V. Sehgal

Recommendations of the American Association of Physicists in Medicine on dosimetry, imaging, and quality assurance procedures for 90Y microsphere brachytherapy in the treatment of hepatic malignancies.

Yttrium-90 microsphere brachytherapy of the liver exploits the distinctive features of the liver anatomy to treat liver malignancies with beta radiation and is gaining more wide spread clinical use. This report provides a general overview of microsphere liver brachytherapy and assists the treatment team in creating local treatment practices to provide safe and efficient patient treatment. Suggestions for future improvements are incorporated with the basic rationale for the therapy and currently used procedures. Imaging modalities utilized and their respective quality assurance are discussed. General as well as vendor specific delivery procedures are reviewed. The current dosimetry models are reviewed and suggestions for dosimetry advancement are made. Beta activity standards are reviewed and vendor implementation strategies are discussed. Radioactive material licensing and radiation safety are discussed given the unique requirements of microsphere brachytherapy. A general, team-based quality assurance program is reviewed to provide guidance for the creation of the local procedures. Finally, recommendations are given on how to deliver the current state of the art treatments and directions for future improvements in the therapy.

Bone mineral density loss in thoracic and lumbar vertebrae following radiation for abdominal cancers.

PURPOSE To investigate the relationship between abdominal chemoradiation (CRT) for locally advanced cancers and bone mineral density (BMD) reduction in the vertebral spine. MATERIALS AND METHODS Data from 272 patients who underwent abdominal radiation therapy from January 1997 to May 2015 were retrospectively reviewed. Forty-two patients received computed tomography (CT) scans of the abdomen prior to initiation and at least twice after radiation therapy. Bone attenuation (in Hounsfield unit) (HU) measurements were collected for each vertebral level from T7 to L5 using sagittal CT images. Radiation point dose was obtained at each mid-vertebral body from the radiation treatment plan. Percent change in bone attenuation (Δ%HU) between baseline and post-radiation therapy were computed for each vertebral body. The Δ%HU was compared against radiation dose using Pearsons linear correlation. RESULTS Abdominal radiotherapy caused significant reduction in vertebral BMD as measured by HU. Patients who received only chemotherapy did not show changes in their BMD in this study. The Δ%HU was significantly correlated with the radiation point dose to the vertebral body (R=-0.472, P<0.001) within 4-8 months following RT. The same relationship persisted in subsequent follow up scans 9 months following RT (R=-0.578, P<0.001). Based on the result of linear regression, 5 Gy, 15 Gy, 25 Gy, 35 Gy, and 45 Gy caused 21.7%, 31.1%, 40.5%, 49.9%, and 59.3% decrease in HU following RT, respectively. Our generalized linear model showed that pre-RT HU had a positive effect (β=0.830) on determining post-RT HU, while number of months post RT (β=-0.213) and radiation point dose (β=-1.475) had a negative effect. A comparison of the predicted versus actual HU showed significant correlation (R=0.883, P<0.001) with the slope of the best linear fit=0.81. Our models predicted HU were within ±20 HU of the actual value in 53% of cases, 70% of the predictions were within ±30 HU, 81% were within ±40 HU, and 90% were within ±50 HU of the actual post-RT HU. Four of 42 patients were found to have vertebral body compression fractures in the field of radiation. CONCLUSIONS Patients who receive abdominal chemoradiation develop significant BMD loss in the thoracic and lumbar vertebrae. Treatment-related BMD loss may contribute to the development of vertebral compression fractures. A predictive model for post-CRT BMD changes may inform bone protective strategies in patients planned for abdominal CRT.

The dosimetric impact of image guided radiation therapy by intratumoral fiducial markers

