F Hacker

A clinical comparison of patient setup and intra-fraction motion using frame-based radiosurgery versus a frameless image-guided radiosurgery system for intracranial lesions

BACKGROUND AND PURPOSE A comparison of patient positioning and intra-fraction motion using invasive frame-based radiosurgery with a frameless X-ray image-guided system utilizing a thermoplastic mask for immobilization. MATERIALS AND METHODS Overall system accuracy was determined using 57 hidden-target tests. Positioning agreement between invasive frame-based setup and image-guided (IG) setup, and intra-fraction displacement, was evaluated for 102 frame-based SRS treatments. Pre and post-treatment imaging was also acquired for 7 patients (110 treatments) immobilized with an aquaplast mask receiving fractionated IG treatment. RESULTS The hidden-target tests demonstrated a mean error magnitude of 0.7mm (SD=0.3mm). For SRS treatments, mean deviation between frame-based and image-guided initial positioning was 1.0mm (SD=0.5mm). Fusion failures were observed among 3 patients resulting in aberrant predicted shifts. The image-guidance system detected frame slippage in one case. The mean intra-fraction shift magnitude observed for the BRW frame was 0.4mm (SD=0.3mm) compared to 0.7mm (SD=0.5mm) for the fractionated patients with the mask system. CONCLUSIONS The overall system accuracy is similar to that reported for invasive frame-based SRS. The intra-fraction motion was larger with mask-immobilization, but remains within a range appropriate for stereotactic treatment. These results support clinical implementation of frameless radiosurgery using the Novalis Body Exac-Trac system.

STEREOTACTIC IRRADIATION OF THE POSTOPERATIVE RESECTION CAVITY FOR BRAIN METASTASIS: A FRAMELESS LINEAR ACCELERATOR-BASED CASE SERIES AND REVIEW OF THE TECHNIQUE

PURPOSE Whole-brain radiation therapy (WBRT) is the standard of care after resection of a brain metastasis. However, concern regarding possible neurocognitive effects and the lack of survival benefit with this approach has led to the use of stereotactic radiosurgery (SRS) to the resection cavity in place of WBRT. We report our initial experience using an image-guided linear accelerator-based frameless stereotactic system and review the technical issues in applying this technique. METHODS AND MATERIALS We retrospectively reviewed the setup accuracy, treatment outcome, and patterns of failure of the first 18 consecutive cases treated at Brigham and Womens Hospital. The target volume was the resection cavity without a margin excluding the surgical track. RESULTS The median number of brain metastases per patient was 1 (range, 1-3). The median planning target volume was 3.49 mL. The median prescribed dose was 18 Gy (range, 15-18 Gy) with normalization ranging from 68% to 85%. In all cases, 99% of the planning target volume was covered by the prescribed dose. The median conformity index was 1.6 (range, 1.41-1.92). The SRS was delivered with submillimeter accuracy. At a median follow-up of 12.7 months, local control was achieved in 16/18 cavities treated. True local recurrence occurred in 2 patients. No marginal failures occurred. Distant recurrence occurred in 6/17 patients. Median time to any failure was 7.4 months. No Grade 3 or higher toxicity was recorded. A long interval between initial cancer diagnosis and the development of brain metastasis was the only factor that trended toward a significant association with the absence of recurrence (local or distant) (log-rank p = 0.097). CONCLUSIONS Frameless stereotactic irradiation of the resection cavity after surgery for a brain metastasis is a safe and accurate technique that offers durable local control and defers the use of WBRT in select patients. This technique should be tested in larger prospective studies.

Automatic marker detection and 3D position reconstruction using cine EPID images for SBRT verification

