Dirkx, M.L.P.

Practical Use of the Extended No Action Level (eNAL) Correction Protocol for Breast Cancer Patients with Implanted Surgical Clips (Article)

2011-03-18T00:00:00Z

Purpose: To describe the practical use of the extended No Action Level (eNAL) setup correction protocol for breast cancer patients with surgical clips and evaluate its impact on the setup accuracy of both tumor bed and whole breast during simultaneously integrated boost treatments. Methods and Materials: For 80 patients, two orthogonal planar kilovoltage images and one megavoltage image (for the mediolateral beam) were acquired per fraction throughout the radiotherapy course. For setup correction, the eNAL protocol was applied, based on registration of surgical clips in the lumpectomy cavity. Differences with respect to application of a No Action Level (NAL) protocol or no protocol were quantified for tumor bed and whole breast. The correlation between clip migration during the fractionated treatment and either the method of surgery or the time elapsed from last surgery was investigated. Results: The distance of the clips to their center of mass (COM), averaged over all clips and patients, was reduced by 0.9 ± 1.2 mm (mean ± 1 SD). Clip migration was similar between the group of patients starting treatment within 100 days after surgery (median, 53 days) and the group starting afterward (median, 163 days) (p = 0.20). Clip migration after conventional breast surgery (closing the breast superficially) or after lumpectomy with partial breast reconstructive techniques (sutured cavity). was not significantly different either (p = 0.22). Application of eNAL on clips resulted in residual systematic errors for the clips' COM of less than 1 mm in each direction, whereas the setup of the breast was within about 2 mm of accuracy. Conclusions: Surgical clips can be safely used for high-accuracy position verification and correction. Given compensation for time trends in the clips' COM throughout the treatment course, eNAL resulted in better setup accuracies for both tumor bed and whole breast than NAL.

Does atlas-based autosegmentation of neck levels require subsequent manual contour editing to avoid risk of severe target underdosage? A dosimetric analysis (Article)

2011-03-01T00:00:00Z

Background and purpose: To investigate the dosimetric impact of not editing auto-contours of the elective neck and organs at risk (OAR), generated with atlas-based autosegmentation (ABAS) (Elekta software) for head and neck cancer patients. Materials and methods: For nine patients ABAS auto-contours and auto-contours edited by two observers were available. Based on the non-edited auto-contours clinically acceptable IMRT plans were constructed (designated 'ABAS plans'). These plans were then evaluated for the two edited structure sets, by quantifying the percentage of the neck-PTV receiving more than 95% of the prescribed dose (V95) and the near-minimum dose (D99) in the neck PTV. Dice coefficients and mean contour distances were calculated to quantify the similarity of ABAS auto-contours with the structure sets edited by observer 1 and observer 2. To study the dosimetric importance of editing OAR auto-contours a new IMRT plan was generated for each patient-observer combination, based on the observer's edited CTV and the non-edited salivary gland auto-contours. For each plan mean doses for the non-edited glands were compared with doses for the same glands edited by the observer. Results: For both observers, edited neck CTVs were larger than ABAS auto-contours (p ≤ 0.04), by a mean of 8.7%. When evaluating ABAS plans on the PTVs of the edited structure sets, V95reduced by 7.2% ± 5.4% (1 SD) (p < 0.03). The mean reduction in D99was 14.2 Gy (range 1-54 Gy). Even for Dice coefficients >0.8 and mean contour distances <1 mm, reductions in D99up to 11 Gy were observed. For treatment plans based on observer PTVs and non-edited auto-contoured salivary glands, the mean doses in the edited glands differed by only -0.6 Gy ± 1.0 Gy (p = 0.06). Conclusions: Editing of auto-contoured neck CTVs generated by ABAS is required to avoid large underdosages in target volumes. Often used similarity measures for evaluation of auto-contouring algorithms, such as dice coefficients, do not predict well for expected PTV underdose. Editing of salivary glands is less important as mean doses achieved for non-edited glands predict well for edited structures.

