Evaluation of the setup margins for cone beam computed tomography-guided cranial radiosurgery: A phantom study.

The aim of this study is to evaluate the setup margins from the clinical target volume (CTV) to planning target volume (PTV) for cranial stereotactic radiosurgery (SRS) treatments guided by cone beam computed tomography (CBCT). We designed an end-to-end (E2E) test using a skull phantom with an embedded 6mm tungsten ball (target). A noncoplanar plan was computed (E2E plan) to irradiate the target. The CBCT-guided positioning of the skull phantom on the linac was performed. Megavoltage portal images were acquired after 15 independent deliveries of the E2E plan. The displacement 2-dimensional (2D) vector between the centers of the square field and the ball target on each portal image was used to quantify the isocenter accuracy. Geometrical margins on each patient׳s direction (left-right or LR, anterior-posterior or AP, superior-inferior or SI) were calculated. Dosimetric validation of the margins was performed in 5 real SRS cases: 3-dimesional (3D) isocenter deviations were mimicked, and changes in CTV dose coverage and organs-at-risk (OARs) dosage were analyzed. The CTV-PTV margins of 1.1mm in LR direction, and 0.7mm in AP and SI directions were derived from the E2E tests. The dosimetric analysis revealed that a 1-mm uniform margin was sufficient to ensure the CTV dose coverage, without compromising the OAR dose tolerances. The effect of isocenter uncertainty has been estimated to be 1mm in our CBCT-guided SRS approach.

Potentials and limitations of guiding liver stereotactic body radiation therapy set-up on liver-implanted fiducial markers.

PURPOSE: We investigated the potentials and limitations of guiding liver stereotactic body radiation therapy (SBRT) set-up on liver-implanted fiducial markers. METHODS AND MATERIALS: Twelve patients undergoing compression-supported SBRT in a stereotactic body frame received fluoroscopy at treatment preparation and before each treatment fraction. In fluoroscopic videos we localized the markers and diaphragm tip at expiration and the spine (measurements on free-breathing and abdominal compression). Day-to-day displacements, rotations (markers only), and deformations were determined. Marker guidance was compared to conventional set-up strategies in treatment set-up simulations. RESULTS: For compression, day-to-day motion of markers with respect to their centers of mass (COM) was sigma = 0.9 mm (random error SD), Sigma = 0.4 mm (systematic error SD), and <2.1 mm (maximum). Consequently, assuming that markers were closely surrounding spherical tumors, marker COM-guided set-up would have required safety margins of approximately 2 mm. Using marker COM as the gold standard, other set-up methods (using no correction, spine registration, and diaphragm tip craniocaudal registration) resulted in set-up errors of 1.4 mm < sigma < 2.8 mm, 2.6 mm < Sigma < 5.1 mm, and 6.3 mm < max < 12.4 mm. Day-to-day intermarker motion of <16.7%, 2.2% median, and rotations between 3.5 degrees and 7.2 degrees were observed. For markers not surrounding the tumor, e.g., 5 cm between respective COMs, these changes could effect residual tumor set-up errors up to 8.4 mm, 1.1 mm median (deformations), and 3.1 mm to 6.3 mm (rotations). Compression did not systematically contribute to deformations and rotations, since similar results were observed for free-breathing. CONCLUSIONS: If markers can be implanted near and around the tumor, residual set-up errors by marker guidance are small compared to those of conventional set-up methods, allowing high-precision tumor radiation set-up. However, substantial errors may result if markers are not implanted precisely, requiring further research to obtain adequate safety margins.

Stereotactic body radiation therapy for liver tumors: impact of daily setup corrections and day-to-day anatomic variations on dose in target and organs at risk.