In previous studies, an electronic portal imaging device (EPID) in cine mode was used for validating respiratory gating and stereotactic body radiation therapy (SBRT) by tracking implanted fiducials. The manual marker tracking methods that were used were time and labor intensive, limiting the utility of the validation. The authors have developed an automatic algorithm to quickly and accurately extract the markers in EPID images and reconstruct their 3D positions. Studies have been performed with gold fiducials placed in solid water and dynamic thorax phantoms. In addition, the authors have examined the cases of five patients being treated under an SBRT protocol for hepatic metastases. For each case, a sequence of images was created by collecting the exit radiation using the EPID. The markers were detected and recognized using an image processing algorithm based on the Laplacian of Gaussian function. To reduce false marker detection, a marker registration technique was applied using image intensity as well as the geometric spatial transformations between the reference marker positions produced from the projection of 3D CT images and the estimated marker positions. An average marker position in 3D was reconstructed by backprojecting, towards the source, the position of each marker on the 2D image plane. From the static phantom study, spatial accuracies of <1 mm were achieved in both 2D and 3D marker locations. From the dynamic phantom study, using only the Laplacian of the Gaussian algorithm, the marker detection success rate was 88.8%. However, adding a marker registration technique which utilizes prior CT information, the detection success rate was increased to 100%. From the SBRT patient study, intrafractional tumor motion (3.1-11.3 mm) in the SI direction was measured using the 2D images. The interfractional patient setup errors (0.1-12.7 mm) in the SI, AP, and LR directions were obtained from the average marker locations reconstructed in 3D and compared to the reference planning CT image. The authors have developed an automatic algorithm to extract marker locations from MV images and have evaluated its performance. The measured intrafractional tumor motion and the interfractional daily patient setup error can be used for off-line retrospective verification of SBRT.

Beam shaping for conformal fractionated stereotactic radiotherapy : A modeling study

PURPOSE The patient population treated with fractionated stereotactic radiotherapy (SRT) is significantly different than that treated with stereotactic radiosurgery (SRS). Generally, lesions treated with SRT are larger, less spherical, and located within critical regions of the central nervous system; hence, they offer new challenges to the treatment planner. Here a simple, cost effective, beam shaping system has been evaluated relative to both circular collimators and an ideal dynamically conforming system for effectiveness in providing conformal therapy for these lesions. METHODS AND MATERIALS We have modeled a simple system for conformal arc therapy using four independent jaws. The jaw positions and collimator angle are changed between arcs but held fixed for the duration of each arc. Eleven previously treated SRT cases have been replanned using this system. The rectangular jaw plans were then compared to the original treatment plans which used circular collimators. The plans were evaluated with respect to tissue sparing at 100%, 80%, 50%, and 20% of the prescription dose. A plan was also done for each tumor in which the beam aperture was continuously conformed to the beams eye view projection of the tumor. This was used as an ideal standard for conformal therapy in the absence of fluence modulation. RESULTS For tumors with a maximum extent of over 3.5 cm the rectangular jaw plans reduced the mean volume of healthy tissue involved at the prescription dose by 57% relative to the circular collimator plans. The ideal conformal plans offered no significant further improvement at the prescription dose. The relative advantage of the rectangular jaw plans decreased at lower isodoses so that at 20% of the prescription dose tissue involvement for the rectangular jaw plans was equivalent to that for the circular collimator plans. At these isodoses the ideal conformal plans gave substantially better tissue sparing. CONCLUSION A simple and economical field shaping device has been shown to provide all of the beam shaping advantage of a hypothetical ideal dynamically conforming system at the prescription level. This system may be immediately implemented in the clinic. It offers a substantial advantage over the currently used circular collimators in the high dose region with equivalent performance in the low dose region.

Low Incidence of Chest Wall Pain with a Risk-Adapted Lung Stereotactic Body Radiation Therapy Approach Using Three or Five Fractions Based on Chest Wall Dosimetry

Purpose To examine the frequency and potential of dose-volume predictors for chest wall (CW) toxicity (pain and/or rib fracture) for patients receiving lung stereotactic body radiotherapy (SBRT) using treatment planning methods to minimize CW dose and a risk-adapted fractionation scheme. Methods We reviewed data from 72 treatment plans, from 69 lung SBRT patients with at least one year of follow-up or CW toxicity, who were treated at our center between 2010 and 2013. Treatment plans were optimized to reduce CW dose and patients received a risk-adapted fractionation of 18 Gy×3 fractions (54 Gy total) if the CW V30 was less than 30 mL or 10–12 Gy×5 fractions (50–60 Gy total) otherwise. The association between CW toxicity and patient characteristics, treatment parameters and dose metrics, including biologically equivalent dose, were analyzed using logistic regression. Results With a median follow-up of 20 months, 6 (8.3%) patients developed CW pain including three (4.2%) grade 1, two (2.8%) grade 2 and one (1.4%) grade 3. Five (6.9%) patients developed rib fractures, one of which was symptomatic. No significant associations between CW toxicity and patient and dosimetric variables were identified on univariate nor multivariate analysis. Conclusions Optimization of treatment plans to reduce CW dose and a risk-adapted fractionation strategy of three or five fractions based on the CW V30 resulted in a low incidence of CW toxicity. Under these conditions, none of the patient characteristics or dose metrics we examined appeared to be predictive of CW pain.

