A Gutierrez

A novel method to correct for pitch and yaw patient setup errors in helical tomotherapy

An accurate means of determining and correcting for daily patient setup errors is important to the cancer outcome in radiotherapy. While many tools have been developed to detect setup errors, difficulty may arise in accurately adjusting the patient to account for the rotational error components. A novel, automated method to correct for rotational patient setup errors in helical tomotherapy is proposed for a treatment couch that is restricted to motion along translational axes. In tomotherapy, only a narrow superior/inferior section of the target receives a dose at any instant, thus rotations in the sagittal and coronal planes may be approximately corrected for by very slow continuous couch motion in a direction perpendicular to the scanning direction. Results from proof-of-principle tests indicate that the method improves the accuracy of treatment delivery, especially for long and narrow targets. Rotational corrections about an axis perpendicular to the transverse plane continue to be implemented easily in tomotherapy by adjustment of the initial gantry angle.

A radiobiological analysis of the effect of 3D versus 4D image-based planning in lung cancer radiotherapy.

Dose distributions generated on a static anatomy may differ significantly from those delivered to temporally varying anatomy such as for abdominal and thoracic tumors, due largely in part to the unavoidable organ motion and deformation effects stemming from respiration. In this work, the degree of such variation for three treatment techniques, namely static conventional, gating and target tracking radiotherapy, was investigated. The actual delivered dose was approximated by planning all the phases of a 4DCT image set. Data from six (n = 6) previously treated lung cancer patients were used for this study with tumor motion ranging from 2 to 10 mm. Complete radiobiological analyses were performed to assess the clinical significance of the observed discrepancies between the 3D and 4DCT image-based dose distributions. Using the complication-free tumor control probability (P+) objective, we observed small differences in P+ between the 3D and 4DCT image-based plans (<2.0% difference on average) for the gating and static conventional regimens and higher differences in P+ (4.0% on average) for the tracking regimen. Furthermore, we observed, as a general trend, that the 3D plan underestimated the P+ values. While it is not possible to draw any general conclusions from a small patient cohort, our results suggest that there exists a patient population in which 4D planning does not provide any additional benefits beyond that afforded by 3D planning for static conventional or gated radiotherapy. This statement is consistent with previous studies based on physical dosimetric evaluations only. The higher differences observed with the tracking technique suggest that individual patient plans should be evaluated on a case-by-case basis to assess if 3D or 4D imaging is appropriate for the tracking technique.

Dosimetric evaluation of multi-pattern spatially fractionated radiation therapy using a multi-leaf collimator and collapsed cone convolution superposition dose calculation algorithm.

PURPOSEnIn this paper, we present an alternative to the originally proposed technique for the delivery of spatially fractionated radiation therapy (GRID) using multi-leaf collimator (MLC) shaped fields. We employ the MLC to deliver various pattern GRID treatments to large solid tumors and dosimetrically characterize the GRID fields.nnnMETHODS AND MATERIALSnThe GRID fields were created with different open to blocked area ratios and with variable separation between the openings using a MLC. GRID designs were introduced into the Pinnacle(3) treatment planning system, and the dose was calculated in a water phantom. Ionization chamber and film measurements using both Kodak EDR2 and Gafchromic EBT film were performed in a SolidWater phantom to determine the relative output of each GRID design as well as its spatial dosimetric characteristics.nnnRESULTSnAgreement within 5.0% was observed between the Pinnacle(3) predicted dose distributions and the measurements for the majority of experiments performed. A higher magnitude of discrepancy (15%) was observed using a high photon beam energy (18MV) and small GRID opening. Skin dose at the GRID openings was higher than the corresponding open field by a factor as high as three for both photon energies and was found to be independent of the open-to-blocked area ratio.nnnCONCLUSIONnIn summary, we reaffirm that the MLC can be used to deliver spatially fractionated GRID therapy and show that various GRID patterns may be generated. The Pinnacle(3) TPS can accurately calculate the dose of the different GRID patterns in our study to within 5% for the majority of the cases based on film and ion chamber measurements. Disadvantages of MLC-based GRID therapy are longer treatment times and higher surface doses.

