based on our immobilization masks and laser-based positioning, majority of the treatment setups were accurate within our acceptably criteria. Both 2DkV and CBCT were insisted to be an effective method to reduce the residual setup error. Results from this study are used as a baseline for further improving the setup accuracy for head and neck IMRT patients at our institution.

Geometric Uncertainties in High Precision Radiotherapy for Brain Tumors: A Prospective Study

62 ( P + 0.7 s) Stroom’s (2 P + 0.7 s) and Van Herk’s (2.5 P + 0.7 s) formula, ( P = systematic error and s = random error). Results:With GTC frame, mean displacement in AP axis is 0.15 mm (± 0.38), CC axis -0.17 mm (± 0.42) and ML axis 0.38 mm (± 0.56). The mean vector displacement for the population is 1.03 mm (± 0.34). The errors were within ±2 mm in ML direction in 99.6% cases, in 99.2% cases in AP and 97.6% cases in CC direction. The random error was 0.73 mm in CC, 1.86 mm in AP and 1.41 in ML axes and it was normally distributed. The margins (in mm) in ML CC and AP directions were 1.55, 0.93, 1.68 (ICRU 62), 2.1, 1.35, 2.06 (Stroom’s) and 2.39, 1.56 and 2.25 (van Herk’s). In BrainLAB, mean errors in ML, CC and AP directions was 0.14 cm (SD 0.27), -0.044cm (SD 0.151) and 0.059 cm (SD 0.150), respectively. The systematic errors in ML, SI and AP directions were 0.03 cm, 0.052 cm and 0.124 cm, respectively. The random errors in ML, SI and AP directions were 0.24 cm, 0.143 cm and 0.106 cm, respectively. The margins (in cm) in ML CC and AP directions were 0.19, 0.15, 0.19 (ICRU 62), 0.23, 0.20, 0.32 (Stroom’s) and 0.24, 0.23, and 0.38 (van Herk’s). In ray cast, the mean error in ML, SI, and AP directions was 0.15 cm (SD 0.14), -0.015 cm (SD 0.326), and 0.15 cm (SD 0.1668). The systematic errors in ML, SI and AP directions were 0.133 cm, 0.2831 cm and 0.0887 cm while random errors in the same directions were 0.160 cm, 0.1784 cm and 0.1429 cm respectively. The margins (in cm) in ML CC and AP directions were 0.24, 0.40, 0.18 (ICRU 62), 0.37, 0.69, 0.27 (Stroom’s) and 0.44, 0.83 and 0.32 (van Herk’s). Conclusions: Immobilization with GTC is superior to BrainLab device for brain. In 97-99% cases the error lies between ± 2 mm. Therefore, a margin of 2 mm around the tumor seems to be adequate. BrainLAB immobilization was better than thermoplastic ray cast in reducing set up errors in radiotherapy for brain tumors. Author Disclosure: S. Das, None; R. Isiah, None; R. B, None; P. S, None; S. John, None.

Cone beam CT evaluation of patient set-up accuracy as a QA tool

Purpose. To quantify by means of cone beam CT the random and systematic uncertainty involved in radiotherapy, and to determine if this information can be used for e.g. technical quality assurance, evaluation of patient immobilization and determination of margins for the treatment planning. Patients and methods. Eighty four cancer patients have been cone beam CT scanned at treatment sessions 1, 2, 3, 10 and 20. Translational and rotational errors are analyzed. Results and conclusions. For the first three treatment sessions the mean translational error in the AP direction is 1 mm; this indicates a small error in the calibration of coronal isocentric laser. The observed SD of the systematic error in each direction is 1 mm if a correction is made after the third fraction with an action limit of 4 mm. The SD of the random errors of the patient group is approximately 1 mm in each direction. The rotational errors have a vanishing mean and a systematic error of 0.5–1.2 degrees and a random error of 0.4–0.7 degrees. The uncertainties from the first three treatment sessions (disregarding rotations) lead to a margin of 4 mm from ITV to PTV for Head-and-Neck patients (all directions) and Thorax patients (AP and lateral directions). In the CC direction, the margin has to be 5 mm for the Thorax patients. The total uncertainty on the patient position grows during the treatment course, especially in the CC direction for patients receiving thoracical irradiation. This may stem from problems in the immobilization of these patients. Consequently, it may be necessary to increase the margins in the CC direction. Once the CBCT scans have been made, the information is available for off-line analysis without any extra workload. Thus, the CBCT data can supplement scheduled QA checks.