Quantifying ITV instabilities arising from 4DCT: a simulation study using patient data.

Treatment planning for patients undergoing radiation therapy is often performed based on four-dimensional computed tomography (4DCT) when respiratory motion is present, as in lung cancer patients. 4DCT is used to define the internal target volume (ITV) that, ideally, incorporates all potential locations of the tumour. In this work, we use the locations of gold fiducial markers implanted in lung tumours of eight patients to represent tumour motion. These fiducial locations are used in a simulation of a four-slice CT scanner to generate the ITV for 10, 20 and 30 mm diameter model tumours. To demonstrate instabilities in the ITV definition based on 4DCT, the ITV calculation was repeated for the same patients for consecutive scan start times, staggered by 1 s. The volumetric difference in the ITV and the per cent of time that the ITV contains in the tumour are both evaluated. The ITV from a single patient was found to vary by 46%-127% for a tumour diameter of 10 mm. The ITV did not cover the entirety of the tumour 11%-74% of the time for a 10 mm tumour diameter.

Outcomes by Tumor Histology and KRAS Mutation Status After Lung Stereotactic Body Radiation Therapy for Early-Stage Non–Small-Cell Lung Cancer

BACKGROUND We analyzed outcomes after lung stereotactic body radiotherapy (SBRT) for early-stage non-small cell lung-carcinoma (NSCLC) by histology and KRAS genotype. PATIENTS AND METHODS We included 75 patients with 79 peripheral tumors treated with SBRT (18 Gy × 3 or 10 to 12 Gy × 5) at our institution from 2009 to 2012. Genotyping for KRAS mutations was performed in 10 patients. Outcomes were analyzed by the Kaplan-Meier method/Cox regression, or cumulative incidence method/Fine-Gray analysis. RESULTS The median patient age was 74 (range, 46 to 93) years, and Eastern Cooperative Oncology Group performance status was 0 to 1 in 63%. Tumor histology included adenocarcinoma (44%), squamous cell carcinoma (25%), and NSCLC (18%). Most tumors were T1a (54%). Seven patients had KRAS-mutant tumors (9%). With a median follow-up of 18.8 months among survivors, the 1-year estimate of overall survival was 88%, cancer-specific survival (CSS) 92%, primary tumor control 94%, and freedom from recurrence (FFR) 67%. In patients with KRAS-mutant tumors, there was a significantly lower tumor control (67% vs. 96%; P = .04), FFR (48% vs. 69%; P = .03), and CSS (75% vs. 93%; P = .05). On multivariable analysis, histology was not associated with outcomes, but KRAS mutation (hazard ratio, 10.3; 95% confidence interval, 2.3-45.6; P = .0022) was associated with decreased CSS after adjusting for age. CONCLUSION In this SBRT series, histology was not associated with outcomes, but KRAS mutation was associated with lower FFR on univariable analysis and decreased CSS on multivariable analysis. Because of the small sample size, these hypothesis-generating results need to be studied in larger data sets.