Radiobiological characterization of post-lumpectomy focal brachytherapy with lipid nanoparticle-carried radionuclides

Post-operative radiotherapy has commonly been used for early stage breast cancer to treat residual disease. The primary objective of this work was to characterize, through dosimetric and radiobiological modeling, a novel focal brachytherapy technique which uses direct intracavitary infusion of β-emitting radionuclides (186Re/188Re) carried by lipid nanoparticles (liposomes). Absorbed dose calculations were performed for a spherical lumpectomy cavity with a uniformly injected activity distribution using a dose point kernel convolution technique. Radiobiological indices were used to relate predicted therapy outcome and normal tissue complication of this technique with equivalent external beam radiotherapy treatment regimens. Modeled stromal damage was used as a measure of the inhibition of the stimulatory effect on tumor growth driven by the wound healing response. A sample treatment plan delivering 50 Gy at a therapeutic range of 2.0 mm for 186Re-liposomes and 5.0 mm for 188Re-liposomes takes advantage of the dose delivery characteristics of the β-emissions, providing significant EUD (58.2 Gy and 72.5 Gy for 186Re and 188Re, respectively) with a minimal NTCP (0.046%) of the healthy ipsilateral breast. Modeling of kidney BED and ipsilateral breast NTCP showed that large injected activity concentrations of both radionuclides could be safely administered without significant complications.

Expected clinical impact of the differences between planned and delivered dose distributions in helical tomotherapy for treating head and neck cancer using helical megavoltage CT images.

Helical Tomotherapy (HT) has become increasingly popular over the past few years. However, its clinical efficacy and effectiveness continues to be investigated. Pre‐treatment patient repositioning in highly conformal image‐guided radiation therapy modalities is a prerequisite for reducing setup uncertainties. A MVCT image set has to be acquired to account for daily changes in the patients internal anatomy and setup position. Furthermore, a comparison should be performed to the kVCT study used for dosimetric planning, by a registration process that results in repositioning the patient according to specific transitional and rotational shifts. Different image registration techniques may lead to different repositioning of the patient and, as a result, to varying delivered doses. This study aims to investigate the expected effect of patient setup correction using the Hi·Art TomoTherapy system by employing radiobiological measures such as the biologically effective uniform dose (D¯¯) and the complication‐free tumor control probability (P+). In this study, a typical case of lung cancer with metastatic head and neck disease was investigated by developing a Helical Tomotherapy plan. For the TomoTherapy Hi·Art plan, the dedicated tomotherapy treatment planning station was used. Three dose distributions (planned and delivered with and without patient setup correction) were compared based on radiobiological measures by using the P+ index and the D¯¯ concept as the common prescription point of the plans, and plotting the tissue response probabilities against the mean target dose for a range of prescription doses. The applied plan evaluation method shows that, in this cancer case, the planned and delivered dose distributions with and without patient setup correction give a P+ of 81.6%, 80.9% and 72.2%, for a D¯¯ to the planning target volume (PTV) of 78.0Gy, 77.7Gy and 75.4Gy, respectively. The corresponding tumor control probabilities are 86.3%, 85.1% and 75.1%, whereas the total complication probabilities are 4.64%, 4.20% and 2.89%, respectively. HT can encompass the often large PTV required while minimizing the volume of the organs at risk receiving high dose. However, the effectiveness of an HT treatment plan can be considerably deteriorated if an accurate patient setup system is not available. Taking into account the dose‐response relations of the irradiated tumors and normal tissues, a radiobiological treatment plan evaluation can be performed, which may provide a closer association of the delivered treatment with the clinical outcome. In such situations, for effective evaluation and comparison of different treatment plans, traditional dose based evaluation tools can be complemented by the use of P+−D¯¯ diagrams. PACS number: 87.55.Qr Quality assurance in radiotherapy