PURPOSE: To assess day-to-day differences between planned and delivered target volume (TV) and organ-at-risk (OAR) dose distributions in liver stereotactic body radiation therapy (SBRT), and to investigate the dosimetric impact of setup corrections. METHODS AND MATERIALS: For 14 patients previously treated with SBRT, the planning CT scan and three treatment scans (one for each fraction) were included in this study. For each treatment scan, two dose distributions were calculated: one using the planned setup for the body frame (no correction), and one using the clinically applied (corrected) setup derived from measured tumor displacements. Per scan, the two dose distributions were mutually compared, and the clinically delivered distribution was compared with planning. Doses were recalculated in equivalent 2-Gy fraction doses. Statistical analysis was performed with the linear mixed model. RESULTS: With setup corrections, the mean loss in TV coverage relative to planning was 1.7%, compared with 6.8% without corrections. For calculated equivalent uniform doses, these figures were 2.3% and 15.5%, respectively. As for the TV, mean deviations of delivered OAR doses from planning were small (between -0.4 and +0.3 Gy), but the spread was much larger for the OARs. In contrast to the TV, the mean impact of setup corrections on realized OAR doses was close to zero, with large positive and negative exceptions. CONCLUSIONS: Daily correction of the treatment setup is required to obtain adequate TV coverage. Because of day-to-day patient anatomy changes, large deviations in OAR doses from planning did occur. On average, setup corrections had no impact on these doses. Development of new procedures for image guidance and adaptive protocols is warranted.

Automated non-coplanar beam direction optimization improves IMRT in SBRT of liver metastasis.

PURPOSE: To investigate whether automatically optimized coplanar, or non-coplanar beam setups improve intensity modulated radiotherapy (IMRT) treatment plans for stereotactic body radiotherapy (SBRT) of liver tumors, compared to a reference equi-angular IMRT plan. METHODS: For a group of 13 liver patients, an in-house developed beam selection algorithm (Cycle) was used for generation of 3D-CRT plans with either optimized coplanar-, or non-coplanar beam setups. These 10 field, coplanar and non-coplanar setups, and an 11 field, equi-angular coplanar reference setup were then used as input for generation of IMRT plans. For all plans, the PTV dose was maximized in an iterative procedure by increasing the prescribed PTV dose in small steps until further increase was prevented by constraint violation(s). RESULTS: For optimized non-coplanar setups, D(PTV, max) increased by on average 30% (range 8-64%) compared to the corresponding reference IMRT plan. Similar increases were observed for D(PTV, 99%) and gEUD(a). For optimized coplanar setups, mean PTV dose increases were only approximately 4%. After re-scaling all plans to the clinically applied dose, optimized non-coplanar configurations resulted in the best sparing of organs at risk (healthy liver, spinal cord, bowel). CONCLUSION: Compared to an equi-angular beam setup, computer optimized non-coplanar setups do result in substantial improvements in IMRT plans for SBRT of liver tumors.

Reduction of respiratory liver tumor motion by abdominal compression in stereotactic body frame, analyzed by tracking fiducial markers implanted in liver.

PURPOSE: To investigate in a three-dimensional framework the effectiveness and reproducibility of reducing the respiratory motion of liver tumors using abdominal compression in a stereotactic body frame. METHODS AND MATERIALS: A total of 12 patients with liver tumors, who were treated with stereotactic body radiotherapy, were included in this study. These patients had three gold fiducial markers implanted in the healthy liver tissue surrounding the tumor. Fluoroscopic videos were acquired on the planning day and before each treatment fraction to visualize the motion of the fiducial markers during free breathing and varying levels of abdominal compression. Software was developed to track the fiducial markers and measure their excursions. RESULTS: Abdominal compression reduced the patient group median excursion by 62% in the craniocaudal and 38% in the anteroposterior direction with respect to the median free-breathing excursions. In the left-right direction, the median excursion increased 15% (maximal increase 1.6 mm). The median residual excursion was 4.1 mm in the craniocaudal, 2.4 mm in the anteroposterior, and 1.8 mm in the left-right direction. The mean excursions were reduced by compression to <5 mm in all patients and all directions, with two exceptions (craniocaudal excursion reduction of 20.5 mm to 7.4 mm and of 21.1 mm to 5.9 mm). The residual excursions reproduced well during the treatment course, and the craniocaudal excursions measured on the treatment days were never significantly (alpha = 0.05) greater than on the planning days. Fine tuning the compression did not considerably change the excursion on the treatment days. CONCLUSIONS: Abdominal compression effectively reduced liver tumor motion, yielding small and reproducible excursions in three dimensions. The compression level established at planning could have been safely used on the treatment days.