Implications of artefacts reduction in the planning CT originating from implanted fiducial markers (Article)

2011-01-01T00:00:00Z

The efficacy of metal artefact reduction (MAR) software to suppress artefacts in reconstructed computed tomography (CT) images originating from small metal objects, like tumor markers and surgical clips, was evaluated. In addition, possible implications of using digital reconstructed radiographs (DRRs), based on the MAR CT images, for setup verification were analyzed. A phantom and 15 patients with different tumor sites and implanted markers were imaged with a multislice CT scanner. The raw image data was reconstructed both with the clinically used filtered-backprojection (FBP) and with the MAR software. Using the MAR software, improvements in image quality were often observed in CT slices with markers or clips. Especially when several markers were located near to each other, fewer streak artefacts were observed than with the FBP algorithm. In addition, the shape and size of markers could be identified more accurately, reducing the contoured marker volumes by a factor of 2. For the phantom study, the CT numbers measured near to the markers corresponded more closely to the expected values. However, the MAR images were slightly more smoothed compared with the images reconstructed with FBP. For 8 prostate cancer patients in this study, the interobserver variation in 3D marker definition was similar (<0.4 mm) when using DRRs based on either FBP or MAR CT scans. Automatic marker matches also showed a similar success rate. However, differences in automatic match results up to 1 mm, caused by differences in the marker definition, were observed, which turned out to be (borderline) statistically significant (p = 0.06) for 2 patients. In conclusion, the MAR software might improve image quality by suppressing metal artefacts, probably allowing for a more reliable delineation of structures. When implanted markers or clips are used for setup verification, the accuracy may slightly be improved as well, which is relevant when using very tight clinical target volume (CTV) to planning target volume (PTV) margins for planning.

Inclusion of the treatment couch in portal dose image prediction for high precision EPID dosimetry (Article)

2011-01-01T00:00:00Z

Purpose: When comparing predicted portal dose images (PDIs) to PDIs acquired by an EPID during treatment delivery, differences are often observed. These differences may be partially attributed to beam attenuation by parts of the treatment couch not taken into account in the PDI prediction. In order to improve the agreement, a model for the treatment couch was derived and included in the PDI prediction. Methods: A CT scan was used to model the couch top. The model for the couch top base was derived by iteratively matching the predicted and measured PDIs for gantry angles of 0°, 45°, and 90°. For PDI prediction, the modeled treatment couch was added to the CT scan of a patient or phantom by using the recorded couch positions from the record and verify system. To validate the couch model, PDI measurements were performed for a range of couch positions and gantry angles, both with and without an anatomical phantom in the beam. Results: After including the couch model in the PDI prediction for beams passing through the couch without phantom, the mean local dose differences between measured and predicted PDIs were reduced from up to 5.5% to less than 1.0% at each gantry angle. Similar results were obtained for measurements with a lung phantom on the couch. Although the couch model was originally derived by using a 6 MV photon beam, the results showed that it is also applicable for a 10 MV beam. Conclusions: A model of the treatment couch was derived and included in the PDI prediction, yielding a substantially improved agreement between measured and predicted PDIs, which makes interpretation of the observed deviations more straightforward.

Clinical Validation of Atlas-Based Auto-Segmentation of Multiple Target Volumes and Normal Tissue (Swallowing/Mastication) Structures in the Head and Neck (Article)

2010-10-06T00:00:00Z

Purpose: To validate and clinically evaluate autocontouring using atlas-based autosegmentation (ABAS) of computed tomography images. Methods and Materials: The data from 10 head-and-neck patients were selected as input for ABAS, and neck levels I-V and 20 organs at risk were manually contoured according to published guidelines. The total contouring times were recorded. Two different ABAS strategies, multiple and single subject, were evaluated, and the similarity of the autocontours with the atlas contours was assessed using Dice coefficients and the mean distances, using the leave-one-out method. For 12 clinically treated patients, 5 experienced observers edited the autosegmented contours. The editing times were recorded. The Dice coefficients and mean distances were calculated among the clinically used contours, autocontours, and edited autocontours. Finally, an expert panel scored all autocontours and the edited autocontours regarding their adequacy relative to the published atlas. Results: The time to autosegment all the structures using ABAS was 7 min/patient. No significant differences were observed in the autosegmentation accuracy for stage N0 and N+ patients. The multisubject atlas performed best, with a Dice coefficient and mean distance of 0.74 and 2 mm, 0.67 and 3 mm, 0.71 and 2 mm, 0.50 and 2 mm, and 0.78 and 2 mm for the salivary glands, neck levels, chewing muscles, swallowing muscles, and spinal cord-brainstem, respectively. The mean Dice coefficient and mean distance of the autocontours vs. the clinical contours was 0.8 and 2.4 mm for the neck levels and salivary glands, respectively. For the autocontours vs. the edited autocontours, the mean Dice coefficient and mean distance was 0.9 and 1.6 mm, respectively. The expert panel scored 100% of the autocontours as a "minor deviation, editable" or better. The expert panel scored 88% of the edited contours as good compared with 83% of the clinical contours. The total editing time was 66 min. Conclusion: Multiple-subject ABAS of computed tomography images proved to be a useful novel tool in the rapid delineation of target and normal tissues. Although editing of the autocontours is inevitable, a substantial time reduction was achieved using editing, instead of manual contouring (180 vs. 66 min).