Total dural irradiation: RapidArc versus static-field IMRT: A case study

The purpose of this study was to compare conventional fixed-gantry angle intensity-modulated radiation therapy (IMRT) with RapidArc for total dural irradiation. We also hypothesize that target volume-individualized collimator angles may produce substantial normal tissue sparing when planning with RapidArc. Five-, 7-, and 9-field fixed-gantry angle sliding-window IMRT plans were generated for comparison with RapidArc plans. Optimization and normal tissue constraints were constant for all plans. All plans were normalized so that 95% of the planning target volume (PTV) received at least 100% of the dose. RapidArc was delivered using 350° clockwise and counterclockwise arcs. Conventional collimator angles of 45° and 315° were compared with 90° on both arcs. Dose prescription was 59.4 Gy in 33 fractions. PTV metrics used for comparison were coverage, V(107)%, D1%, conformality index (CI(95)%), and heterogeneity index (D(5)%-D(95)%). Brain dose, the main challenge of this case, was compared using D(1)%, Dmean, and V(5) Gy. Dose to optic chiasm, optic nerves, globes, and lenses was also compared. The use of unconventional collimator angles (90° on both arcs) substantially reduced dose to normal brain. All plans achieved acceptable target coverage. Homogeneity was similar for RapidArc and 9-field IMRT plans. However, heterogeneity increased with decreasing number of IMRT fields, resulting in unacceptable hotspots within the brain. Conformality was marginally better with RapidArc relative to IMRT. Low dose to brain, as indicated by V5Gy, was comparable in all plans. Doses to organs at risk (OARs) showed no clinically meaningful differences. The number of monitor units was lower and delivery time was reduced with RapidArc. The case-individualized RapidArc plan compared favorably with the 9-field conventional IMRT plan. In view of lower monitor unit requirements and shorter delivery time, RapidArc was selected as the optimal solution. Individualized collimator angle solutions should be considered by RapidArc dosimetrists for OARs dose reduction. RapidArc should be considered as a treatment modality for tumors that extensively involve in the skull, dura, or scalp.

Evaluation of initial setup accuracy and intrafraction motion for spine stereotactic body radiation therapy using stereotactic body frames

PURPOSE The purposes of this study were (1) to evaluate the initial setup accuracy and intrafraction motion for spine stereotactic body radiation therapy (SBRT) using stereotactic body frames (SBFs) and (2) to validate an in-house-developed SBF using a commercial SBF as a benchmark. METHODS AND MATERIALS Thirty-two spine SBRT patients (34 sites, 118 fractions) were immobilized with the Elekta and in-house (BHS) SBFs. All patients were set up with the Brainlab ExacTrac system, which includes infrared and stereoscopic kilovoltage x-ray-based positioning. Patients were initially positioned in the frame with the use of skin tattoos and then shifted to the treatment isocenter based on infrared markers affixed to the frame with known geometry relative to the isocenter. ExacTrac kV imaging was acquired, and automatic 6D (6 degrees of freedom) bony fusion was performed. The resulting translations and rotations gave the initial setup accuracy. These translations and rotations were corrected for by use of a robotic couch, and verification imaging was acquired that yielded residual setup error. The imaging/fusion process was repeated multiple times during treatment to provide intrafraction motion data. RESULTS The BHS SBF had greater initial setup errors (mean±SD): -3.9±5.5mm (0.2±0.9°), -1.6±6.0mm (0.5±1.4°), and 0.0±5.3mm (0.8±1.0°), respectively, in the vertical (VRT), longitudinal (LNG), and lateral (LAT) directions. The corresponding values were 0.6±2.7mm (0.2±0.6°), 0.9±5.3mm (-0.2±0.9°), and -0.9±3.0mm (0.3±0.9°) for the Elekta SBF. The residual setup errors were essentially the same for both frames and were -0.1±0.4mm (0.1±0.5°), -0.2±0.4mm (0.0±0.4°), and 0.0±0.4mm (0.0±0.4°), respectively, in VRT, LNG, and LAT. The intrafraction shifts in VRT, LNG, and LAT were 0.0±0.4mm (0.0±0.3°), 0.0±0.5mm (0.0±0.4°), and 0.0±0.4mm (0.0±0.3°), with no significant difference observed between the 2 frames. CONCLUSIONS These results showed that the combination of the ExacTrac system with either SBF was highly effective in achieving both setup accuracy and intrafraction stability, which were on par with that of mask-based cranial radiosurgery.

Evaluation of a new IR-guided system for mechanical QA of linear accelerators

The authors report the development of a new procedure for mechanical quality assurance of linear accelerators using an infrared-guided system. The system consists of an infrared (IR) camera and an IR-reflective marker that can be attached to a gantry, a collimator, or a treatment table. The trace of this marker can be obtained in three dimensions (3D) for a full or partial rotation of the mechanical devices. The software is written to localize rotational axes of the gantry, collimator, and the treatment table based on the marker traces. The separation of these axes characterizes the size of the sphere defining the mechanical isocenter. Additional information on anomalies in gantry movement such as degree of gantry sag and hysteresis can also be obtained. An intrinsic uncertainty of the system to localize rotational axis is 0.35mm or less. Tests on a linear accelerator demonstrated the ability of this system to detect the separation between rotational axes of less than 1mm and to confirm orthogonality of the planes of gantry, collimator, and table rotation.