Citations

The purpose of this study was to introduce a planning strategy for dynamic conformal arc therapy (DCAT), named negative margin technique (NMT), and evaluate its dosimetric gain in lung stereotactic body radiation therapy (SBRT). In DCAT, the field aperture is continuously conformed to the planning target volume (PTV) with an aperture margin (AM) to compensate for the penumbra effect with gantry rotation. It is a common belief the AM should be positive (or at least ‘zero'). However, the radial penumbra width becomes significantly wider because of continuously overlapped beams in arc delivery. Therefore, we hypothesize if the ‘negative margin’ is applied in the radial direction, it would improve the PTV dose conformation while reducing normal tissue dose. For verification, trial plans were made using the NMT and compared with ‘zero margin (ZM)’ plans for five lung SBRT cases representing different situations depending on the location of the PTV and organs at risk. All plans met 95% PTV coverage with the prescription dose and spared the spinal cord below the tolerance. Two conventional conformation indices (the ratio of prescription isodose volume to the PTV (CI100) and the ratio of 50% prescription isodose volume to the PTV (CI50)) and a modified conformation index were investigated. The maximum dose at 2 cm from the PTV and the percent of lung volume receiving 20 Gy were also evaluated. Another planning simulation was performed with a total of ten randomly selected lung SBRT cases to mimic actual practice. In this simulation, optimization with ZM was first performed and further optimization using the NMT was processed for cases that could not meet a goal of with the ZM optimization. In all cases, both the and values were significantly reduced (overall, and for and , respectively). The modified conformation index values also showed similar improvement (overall, increase). Reduction of was also observed in all cases . values decreased in all cases but one (, excluding the increased case). In the random group simulation, it was possible to achieve the goal with just one NMT trial for five out of six cases that did not meet the goal in the ZM optimization. Interestingly, however, one case needed as many as six iterations to get the goal. The NMT turned out to be an effective planning strategy that could bring significant improvement of dose conformation. The NMT can be easily implemented in most clinics with no prerequisite.PACS number: 87.55.D‐

A multiple‐slit collimator (MSC) design was introduced for scatter reduction in cone‐beam computed tomography (CBCT). Unlike most other collimators, the open and closed septa of the proposed MSC are placed in an equi‐angular interval on a circular track of the central sagittal plane. Therefore, one gantry rotation provides only the half of necessary dataset and two gantry rotations are needed to obtain full information. During the first gantry rotation, the MSC position relative to the source is fixed. For the second rotation, the MSC is rotated by the equi‐angle interval. We assume signals under the closed septa are totally attributed to scatter radiation. Then, scatter contributions under open septa are determined by interpolating them.Monte Carlo (MC) simulations for two virtual phantoms (one with a simple geometry and the other with two heterogeneities simulating the bone and the lung) were performed to evaluate the effectiveness of the system. Using the method developed, we could obtain images with significant scatter reduction. Contrast ratio (CR) improvement factors were 1.165 in a 2D projection view, and 1.210 and 1.223 at the central and peripheral slice of the reconstructed CBCT image of the simple geometry phantom.This preliminary study demonstrated that the proposed MSC, together with the imaging process technique, had a great potential to reduce scatter contribution in CBCT. Further studies will be performed to investigate the effect of various factors, such as reducing the detector size, increasing the number of history of MC simulation, and including many structures with different densities.PACS number: 87.57.C

The main purpose of this work was to quantify patient organ doses from the two kilovoltage cone beam computed tomography (CBCT) systems currently available on medical linear accelerators, namely the X‐ray Volumetric Imager (XVI, Elekta Oncology Systems) and the On‐Board Imager (OBI, Varian Medical Systems). Organ dose measurements were performed using a fiber‐optic coupled (FOC) dosimetry system along with an adult male anthropomorphic phantom for three different clinically relevant scan sites: head, chest, and pelvis. The FOC dosimeter was previously characterized at diagnostic energies by Hyer et al. [Med Phys 2009;36(5):1711–16] and a total uncertainty of approximately 4% was found for in‐phantom dose measurements. All scans were performed using current manufacturer‐installed clinical protocols and appropriate bow‐tie filters. A comparison of image quality between these manufacturer‐installed protocols was also performed using a Catphan 440 image quality phantom. Results indicated that for the XVI, the dose to the lens of the eye (1.07 mGy) was highest in a head scan, thyroid dose (19.24 mGy) was highest in a chest scan, and gonad dose (29 mGy) was highest in a pelvis scan. For the OBI, brain dose (3.01 mGy) was highest in a head scan, breast dose (5.34 mGy) was highest in a chest scan, and gonad dose (34.61 mGy) was highest in a pelvis scan. Image quality measurements demonstrated that the OBI provided superior image quality for all protocols, with both better spatial resolution and low‐contrast detectability. The measured organ doses were also used to calculate a reference male effective dose to allow further comparison of the two machines and imaging protocols. The head, chest, and pelvis scans yielded effective doses of 0.04, 7.15, and 3.73 mSv for the XVI, and 0.12, 1.82, and 4.34 mSv for the OBI, respectively.PACS number: 87.57.uq