Evaluation of the 'dose of the day' for IMRT prostate cancer patients derived from portal dose measurements and cone-beam CT (Article)

2010-08-01T00:00:00Z

Purpose: High geometrical and dosimetrical accuracies are required for radiotherapy treatments where IMRT is applied in combination with narrow treatment margins in order to minimize dose delivery to normal tissues. As an overall check, we implemented a method for reconstruction of the actually delivered 3D dose distribution to the patient during a treatment fraction, i.e., the 'dose of the day'. In this article results on the clinical evaluation of this concept for a group of IMRT prostate cancer patients are presented. Materials and methods: The actual IMRT fluence maps delivered to a patient were derived from measured EPID-images acquired during treatment using a previously described iterative method. In addition, the patient geometry was obtained from in-room acquired cone-beam CT images. For dose calculation, a mapping of the Hounsfield Units from the planning CT was applied. With the fluence maps and the modified cone-beam CT the 'dose of the day' was calculated. The method was validated using phantom measurements and evaluated clinically for 10 prostate cancer patients in 4 or 5 fractions. Results: The phantom measurements showed that the delivered dose could be reconstructed within 3%/3 mm accuracy. For prostate cancer patients, the isocenter dose agreed within -0.4 ± 1.0% (1 SD) with the planned value, while for on average 98.1% of the pixels within the 50% isodose surface the actually delivered dose agreed within 3% or 3 mm with the planned dose. For most fractions, the dose coverage of the prostate volume was slightly deteriorated which was caused by small prostate rotations and small inaccuracies in fluence delivery. The dose that was delivered to the rectum remained within the constraints used during planning. However, for two patients a large degrading of the dose delivery was observed in two fractions. For one patient this was related to changes in rectum filling with respect to the planning CT and for the other to large intra-fraction motion during treatment delivery, resulting in mean underdosages of 16% in the prostate volume. Conclusions: A method to accurately assess the 'dose of the day' was evaluated for prostate cancer patients treated with IMRT. To correct for observed dose deviations off-line dose-adaptive strategies will be developed.

Dosimetric validation of a commercial Monte Carlo based IMRT planning system (Article)

2010-02-08T00:00:00Z

Purpose: Recently a commercial Monte Carlo based IMRT planning system (Monaco version 1.0.0) was released. In this study the dosimetric accuracy of this new planning system was validated. Methods: Absolute dose profiles, depth dose curves, and output factors calculated by Monaco were compared with measurements in a water phantom. Different static on-axis and off-axis fields were tested at various source-skin distances for 6, 10, and 18 MV photon beams. Four clinical IMRT plans were evaluated in a water phantom using a linear diode detector array and another six IMRT plans for different tumor sites in solid water using a 2D detector array. In order to evaluate the accuracy of the dose engine near tissue inhomogeneities absolute dose distributions were measured with Gafchromic EBT film in an inhomogeneous slab phantom. For an end-to-end test a four-field IMRT plan was applied to an anthropomorphic lung phantom with a simulated tumor peripherally located in the right lung. Gafchromic EBT film, placed in and around the tumor area, was used to evaluate the dose distribution. Results: Generally, the measured and the calculated dose distributions agreed within 2% dose difference or 2 mm distance-to-agreement. But mainly at interfaces with bone, some larger dose differences could be observed. Conclusions: Based on the results of this study, the authors concluded that the dosimetric accuracy of Monaco is adequate for clinical introduction.

Dosimetric validation of a commercial Monte Carlo based IMRT planning system (Article)

2010-02-08T00:00:00Z

Evaluation of the dosimetric impact of non-exclusion of the rectum from the boost PTV in IMRT treatment plans for prostate cancer patients (Article)