PURPOSE Pancreatic fiducials have proven superior over other isocenter localization surrogates, including anatomical landmarks and intratumoral or adjacent stents. The more clinically relevant dosimetric impact of image guided radiation therapy (IGRT) using intratumoral fiducial markers versus bony anatomy has not yet been described and is therefore the focus of the current study. METHODS AND MATERIALS Using daily orthogonal kV or cone beam computed tomography (CBCT) images and positional and dosimetric data were analyzed for 12 consecutive patients treated with fiducial based IGRT and volumetric modulated arc therapy to the intact pancreas. The shifts from fiducial to bone (ΔFid-Bone) required to realign the daily fiducial-matched pretreatment images (kV, CBCTs) to the planning computed tomography (CT) using bony anatomic landmarks were recorded. The isocenter was then shifted by (ΔFid-Bone) for 5 evenly spaced treatments, and the dosimetric impact of ΔFid-Bone was calculated for planning target volume coverage (PTV50.4 and PTV47.9) and organs at risk (liver, kidney, and stomach/duodenum). RESULTS The ΔFid-Bone were greatest in the superoinferior direction (ΔFid-Bone anteroposterior, 2.7 ± 3.0; left-right, 2.8 ± 2.8; superoinferior, 6.3 ± 7.9 mm; mean ± standard deviation; P = .03). PTV50.4 coverage was reduced by 13% (fiducial plan 95 ± 2.0 vs bone plan 82 ± 12%; P = .005; range, 5%-52%; >5% loss in all; and >10% loss in 42% of patients), and to a lesser degree for PTV47.9 (difference, -8%; range, 1%-30%; fiducial plan 100 ± 0.3% vs bone plan 92 ± 7.6%; P = .003; with reductions of >5% in 66% and >10% in 33% of patients). The dosimetric impact of ΔFid-Bone on the organs at risk was not significant. Positional shifts for kV- and CBCT-based realignments were nearly identical. CONCLUSION Compared with matching by fiducial markers, IGRT matched by bony anatomy substantially reduces the PTV50.4 and PTV47.9 coverage, supporting the use of intratumoral pancreatic markers for improved targeting in IGRT for pancreatic cancer.

SU‐FF‐T‐319: Novel Kyphoplasty/brachytherapy Technique for the Treatment of Vertebral Metastases: A Monte Carlo Feasibility Study

Purpose: Spinal metastases often requires Kyphoplasty (used to stabilize a collapsed vertebral body and relieve pain by injecting a cement mixture into the vertebral body) followed by palliative fractionated external beam radiotherapy. We propose to combine these two treatments into a single procedure (spinal brachytherapy) by adding a β‐emitting radionuclide to the kyphoplasty cement, thereby irradiating the bone from within. The potential advantages of this approach include delivery of a higher dose to the vertebral body, limiting the radiationdose to the spinal cord (due to the limited range of β‐particles in tissue) and eliminating the need for repeat patient visits for palliative radiotherapy.Method and Materials: The feasibility of the proposed procedure was studied by creating a CT‐based voxelized model of a T6 vertebral body, coupled with radiation transport using Monte Carlo N‐Particle code (MCNPX Version 2.5). A cylindrical source representing the injected cement was introduced into the voxelized model. Four β‐emitters, Y‐90, P‐32, Sr‐89 and Re‐188 were chosen as candidate isotopes for the simulations. Each simulation consisted of thirty million histories. The dose deposited in each voxel was used to generate isodose curves and dose‐volume histograms, yielding comparisons of dose to the vertebral body and the spinal cord. The activity of each radionuclide necessary to deliver a dose of 60 Gy to the vertebral body was calculated. The corresponding dose to the spinal cord was also determined. Results:Dose‐volume histograms and isodose line overlays for the different radionuclides indicate activities ranging from 0.5 – 950 mCi, will deliver the target dose to the vertebral body. Conclusions: Present work indicates that β‐emitting radionuclides can be safely used to irradiate the vertebral body while sparing the spinal cord. Future work will involve determining the radionuclide most suitable for this application based on different dosimetric and design considerations.

SU‐GG‐J‐34: Comprehensive Clinical Commissioning and Quality Assurance Procedures of a Big Bore CT Simulator in a Radiation Oncology Department

Purpose: To perform acceptance testing and clinical commissioning of a 16‐slice big bore CT‐simulator (85 cm) and comprehensive quality assurance (QA) procedures for radiation treatment planning using several phantoms. Method and Materials: This study focused on the performance evaluation of this CT simulator in a radiation oncology environment through the acceptance testing and commissioning for clinical use. Parameters derived from commissioning were used as reference values for a comprehensive QA program. Several phantoms were used in this project to evaluate the CT simulator image quality, like ACR phantom, Vendor (Philips) provided phantom and CATPHAN 504. CTDI phantom was used to evaluate the radiation dose to patients during scans. RMI phantom was used to evaluate the CT number linearity for use in the treatment planning system for density correction. Results: For clinical commissioning, the radiation dose reported in this study is lower compared to published data based on CTDI measurements. The results from ACR phantom showed that image quality specifications were met. The highest spatial frequency for two parts of the ACR phantom; abdomen and chest, there were 8 and 9 lp/cm; respectively visualized. The manufacture specification is 5 lp/cm. CT number linearity and low contrast resolution tests were also within acceptance requirements. Image quality evaluation parameters from different phantoms were compared. These showed good self consistency using this CT simulator. Conclusion: A comprehensive analysis of the test results indicates consistent and reproducible operation of the big bore CT‐simulator. This big bore CT simulator is well suited for use in a radiation oncology setting.

