O Gayou

Stereotactic body radiotherapy (SBRT) with or without surgery for primary and metastatic liver tumors

OBJECTIVES We report single center experience on the outcome and toxicity of SBRT alone or in combination with surgery for inoperable primary and metastatic liver tumors between 2007 and 2014. PATIENTS AND METHODS Patients with 1-4 hepatic lesions and tumor diameter ≤9 cm received SBRT at 46.8Gy ± 3.7 in 4-6 fractions. The primary end point was local control with at least 6 months of radiographic followup, and secondary end points were toxicity and survival. RESULTS Eighty-seven assessable patients (114 lesions) completed liver SBRT for hepatoma (39) or isolated metastases (48) with a median followup of 20.3 months (range 1.9-64.1). Fourteen patients underwent liver transplant with SBRT as a bridging treatment or for tumor downsizing. Eight patients completed hepatic resections in combination with planned SBRT for unresectable tumors. Two-year local control was 96% for hepatoma and 93.8% for metastases; it was 100% for lesions ≤4 cm. Two-year overall survival was 82.3% (hepatoma) and 64.3% (metastases). No incidence of grade >2 treatment toxicity was observed. CONCLUSION In this retrospective analysis we demonstrate that liver SBRT alone or in combination with surgery is safe and effective for the treatment of isolated inoperable hepatic malignancies and provides excellent local control rates.

Influence of acquisition parameters on MV-CBCT image quality

The production of high quality pretreatment images plays an increasing role in image‐guided radiotherapy (IGRT) and adaptive radiation therapy (ART). Megavoltage cone‐beam computed tomography (MV‐CBCT) is the simplest solution of all the commercially available volumetric imaging systems for localization. It also suffers the most from relatively poor contrast due to the energy range of the imaging photons. Several avenues can be investigated to improve MV‐CBCT image quality while maintaining an acceptable patient exposure: beam generation, detector technology, reconstruction parameters, and acquisition parameters. This article presents a study of the effects of the acquisition scan length and number of projections of a Siemens Artiste MV‐CBCT system on image quality within the range provided by the manufacturer. It also discusses other aspects not related to image quality one should consider when selecting an acquisition protocol. Noise and uniformity were measured on the image of a cylindrical water phantom. Spatial resolution was measured using the same phantom half filled with water to provide a sharp water/air interface to derive the modulation transfer function (MTF). Contrast‐to‐noise ratio (CNR) was measured on a pelvis‐shaped phantom with four inserts of different electron densities relative to water (1.043, 1.117, 1.513, and 0.459). Uniformity was independent of acquisition protocol. Noise decreased from 1.96% to 1.64% when the total number of projections was increased from 100 to 600 for a total exposure of 13.5 MU. The CNR showed a∓5% dependence on the number of projections and 10% dependence on the scan length. However, these variations were not statistically significant. The spatial resolution was unaffected by the arc length or the sampling rate. Acquisition parameters have little to no effect on the image quality of the MV‐CBCT system within the range of parameters available on the system. Considerations other than image quality, such as memory storage, acquisition speed, and individual projection image quality, speak in favor of the use of a coarse sampling rate on the short scan. PACS numbers: 87.57.C‐; 87.57.nf

Use of implanted gold fiducial markers with MV-CBCT image-guided IMRT for pancreatic tumours

Visualisation of soft tissues such as pancreatic tumours by mega‐voltage cone beam CT (MV‐CBCT) is frequently difficult and daily localisation is often based on more easily seen adjacent bony anatomy. Fiducial markers implanted into pancreatic tumours serve as surrogates for tumour position and may more accurately represent absolute tumour position. Differences in daily shifts based on alignment to implanted fiducial markers vs. alignment to adjacent bony anatomy were compared.

SU‐D‐BRA‐02: Penumbra Measurement for a Flattening‐Filter‐Free Artiste Linear Accelerator

Purpose: The primary benefit of removing the flattening filter of a linear accelerator is to increase the dose rate by a factor of 3 to 6. Additionally, change in scatter conditions in the head of the accelerator may also decrease the penumbra, which could lead to better target conformality. The purpose of this work is to evaluate this effect on a Siemens Artiste linear accelerator. Methods: Profiles of 10 ×10 cm2 flat and flattening‐filter‐free (FFF) fields were measured in water using a scanning diode with 0.3 mm steps. The penumbra created by both the 160‐leaf multileaf collimator(MLC) and the backup jaw (Y‐jaw) was measured. The profile of the FFF beam was renormalized so that for both flat and FFF beams, the penumbra was defined as the distance between the points with 20% and 80% of the central flat beam dose. The penumbra was measured every 5 cm along the major axes and around the central axis at four depths from 1.5 to 20 cm. Results: The average penumbra defined by the MLC and Y‐jaw was 4.9 mm and 5.9 mm, respectively for the flat beam, and 4.9 mm and 5.2 mm for the FFF beam. The flat beam penumbra increased with depth from 4.6 to 9.2 mm and 5.5 to 10.1 mm for the MLC and Y‐jaw respectively. The corresponding increase for the FFF beam penumbra was 4.6 to 9.2 mm and 4.6 to 8.8 mm. The average paired difference was <0.1 mm for the MLC and 0.8 mm for the Y‐jaw. Conclusion: The MLC penumbra was not affected by removing the flattening filter, while the decreased scattering in the FFF beam reduced the Y‐jaw penumbra by 0.8 mm. Removing the flattening filter does not clinically significantly improve the way the MLC can conform to the target. This research was partially supported by Siemens Medical Solutions.