2009-07-01T00:00:00Z

Purpose: In dose escalation trial, for prostate cancer patients, zero CTV-PTV margins towards the rectum are often applied in the boost phase in order to avoid excessive dose delivery to the rectum. In this study, the dosimetric impact of non-exclusion of the rectum from the boost PTV is evaluated. Treatment plans created according to the protocol used in our institute for patients in a Dutch hypofractionated trial (HYPO), where the rectum is excluded from the boost PTV, were compared to plans designed with a modified version of this protocol (HYPO-exp) for which the rectal exclusion was not performed. Differences in target coverage and rectum dose were quantified. Methods and materials: Treatment plans were generated for 36 prostate cancer patients. In the HYPO plans, the CTV-PTV margins around the prostate were 6 mm (7.5 mm at the caudal side) and 10 mm around the seminal vesicles (PTV1). For the boost phase, these margins were reduced to 5 mm, but no margin was taken at the overlap with the rectum (PTV2). The margin prescription for HYPO-exp was identical to that for HYPO, except that the zero CTV-PTV margin towards the rectum was omitted. For the HYPO and HYPO-exp plans, a simultaneous integrated boost technique using IMRT was applied to deliver 72.2 Gy to PTV1 and 78 Gy to PTV2. For all plans, the dose to the rectum was compared using V50, V60, V70, the equivalent uniform dose (EUD), considering α = 9 and 1, respectively, and normal tissue complication probabilities (NTCPs). In addition, the dose coverage of PTV1 and PTV2 and the minimum dose in those volumes were quantified. To assess the clinical impact of differences in dose delivery to the rectum, both IMRT plans were also compared to a plan (DESC) based on the treatment protocol applied in our institute in a former national dose escalation trial, which in the meantime has a median follow-up of six years. Results: Compared to HYPO, V70and the rectal EUD calculated with α = 9 were slightly higher for HYPO-exp, but the differences were not statistically significant. V50, V60and the rectal EUD calculated with α = 1 were similar for both the IMRT plans. In contrast, each of these parameters was significantly lower compared to DESC (p < 0.001). The coverage of the boost PTV, used in HYPO-exp, by at least 95% of the prescribed dose was significantly better for HYPO-exp than for HYPO (p < 0.001). In the overlap of this volume with the rectum, the minimum dose increased by 1.1 ± 1.2 Gy for HYPO-exp (p = 0.002) and the mean dose by 1.2 ± 1.5 Gy (p = 0.001). Conclusion: By omitting the zero margin towards the rectum, underdosages in the target volume are reduced significantly, while a clinically relevant increase in rectum exposure is not observed.

Portal dose image prediction for in vivo treatment verification completely based on EPID measurements (Article)

2009-03-05T00:00:00Z

A high dosimetric accuracy is required for radiotherapy treatments where IMRT in combination with narrow treatment margins is applied to achieve optimally conformal dose distributions. In order to routinely verify the in vivo fluence delivery (i.e., during the actual patient treatment), our method for predicting portal dose images with a patient in the beam was validated. A unique feature of this method is that it is fully based on calibration measurements with an EPID. The portal dose image (PDI) behind a patient is dependent on the transmission of primary radiation through the patient and a contribution of scattered radiation from the patient. To derive both components, the patient geometry as observed in the planning CT scan is converted into an equivalent homogeneous phantom. A limited set of EPID measurements is required to derive the input parameters of this model. The accuracy of the in vivo PDI prediction was verified using measurements behind phantoms and four prostate cancer patients treated with IMRT. Behind homogeneous slab phantoms, the local differences between measured and predicted PDIs were within 2% inside the field, while behind a lung and a pelvic phantom, the agreement was within 3% or within 3 mm in regions with steep gradients. Outside the fields, the PDIs agreed within 2% of the global dose maximum. Evaluation of the in vivo PDI measurements behind patients showed that, on average, 87% of the pixels inside the field fulfilled the 3% local dose and 3 mm distance-to-agreement criteria. For half of the failing pixels the differences occurred due to changes in patient geometry with respect to the planning CT or due to beam attenuation by the treatment couch that was not accounted for. A fully EPID-based method for predicting portal dose images using the planning CT scan has been implemented and validated for phantoms and clinical patients.

Surgical clips for position verification and correction of non-rigid breast tissue in simultaneously integrated boost (SIB) treatments (Article)