Simultaneous tumour dose escalation and liver sparing in Stereotactic Body Radiation Therapy (SBRT) for liver tumours due to CTV-to-PTV margin reduction.

PURPOSE: To quantify potential benefits of CTV-to-PTV margin reduction for SBRT of liver tumours, as allowed by enhanced treatment precision. MATERIALS AND METHODS: For 14 patients plans were generated for the clinical margin and for 3 tighter margins. An in-house developed algorithm was used to optimise beam directions, shapes, and weights for generation of the plan with the highest isocenter dose (D(iso)), while keeping the minimum PTV dose at least 65%xD(iso) and strictly adhering to all imposed hard OAR constraints. Each plan contains 10 optimal beam directions, automatically selected from up to 252 coplanar and non-coplanar input directions. RESULTS: Apart from the expected tumour dose escalation (D(iso), EUD(PTV), gEUD(PTV)) with decreasing margin, a simultaneous improved sparing of the normal liver (D33%, D50%, D(mean)) was also observed. The smaller the margin was, the bigger both effects were. For renormalized plans with D(iso) equal to the clinical value (3x19.2Gy), and a margin reduction of 50% (2.5mm laterally, 5mm longitudinally), normal liver D33% and D50% reduced on average by 22% (maximum 38%), and 26% (maximum 47%), respectively. CONCLUSIONS: Using an algorithm for beam direction, shape and weight optimisation, large increases in the therapeutic ratio of liver plans could be obtained for reduced margins.

Quality of life after stereotactic body radiation therapy for primary and metastatic liver tumors.

PURPOSE: Stereotactic body radiation therapy (SBRT) provides a high local control rate for primary and metastatic liver tumors. The aim of this study is to assess the impact of this treatment on the patient's quality of life. This is the first report of quality of life associated with liver SBRT. METHODS AND MATERIALS: From October 2002 to March 2007, a total of 28 patients not suitable for other local treatments and with Karnofsky performance status of at least 80% were entered in a Phase I-II study of SBRT for liver tumors. Quality of life was a secondary end point. Two generic quality of life instruments were investigated, EuroQol-5D (EQ-5D) and EuroQoL-Visual Analogue Scale (EQ-5D VAS), in addition to a disease-specific questionnaire, the European Organization for Research and Treatment of Cancer Core Quality of Life Questionnaire (EORTC QLQ C-30). Points of measurement were directly before and 1, 3, and 6 months after treatment. Mean scores and SDs were calculated. Statistical analysis was performed using paired-samples t-test and Student t-test. RESULTS: The calculated EQ-5D index, EQ-5D VAS and QLQ C-30 global health status showed that mean quality of life of the patient group was not significantly influenced by treatment with SBRT; if anything, a tendency toward improvement was found. CONCLUSIONS: Stereotactic body radiation therapy combines a high local control rate, by delivering a high dose per fraction, with no significant change in quality of life. Multicenter studies including larger numbers of patients are recommended and under development.

PTV dose prescription strategies for SBRT of metastatic liver tumours.

PURPOSE: Recently we have demonstrated that our in-house developed algorithm for automated plan generation for fully non-coplanar SBRT of liver patients (designated Cycle) yields plans that are superior to conventionally generated plans of experienced dosimetrists. Here we use Cycle in the comparison of plans with prescription isodoses of 65% or 80% of the isocentre dose. METHODS: Plans were generated using CT-data of 15 previously treated patients. For each patient, both for the 65%- and the 80% strategy, Cycle was used to generate a plan with the maximum isocentre dose, D(isoc), while strictly obeying a set of hard constraints for the organs at risk (OAR). Plans for the two strategies were compared using D(isoc), D(PTV,99%) (the minimum dose delivered to 99% of the PTV), and the generalised equivalent uniform dose, gEUD(PTV)(a), for several values of the parameter a. Moreover, for the OARs, the distance to the constraint values was analysed. RESULTS: The 65% strategy resulted in treatment plans with a higher D(isoc) (average 17.6%, range 7.6-31.1%) than the 80% strategy, at the cost of a somewhat lower D(PTV,99%) (average -2.0%, range -9.6% to 9.3%). On average, voxels with a dose in the 65% strategy, lower than the minimum PTV dose in the 80% strategy, were within 0.2cm from the PTV surface. For a-10, the 65% strategy yielded on average a significantly (P<0.01) higher gEUD(PTV)(a) than the 80% strategy, whereas for highly negative a-values the 80% approach was slightly better, although not significantly. Large variations between patients were observed. Generally, for the OAR the approach to the constraint levels was similar for the two strategies. CONCLUSION: On average, PTV dose delivery is superior with the 65% strategy. However, apart from the isocentre dose, for each applied PTV dose parameter at least one patient would have been better off with the 80% dose prescription strategy.