Radiation dermatitis caused by a bolus effect from an abdominal compression device.

American Association of Physicists in Medicine (AAPM) Task Group 176 evaluated the dosimetric effects caused by couch tops and immobilization devices. The report analyzed the extensive physics-based literature on couch tops, stereotactic body radiation therapy (SBRT) frames, and body immobilization bags, while noting the scarcity of clinical reports of skin toxicity because of external devices. Here, we present a clinical case report of grade 1 abdominal skin toxicity owing to an abdominal compression device. We discuss the dosimetric implications of the utilized treatment plan as well as post hoc alternative plans and quantify differences in attenuation and skin dose/build-up between the device, a lower-density alternative device, and an open field. The description of the case includes a 66-year-old male with HER2 amplified poorly differentiated distal esophageal adenocarcinoma treated with neoadjuvant chemo-radiation and the use of an abdominal compression device. Radiation was delivered using volumetric modulated arc therapy (VMAT) with 2 arcs using abdominal compression and image guidance. The total dose was 50.4Gy delivered over 40 elapsed days. With 2 fractions remaining, the patient developed dermatitis in the area of the compression device. The original treatment plan did not include a contour of the device. Alternative post hoc treatment plans were generated, one to contour the device and a second with anterior avoidance. In conclusion, replanning with the device contoured revealed the bolus effect. The skin dose increased from 27 to 36Gy. planned target volume (PTV) coverage at 45Gy was reduced to 76.5% from 95.8%. The second VMAT treatment plan with an anterior avoidance sector and more oblique beam angles maintained PTV coverage and spared the anterior wall, however at the expense of substantially increased dose to lung. This case report provides an important reminder of the bolus effect from external devices such as abdominal compression. Special consideration must be given to contour and/or avoiding beam entrance to the device, and to the use of such devices in patients who may have heightened radiosensitivity, such as those who are human immunodeficiency virus (HIV)-positive.

Response to Letter to the Editor: Regarding “The dosimetric impact of image guided radiation therapy by intratumoral fiducial markers”

We thank the authors of the letter for their interest in our work.1 They make an interesting suggestion. The intention of the clinical prescription is to deliver to the planning target volume (PTV) the clinically required dose. This approach guarantees that the clinical target volume, which is inherently difficult to define accurately,2 receives the prescribed dose as required for appropriate clinical management throughout treatment. As indicated in our work, and also acknowledged in the letter, PTVs were generated by expanding internal target volumes (ITVs) by 5mm in all directions to account for setup uncertainties. Ideally, with accurate image guidance for each treatment and fiducial marker alignment, 5 mm would provide a sufficient margin for ITVs to be covered by the prescribed dose, with consideration of uncertainties throughout treatment. However, our findings indicate that without the use of fiducial markers, PTV margins would have to be 27 mm in superior inferior (SI), 9 mm in anterior posterior (AP), and 8mm in left right (LR), respectively, to account for 95% interfractional patient position variations. This magnitude is much greater than the intended 5 mm expansion with an appropriate image-guided radiation therapy technique. Therefore, inadequate coverage to the ITV is expected when bony anatomy is used for online matching. To obtain more quantitative data, we reanalyzed our data for ITV coverage (V100%). Our findings indicate that the novel method utilizing mean absolute shifts of all fractions (Bone PlanEST) was equivalent to dose summation for all fractions for which we extracted data on ITV

SU-E-T-216: Comparison of Volumetrically Modulated Arc Therapy Treatment Using Flattening Filter Free Beams Vs. Flattened Beams for Partial Brain Irradiation