2009-01-01T00:00:00Z

Background and purpose: The aim of this study is to investigate whether surgical clips in the lumpectomy cavity are representative for position verification of both the tumour bed and the whole breast in simultaneously integrated boost (SIB) treatments. Materials and methods: For a group of 30 patients treated with a SIB technique, kV and MV planar images were acquired throughout the course of the fractionated treatment. The 3D set-up error for the tumour bed was derived by matching the surgical clips (3-8 per patient) in two almost orthogonal planar kV images. By projecting the 3D set-up error derived from the planar kV images to the (u, v)-plane of the tangential beams, the correlation with the 2D set-up error for the whole breast, derived from the MV EPID images, was determined. The stability of relative clip positions during the fractionated treatment was investigated. In addition, for a subgroup of 15 patients, the impact of breathing was determined from fluoroscopic movies acquired at the linac. Results: The clip configurations were stable over the course of radiotherapy, showing an inter-fraction variation (1 SD) of 0.5 mm on average. Between the start and the end of the treatment, the mean distance between the clips and their center of mass was reduced by 0.9 mm. A decrease larger than 2 mm was observed in eight patients (17 clips). The top-top excursion of the clips due to breathing was generally less than 2.5 mm in all directions. The population averages of the difference (±1 SD) between kV and MV matches in the (u, v)-plane were 0.2 ± 1.8 mm and 0.9 ± 1.5 mm, respectively. In 30% of the patients, time trends larger than 3 mm were present over the course of the treatment in either or in both kV and MV match results. Application of the NAL protocol based on the clips reduced the population mean systematic error to less than 2 mm in all directions, both for the tumour bed and the whole breast. Due to the observed time trends, these systematic errors can be further reduced to about 1 mm by using an eNAL protocol instead. Conclusions: The relative positions of implanted surgical clips in the lumpectomy cavity after breast-conserving surgery remain stable during the course of radiotherapy treatment. Application of a NAL or eNAL set-up correction protocol based on surgical clips allows for adequate treatment of both the tumour bed and the whole breast with tight CTV-PTV margins.

Correction of conebeam CT values using a planning CT for derivation of the "dose of the day" (Article)

2007-11-01T00:00:00Z

Background and purpose: Verification of the actually delivered 3D dose distribution during each treatment fraction ("dose of the day") is the most complete and clinical relevant "in-vivo" check of an IMRT treatment. To do this, during patient treatment portal dose images are routinely acquired with our electronic portal imaging device to derive the delivered fluence map for each treatment field. In addition, a conebeam CT scan is acquired just prior to treatment to derive the patient geometry at the time of treatment. However, the use of conebeam CT scans for dose calculation is hampered by inaccuracies in the conversion of CT values to electron densities due to an enlarged scatter contribution. Materials and methods: In this work, a method is described for mapping of Hounsfield Units of the planning CT to the conebeam CT scan, while accounting for non-rigidity in the anatomy, e.g. related to weight loss, in an approximate way. The method was validated for head and neck cancer patients by comparing dose distributions calculated using adjusted Hounsfield Units with a golden standard. Results and conclusions: The observed dose differences were less than 1% in the majority of points, and in at least 96% of the points a 3D γ analysis resulted in γ values of less than 1 when applying a 2%/2 mm criterion, showing that this straightforward approach allows for an accurate dose calculation based on conebeam CT scans.

3D dose reconstruction for clinical evaluation of IMRT pretreatment verification with an EPID (Article)

2007-02-01T00:00:00Z

Background and purpose: Pretreatment verification with an electronic portal imaging device is an important part of our patient-specific quality assurance program for advanced treatment techniques. Up to now, this verification has been performed for over 400 IMRT patient plans. For every treatment field, a 2D portal dose image (PDI) is measured and compared with a predicted PDI. Often it is not straightforward to interpret dose deviations found in these 2D comparisons in terms of clinical implications for the patient. Therefore, a method to derive the 3D patient dose based on the measured PDIs was implemented. Methods and materials: For reconstruction of the 3D patient dose, the actual fluences delivered by the accelerator are derived from measured portal dose images using an iterative method. The derived fluence map for each beam direction is then used as input for the treatment planning system to generate an adapted 3D patient dose distribution. The accuracy of this method was assessed by measurements in a water phantom. Clinical evaluation of the 3D dose reconstruction was performed for 17 IMRT patients with different tumor sites. Dose differences with respect to the original treatment plan were evaluated in individual CT slices using dose difference maps and a 3D γ analysis and by comparing dose-volume histograms (DVHs). Results: The measurements indicated that the accuracy of the 3D dose reconstruction was within 2%/2 mm. For the patients observed dose differences with respect to the original plan were generally within 2%, except at the field edges and in the sharp dose gradients around the planning target volume (PTV). Gamma analysis showed that the dose differences were within 2%/2 mm for more than 95% of the points in all cases. Differences in DVH parameters for the PTV and organs at risk were also within 2% in nearly all cases. Conclusion: A method to derive actual delivered fluence maps from measured PDIs and to use them to reconstruct the 3D patient dose was implemented. The reconstruction eases the estimation of the clinical relevance of observed dose differences in the pretreatment measurements.

Static and dynamic beam intensity modulation in radiotherapy using a multileaf collimator (Doctoral Thesis)

2000-12-08T00:00:00Z