Target coverage in image-guided stereotactic body radiotherapy of liver tumors.

PURPOSE: To determine the effect of image-guided procedures (with computed tomography [CT] and electronic portal images before each treatment fraction) on target coverage in stereotactic body radiotherapy for liver patients using a stereotactic body frame (SBF) and abdominal compression. CT guidance was used to correct for day-to-day variations in the tumor's mean position in the SBF. METHODS AND MATERIALS: By retrospectively evaluating 57 treatment sessions, tumor coverage, as obtained with the clinically applied CT-guided protocol, was compared with that of alternative procedures. The internal target volume-plus (ITV(+)) was introduced to explicitly include uncertainties in tumor delineations resulting from CT-imaging artifacts caused by residual respiratory motion. Tumor coverage was defined as the volume overlap of the ITV(+), derived from a tumor delineated in a treatment CT scan, and the planning target volume. Patient stability in the SBF, after acquisition of the treatment CT scan, was evaluated by measuring the displacement of the bony anatomy in the electronic portal images relative to CT. RESULTS: Application of our clinical protocol (with setup corrections following from manual measurements of the distances between the contours of the planning target volume and the daily clinical target volume in three orthogonal planes, multiple two-dimensional) increased the frequency of nearly full (> or = 99%) ITV(+) coverage to 77% compared with 63% without setup correction. An automated three-dimensional method further improved the frequency to 96%. Patient displacements in the SBF were generally small (< or = 2 mm, 1 standard deviation), but large craniocaudal displacements (maximal 7.2 mm) were occasionally observed. CONCLUSION: Daily, CT-assisted patient setup may substantially improve tumor coverage, especially with the automated three-dimensional procedure. In the present treatment design, patient stability in the SBF should be verified with portal imaging.

Stereotactic body radiation therapy for primary and metastatic liver tumors: A single institution phase i-ii study.

The feasibility, toxicity and tumor response of stereotactic body radiation therapy (SBRT) for treatment of primary and metastastic liver tumors was investigated. From October 2002 until June 2006, 25 patients not suitable for other local treatments were entered in the study. In total 45 lesions were treated, 34 metastases and 11 hepatocellular carcinoma (HCC). Median follow-up was 12.9 months (range 0.5-31). Median lesion size was 3.2 cm (range 0.5-7.2) and median volume 22.2 cm3 (range 1.1-322). Patients with metastases, HCC without cirrhosis, and HCC < 4 cm with cirrhosis were mostly treated with 3 x 12.5 Gy. Patients with HCC > or =4 cm and cirrhosis received 5 x 5 Gy or 3 x 10 Gy. The prescription isodose was 65%. Acute toxicity was scored following the Common Toxicity Criteria and late toxicity with the SOMA/LENT classification. Local failures were observed in two HCC and two metastases. Local control rates at 1 and 2 years for the whole group were 94% and 82%. Acute toxicity grade > or =3 was seen in four patients; one HCC patient with Child B developed a liver failure together with an infection and died (grade 5), two metastases patients presented elevation of gamma glutamyl transferase (grade 3) and another asthenia (grade 3). Late toxicity was observed in one metastases patient who developed a portal hypertension syndrome with melena (grade 3). SBRT was feasible, with acceptable toxicity and encouraging local control. Optimal dose-fractionation schemes for HCC with cirrhosis have to be found. Extreme caution should be used for patients with Child B because of a high toxicity risk.