125 Background: Treatment for locally advanced esophageal carcinoma is radiation and chemotherapy, with or without surgery. Radiation has traditionally been delivered with 3D conformal radiation therapy (3D CRT). This study evaluates late toxicity in patients treated with IMRT as well as early outcomes and acute toxicity. Methods: This is a retrospective review of 32 patients with esophageal carcinoma treated with IMRT at Mayo Clinic Florida from 2008 -2012. Pathology includes squamous cell and adenocarcinomas. Tumor sites include middle and lower thoracic and GE junction. Clinical stages are TX-T3, N0-3, M0-1. All patients received at least one cycle of concurrent chemotherapy. IMRT dose was 50.4 Gy in 28 fractions prescribed to a target volume including the tumor and regional lymphatics. IMRT plans utilized coplaner beams in a 7-9 beam arrangement or volumetric modulated arc therapy. Results: Median follow-up is 8.9 months (range 2.4-23.0) for all patients and 13.1 months (range 2.8-23.0 months) in surviving patients. Median patient age is 69 (range 46-87). Trimodality treatment was completed in 20 patients (62.5%). Surgery was either an open or minimally invasive esophagogastrectomy. The incidence of grade 3 or greater late toxicity at 1 year was 48% in surgery patients and 26% in non-surgery patients. The most common grade 3 or higher toxicity was esophageal strictures in 25%. The incidence of any grade 3 or greater acute toxicity was 65% in the surgery patients and 75% in the non-surgery patients. Overall survival (OS) for all patients at 18 months is 57% (CI 37-86%) and progression-free survival (PFS) is 60% (36-99%). OS and PFS for trimodality therapy at 12 months is 83% (66-100%) and 81% (63-100%) respectively and for bimodality therapy is 34% (12-93%) and 70% (33-100%) respectively. Conclusions: Increased late toxicity occurs in surgery patients, and increased acute toxicity in non-surgery patients. Lower survival in non-surgery patients may be due to early progression, morbidities which preclude surgery or improved survival with surgery. Overall, IMRT is a feasible treatment modality, which may be equally efficacious to 3D CRT for the treatment of esophageal carcinoma.

211 Background: The current standard of care for locally advanced esophageal cancer includes chemoradiotherapy with or without surgery. Radiation is usually delivered via a 3D technique. IMRT has been utilized in the treatment of multiple tumors and demonstrated similar efficacy while offering the possibility of decreased toxicity. Methods: Thirty-six patients were treated with IMRT and chemotherapy. Twenty-one patients underwent surgical resection. Eleven underwent open surgery and the remainder underwent minimally invasive surgery. Chemotherapy consisted primarily of 5-FU with oxaliplatin or cisplatin. All but two patients received 50.4 Gy; one patient received 41.4 Gy without surgery and one patient discontinued treatment after 25.2 Gy. Eleven patients required a treatment break during radiotherapy. The median age was 69 (range 46-87). Approximately two-thirds of tumors were adenocarcinomas located in the lower thorax. Two thirds of patients were staged as T3 and had positive lymph nodes. The median tumor size was 5 cm (range 2-13). Results: With a median follow-up of 21.3 months (range 2.4-44.8) and 33.9 months for survivors (range 3.7-44.8), overall survival at 24 months was 55%. The 24 month overall survival was 75% vs 24% for surgical and non-surgical patients, respectively. Seven patients had a complete pathologic response. Twenty-four patients experienced grade 3 or higher acute toxicity and there was one grade 5 toxicity. Acute toxicity was similar between surgery and non-surgery patients. Fourteen patients experienced grade 3 or higher late toxicity; 9 surgery and 5 non-surgery patients. The most frequent late toxicity was grade 3 stricture (21%). On multivariate analysis, advanced age (RR [10 year increase] 2.01, p=0.032) and heart maximum dose >55 Gy (RR 3.73, p=0.011) were associated with decreased survival. Conclusions: Patients who undergo surgery after chemoradiotherapy demonstrate improved survival; however, this may be related to underlying comorbidities that preclude surgery. IMRT appears to be a reasonable treatment option that may reduce complications from radiotherapy. Careful attention should be given to heart dose during treatment planning.

284 Background: Frameless stereotactic radiosurgery (SRS) is single fraction radiation therapy delivered to an intracranial lesion. Rather than with a rigid head frame, frameless SRS uses a thermoplastic mask for immobilization. The benefit of this mask includes the ability to simulate and treat the patient at different times, ease of patient set-up and improved patient comfort. Clinically, frameless SRS is used for primary brain tumors and metastatic lesions. The length of the treatments may vary based on a number of factors, including total treatment dose, number of monitor units, number and arrangement of beams, and equipment manipulations. This is a single institution review of patients treated with SRS with positional checks made before, during and at the completion of treatment to evaluate intrafraction motion. Methods: Patients were treated with SRS at Mayo Clinic Florida between 2010 and 2011. All patients were immobilized with a thermoplastic mask and treated on a Varian IX Clinac. KV Cone beam CT was taken prior to the initiation of treatment to verify the treatment position. Cone beam CT’s were also performed at the midpoint and conclusion of treatment. The same physician analyzed all cone beam CT’s. Adjustments were recorded in each dimension and analyzed to determine intrafraction motion. Results: Intrafraction motion was evaluated during six separate SRS cases. The mean displacements at mid-treatment were 0.67±0.52 mm, 0.50±0.55 mm, 0.83±0.75 mm, and 1.35±0.78 mm in the medial/lateral, superior/inferior, anterior/posterior, and vectoral direction, respectively. The mean post-treatment displacements were 0.17±0.41 mm, 0.67±0.82 mm, 0.50±0.84 mm, and 1.11±0.96 mm in the medial/lateral, superior/inferior, anterior/posterior, and vectoral direction, respectively. Estimated overall margins to accommodate intrafraction motion in 90% of patients were 2.63 mm and 2.69 mm at mid-treatment and post-treatment, respectively. Conclusions: The majority of intrafraction motion occurred during the first portion of treatment. Intrafraction motion during the course of frameless SRS treatment is not negligible and approximately a 2.5 mm margin should be taken into account during the planning process.