Interfractional set-up errors evaluation by daily electronic portal imaging of IMRT in head and neck cancer patients

Introduction. Interfractional set-up errors were assessed from daily portal images (PI) registration for head and neck cancer patients. We aimed to evaluate whether a daily PI is worthwhile and we derived the Planning Target Volume (PTV) margins from the estimation of systematic and random errors. Material and methods. Twenty patients were treated in supine position with a fixed 5-point mask immobilisation system and head-and-knee supports. DRRs (Digitally Reconstructed Radiograph) were obtained from the planning CT-scan and considered the reference images to be compared with two orthogonal PI by matching bone anatomy landmarks. A total of 567 PI were done. For the set-up errors analysis, we determined the systematic, random, and overall standard deviations (SD), as well as the overall means in three directions (cranio caudal CC, medio lateral ML and anterior posterior AP). PTV-margins were calculated according to three methods. Differences of SD regarding the overall displacements among portals performed every day and each 2, 3, or 4 days were tested. Results. The systematic set-up errors were less than 1 mm in the three directions whereas the random set-up errors were around 2 mm. PTV margins varied from 3 to 4 mm in the 3 directions. Corrections were significant in the CC direction only, in which the set-up error increased significantly when the scenario of one PI every 3 fractions was adopted. Conclusions. It is of practical importance to apply on-line protocols with contouring of the bony landmarks on the PI in order to decrease the systematic mean error in this patient group. This study suggested that a PI in AP and ML directions once a week and every two days in the CC direction would be adequate to overcome the problem of set-up errors.

Repositioning accuracy of fractionated stereotactic irradiation: assessment of isocentre alignment for different dental fixations by using sequential CT scanning

Background and purpose To quantify the accuracy and reproducibility of patient repositioning in fractionated stereotactic conformal radiotherapy (SCRT) using dental fixations in conjunction with a stereotactic head mask. Patients and methods One hundred and fourteen verification CT scans were performed on 57 patients in order to check set-up alignment. The first scan was done immediately after the first treatment. Twelve patients were checked for alignment accuracy with weekly CT scans over a period of 3–6 weeks, all others had 1–2 scans. Two different dental fixations were used in combination with a non-invasive mask system: an upper jaw support (35 patients) and a customised bite-block (17 patients). Five patients were treated with no additional fixation. Co-registration to the planning CT was used to assess alignment of the isocentre to the reference markers. Additionally, the intra-operator variability of image co-registration was assessed. Results There was a significant improvement of the overall alignment in using the bite-block instead of the upper jaw support ( P<0.001). The mean deviation was for the bite-block 2.2±1.1 mm (1 SD), for the upper jaw support 3.3±1.8 mm and 3.7±2.8 mm for the mask alone. Overall isocentre deviations independent of the method of fixation were 2.8 mm (1.7 mm, 1 SD). Displacements in CC direction were significantly less for the bite-block compared to the upper jaw support ( P=0.03). The addition of an upper jaw support significantly reduced lateral rotations compared to the mask system alone ( P=0.03). The intra-operator variability of image co-registration was 1.59±0.49 mm (1 SD). Conclusion The reproducibility of patient positioning using a re-locatable head mask system combined with a bite-block is within the reported range for similar devices and is preferable to a simple upper jaw support. In order to further reduce the margin for the planning target volume an intra-oral dental fixation is recommended.