Megavoltage conebeam CT cine as final verification of treatment plan in lung stereotactic body radiotherapy

To analyse the clinical impact of megavoltage conebeam computed tomography (MV‐CBCT) cine on internal target volume (ITV) coverage in lung stereotactic body radiotherapy (SBRT).

TU-F-12A-08: Verification of APBI Balloon Integrity Using Ultrasound Imaging

PURPOSE An overview of the ultrasound imaging artifacts encountered in Mammosite balloon APBI is presented. The goal was to determine whether clinically relevant changes in balloon size resulting from leakage can be detected by ultrasound imaging and also to compare the magnitude of size variations using both ultrasound and CT imaging. METHODS Ultrasound imaging of a Mammosite balloon phantom was performed to better understand measurement errors and accuracy. The dose to the prescription point as a function of balloon diameter was computed for different sized balloons. The results were compared to phantom measurements of balloon diameter versus filling volume to assess the dose change that would result from tissue moving inward with a shrinking balloon boundary. In APBI patients undergoing a course of 10 treatment fractions, an assessment was made of the variability and accuracy of balloon size measured with ultrasound imaging compared to CT. RESULTS Ultrasound artifacts combine to form a false image of the distal balloon boundary. Proper US probe orientation and choice of measurement point locations improves distance measurement accuracy. A 1 mm change in balloon diameter is measurable with ±0.1 mm error; this 1 mm change corresponding to <4% change in dose at a distance 1 cm from the balloon. In patients, the implanted balloon diameters ranged from 3.4 to 5.8 cm. Measurement errors relative to CT averaged less than 1.4 mm and variability (standard deviation) over the course of treatment averaged 1.9 mm. CONCLUSION Complex and misleading image artifacts can be minimized by proper probe orientation, and accurate balloon diameter measurement requires consistent image analysis methods. Ultrasound imaging can detect dosimetrically relevant changes in the size of a leaking Mammosite balloon, therefore ultrasound imaging can be used for verification of balloon integrity over the course of APBI treatment.

SU-E-J-198: Preliminary Study On Liver Radiation Response to Stereotactic Body Radiotherapy in Patients with Hepatocellular Carcinoma Based On MRI

Purpose: To develop a method to study the MRI response of tumor and hepatic parenchyma to stereotactic body radiotherapy (SBRT) in patients with hepatocellular carcinoma (HCC). Methods: Two adult patients with HCC who received SBRT were recruited in this IRB‐approved study. T1‐weighted delayed phase contrast Eovist enhanced MRI were performed immediately prior to and three months after SBRT. The time between contrast injection and MRI acquisition was identical for both pre‐and post‐SBRT imaging sessions. The patients received 3D conformal SBRT with a total dose of 40 Gy in 5 fractions. Both MRI images were rigidly registered to the planning computed tomography (CT) data set and the 3‐dimensional (3D) dosimetry data, focusing on local alignment in the tumor region. The average MRI intensity in the spleen was calculated for the pre‐and post‐SBRT images, and subtracted to reduce the effects of different imaging conditions. For each 4.9mm by 4.9mm by 6mm voxel in a region of interest (ROI) delimited by the liver contour and the 20 Gy isodose line, the difference in MRI intensity between pre‐and post‐SBRT images was generated and correlated to radiation dose deposited in that voxel. Results: The Pearson correlation coefficient between MRI intensity change and dose was 0.58 (p<0.0001) for patient A and 0.33 (p<0.0001) for the patient B. Conclusion: We have developed a method to study tumor and hepatic parenchyma response to radiation dose in HCC patients treated with SBRT. Our preliminary data suggest a correlation between the radiologic response and dose in HCC p333atients treated with SBRT, and warrant further study with more patients.