Purpose: Flattening Filter Free (FFF) beams offer the potential for higher dose rates, short treatment time, and lower out of field dose. Therefore, the aim of this study was to investigate the dosimetric effects and out of field dose of Volumetric Modulated Arc Therapy (VMAT) plans using FFF vs Flattening Filtering (FF) beams for partial brain irradiation. Methods: Ten brain patients treated with a 6FF beam from a Truebeam STX were analyzed retrospectively for this study. These plans (46Gy at 2 Gy per fraction) were re-optimized for 6FFF beams using the same dose constraints as the original plans. PTV coverage, PTV Dmax, total MUs, and mean dose to organs-at-risk (OAR) were evaluated. In addition, the out-of-field dose for 6FF and 6FFF plans for one patient was measured on an anthropomorphic phantom. TLDs were placed inside (central axis) and outside (surface) the phantom at distances ranging from 0.5 cm to 17 cm from the field edge. Paired T-test was used for statistical analysis. Results: PTV coverage and PTV Dmax were comparable for the FF and FFF plans with 95.9% versus 95.6% and 111.2% versus 111.9%, respectively. Mean dose to the OARs were 3.7% less for FFF than FF plans (p<0.0001). Total MUs were, on average, 12.5% greater for FFF than FF plans with 481±55 MU (FFF) versus 429±50 MU (FF), p=0.0003. On average, the measured out of field dose was 24% less for FFF compared to FF, p<0.0001. A similar beam-on time was observed for the FFF and FF treatment. Conclusion: It is beneficial to use 6FFF beams for regular fractionated brain VMAT treatments. VMAT treatment plans using FFF beams can achieve comparable PTV coverage but with more OAR sparing. The out of field dose is significant less with mean reduction of 24%.

SU‐E‐J‐102: The Impact of the Number of Subjects for Atlas‐Based Automatic Segmentation

PURPOSE To determine the impact of atlas size on the performance of atlas-based automatic segmentation (ABAS) in delineation of organs at risk for adaptive radiation therapy. METHODS A total of 25 patients who had undergone intensity modulated radiation therapy for various head and neck cancers were retrospectively selected for inclusion in a library to be used for ABAS with the MIM VISTA software package (MIM Software, Cleveland OH). Treatment planning computed tomography (CT) scans and subsequent organ at risk (OAR) contours generated as part of the treatment planning process for these patients were added to the library. This library of 25 patients was then successively pruned to generate 5 atlases with 25, 20, 15, 10, and 5 patient subjects respectively. Atlas based segmentation was performed on 10 retrospectively selected treatment planning CT scans to automatically generate right and left parotid glands and brainstem contours. These planning CT scans belonged to a unique set of 10 patient subjects different from the ones used for generating the atlases. One physician (JW), who was blinded to the ABAS results, manually delineated gold-standard contours for the right and left parotid glands and brainstem. Dice similarity coefficients were calculated and analyzed as a function of atlas subject size. RESULTS For the sites selected in this study, the performance of ABAS was relatively insensitive to atlas size. Furthermore, some patient subjects were repeatedly selected implying that the adoption of a single standard patient for ABAS may be of benefit. CONCLUSIONS Our preliminary results indicate that the performance of the atlas based segmentation module in MIM VISTA Version 5.2 for the organs studied here may be relatively insensitive to the atlas size.

SU‐E‐T‐381: Dosimetric Analysis of Patients Treated with Accelerated Partial Breast Irradiation Using the Mammosite® and SAVI Applicators

Purpose: To present dosimetric data for patients undergoing Accelerated Partial Breast Irradiation (APBI) using the Mammosite® and Strut‐Adjusted Volume Implant (SAVI) applicators. Methods: We have treated 35 patients with Accelerated Partial Breast Irradiation (APBI) using high dose rate brachytherapy. These patients were treated per guidelines specified in the NSABP B‐39/RTOG 0413 protocol. The patients undergoing APBI have been treated with Mammosite® applicator (N=20) and SAVI applicator (N=15). The Mammosite® applicator used for all 20 patients was a single channel applicator. Single or multiple dwell positions were used as warranted by the type of balloon used and other clinical factors. The SAVI Applicator is a single‐entry, multi‐catheter device available in different sizes with 7, 9 and 11 catheters. The use of multiple catheters facilitates dose sculpting to improve treatment plan quality as well as reduce the skindose. Each treatment plan was evaluated for conformance of the dose to the PTV using commonly used dosimetric parameters. These parameters included balloon/cavity volume, PTV volume, V90, V100, V150 and V200. Results: The median V90 for the treatment plans delivered using the Mammosite® applicator was 99.7% (96.7%–99.8%) whereas it was 98.8% (96%–100.0%) for the patients treated with the SAVI applicator. Other data including the V100, V150, V200 and skindose will be presented. The impact/ importance of the data presented is that it provides useful information about the range of dosimetric parameters to be expected in treatment planning of APBI cases using these applicators. Conclusions: : Our results indicate that both the Mammosite® and SAVI applicators allow treatment planning and delivery of APBI per the guidelines specified in the NSABP B‐39/RTOG 0413 protocol.