SU‐E‐J‐152: Fluoroscopic Treatment Verification for Gated Stereotactic Body Radiation Therapy of a Tumor Located Near the Dome of the Liver: A

Purpose: To describe a method using pretreatment and intrafraction fluoroscopic verification of the gated treatment of a liver lesion located near the diaphragm. Methods: A 74‐year‐old female patient, who had previously received radiation to the lung, presented with a 13.8 cc tumor located 0.5 cm inferior to the superior border of the liver, moving 2 cm in the cranio‐caudal direction with breathing. A planning CT with contrast in full exhale breath hold (FEBH) was acquired to reduce motion artifact, followed by a 4‐dimensional (4D) CT. The gross target volume (GTV) was contoured on the FEBH CT and propagated only to the 25% inspiration (25In) and 25% expiration (25Ex) phases of the 4D‐CT, in order to reduce lung dose. An internal target volume (ITV) was created encompassing all 3 phases. A plan was created to deliver 50 Gy to the PTV (ITV+0.3cm laterally and inferiorly) in 5 fractions using 3D conformal planning gated on the FE, 25In and 25Ex phases. Digitally reconstructed radiographs (DRR) were created for each beam angle and for each of the 3 respiratory phases included in the gating window. Prior to treatment, a mega‐voltage (MV)‐CBCT was acquired and its projections were analyzed in cine‐mode. Intrafraction motion verification was monitored using MV‐fluoroscopy. On both cine and fluoroscopy movies, the respiratory phase was identified using the position of the diaphragm, and the frames were compared to the corresponding DRRs. Results: Qualitative analysis on both the pretreatment cine and intrafraction fluoroscopy movies showed a clinically acceptable agreement between plan and treatment position. Conclusion: We described a treatment verification method for gated SBRT of lesions near the dome of the liver, using pre‐treatment and intrafraction fluoroscopic movies. This method is a good gated treatment quality assurance tool allowing to confidently spare lung tissue.

SU‐E‐T‐329: Measurement of I‐125 Brachytherapy Dose in Lung

Purpose: Sublobar resection and intraoperative placement of I‐125 seeds along the staple line is an accepted treatment for patients with high risk stage I non‐small cell lungcancer. The dosimetry standard for such an implant is to follow the TG43U1 protocol, which assigns water as the universal medium for dose calculation. This may not be appropriate for lungtissue. The purpose of this study is to characterize the dose distribution in lung relative to water using radiochromic film. Methods: Seven Amersham 6711 I‐125 seeds were sandwiched between slabs of Solid Water® on one side and lung‐equivalent material on the other side. A sheet of Gafchromic EBT2 film was inserted parallel to the sources axes at the depth of measurement in the water and the lung simultaneously. Separate measurements were performed for three depths of 0.5, 1.0 and 1.5 cm. The seeds were far enough apart from each other so as not to influence the measurement from the neighboring seeds. The exposures lasted 2 hours, 5.5 hours and 15 hours for 0.5 cm, 1.0 cm and 1.5 cm, respectively. The total dose in water for these exposure times was 52 cGy, 35 cGy and 39 cGy, respectively. The ratios of lung over Solid Water® doses were averaged over the 7 seeds. Results: The lung to Solid Water® dose ratio was 1.071±0.062, 1.205±0.090 and 1.259±0.053 for the depths of 0.5, 1.0 and 1.5 cm, respectively. Conclusions: The dose in lung is quite similar to the dose in water at the depth of 0.5 cm, which is the prescription depth for a typical lung low dose rate brachytherapy implant. However, the attenuation in lung is smaller than in water, leading to an underestimation of lungdose at 1.5 cm by approximately 25% when TG43 parameters are used.

SU‐GG‐T‐522: Investigation of Intracranial Localization for SRS and SRT Using MV‐CBCT

Purpose: To investigate the accuracy and precision of Mega‐Voltage Cone Beam Computed Tomography (MV‐CBCT) localization for intracranial stereotactic treatment deliveries. Method and Materials: A Rando head phantom containing a metal ball bearing (BB) was mounted with a head frame. A treatment plan targeting the BB was generated and three localization methods were compared. The first, was the standard technique using laser micro‐alignment to the isocenter projected onto target positioning sheets. Second, a standard MV‐CBCT was acquired, using skull based registration with the treatment planningCT and millimeter precision correction using the motorized treatment couch. For the third, a micro‐adjustment of the head frame position using a Vernier scale was used to apply the MV‐CBCT correction with a 0.1 mm accuracy. After localization with each method was achieved, a series of 1.0 × 1.0 cm fields from gantry angles 0, 90, 180 and 270 degrees was used to asses target alignment by measuring the BB position relative to the beam central axis. Results: The standard localization technique resulted in an average displacement of the BB with respect to the beam central axis of 1.2 ± 0.1 mm. Skull‐based localization with MV‐CBCT using millimeter precision couch offsets resulted in a displacement of the BB with respect to the beam central axis of 2.3 mm. Localization with MV‐CBCT and micro‐adjustment resulted in a displacement of the BB with respect to the beam central axis of 1.2 ± 0.2 mm. Conclusion: After modifications were applied to the MV‐CBCT localization technique, accuracy was as good as the standard approach for head frame cases. When MV‐CBCT localization is applied to a frameless case, the anatomy‐based alignment eliminates the variability of interfraction patient positioning within the immobilizing mask. Conflict of Interest: Research partially funded by Siemens Medical Solutions.