MO-AB-BRA-04: Radiation Measurements with a DNA Double-Strand-Break Dosimeter

PURPOSEnMany types of dosimeters are used to measure radiation, but none of them directly measures the biological effect of this dose. The purpose here is to create a dosimeter that can measure the probability of double-strand breaks (DSB) for DNA, which is directly related to the biological effect of radiation.nnnMETHODSnThe dosimeter has DNA strands, which are labeled on one end with biotin and on the other with fluorescein. The biotin attaches these strands to magnetic beads. We suspended the DNA dosimeter in phosphate-buffered saline (PBS) as it matches the internal environment of the body. We placed small volumes (50µL) of the DNA dosimeter into tubes and irradiated these samples in a water-equivalent plastic phantom with several doses (three samples per dose). After irradiating the samples, a magnet was placed against the tubes. The fluorescein attached to broken DNA strands was extracted (called the supernatant) and placed into a different tube. The fluorescein on the unbroken strands remained attached to the beads in the tube and was re-suspended with 50µL of PBS. A fluorescence reader was used to measure the fluorescence for both the re-suspended beads and supernatant. To prove that we are measuring DSB, we tested dosimeter response with two different lengths of attached DNA strands (1 and 4 kilo-base pair).nnnRESULTSnThe probability of DSB at the dose levels of 5, 10, 25, and 50 Gy were 0.05, 0.08, 0.12, and 0.19, respectively, while the coefficients of variation were 0.14, 0.07, 0.02, and 0.01, respectively. The 4 kilo-base-pair dosimeter produced 5.3 times the response of the 1 kilo-base-pair dosimeter.nnnCONCLUSIONnThe DNA dosimeter yields a measurable response to dose that scales with the DNA strand length. The goal now is to refine the dosimeter fabrication to reproducibly create a low coefficient of variation for the lower doses. This work was supported in part by Yarmouk University (Irbid, Jordan) and CPRIT (RP140105).

SU‐FF‐T‐150: Consequence of CT Couch Sag in Radiation Therapy

Purpose: To characterize couch sag on a CT scanner and determine the impact on patient treatments. A CT couch sags as it enters the gantry, and the amount depends on couch design, material, patient weight and distribution. Method and Materials: Four individuals were placed on a CT scanner couch (GE Discovery Lightspeed). A dial indicator was fixed to the bore of the CT scanner to measure couch deflection at the scanning plane when the couch was extended to various lengths. A second experiment was performed to measure sag at various positions for a fixed couch extension. A steel I‐beam with the dial indicator mounted on a rail was placed underneath the CT and tomotherapy couches. Two uniform loads were placed on the couch for three couch extensions and measurements were recorded at discrete locations. Results: Two forms of couch sag relevant to radiotherapy were observed: absolute and intra‐target sag. Absolute sag was shown to be 8.0±0.5 mm. Intra‐target sag depended on the S‐I length of the target and could be 2.3±0.5 mm. Both CT and tomotherapy couches displayed similar sag patterns. Differences in sag between the couches were more pronounced at short cantilevered distances with a maximum difference of 1 mm. Conclusion: A significant amount of sag occurs at the scanning plane resulting in a shearing of the images used for treatment planning. Absolute sag may be corrected by a couch height adjustment but intra‐target sag is a concern for elongated targets. The impact of sag on tomotherapy treatments is minimal since the relative difference in sag patterns between CT and tomotherapy couches is small. However, differences between CT and tomotherapy couch sag have become a concern for facilities treating total marrow irradiation (TMI) fields with tomotherapy. Conflict of Interest: Mackie has a financial interest in TomoTherapyInc.

SU-F-T-458: Tracking Trends of TG-142 Parameters Via Analysis of Data Recorded by 2D Chamber Array

PURPOSEnWith increasing QA demands of medical physicists in clinical radiation oncology, the need for an effective method of tracking clinical data has become paramount. A tool was produced which scans through data automatically recorded by a 2D chamber array and extracts relevant information recommended by TG-142. Using this extracted information a timely and comprehensive analysis of QA parameters can be easily performed enabling efficient monthly checks on multiple linear accelerators simultaneously.nnnMETHODSnA PTW STARCHECK chamber array was used to record several months of beam outputs from two Varian 2100 series linear accelerators and a Varian NovalisTx-. In conjunction with the chamber array, a beam quality phantom was used to simultaneously to determine beam quality. A minimalist GUI was created in MatLab that allows a user to set the file path of the data for each modality to be analyzed. These file paths are recorded to a MatLab structure and then subsequently accessed by a script written in Python (version 3.5.1) which then extracts values required to perform monthly checks as outlined by recommendations from TG-142. The script incorporates calculations to determine if the values recorded by the chamber array fall within an acceptable threshold.nnnRESULTSnValues obtained by the script are written to a spreadsheet where results can be easily viewed and annotated with a pass or fail and saved for further analysis. In addition to creating a new scheme for reviewing monthly checks, this application allows for able to succinctly store data for follow up analysis.nnnCONCLUSIONnBy utilizing this tool, parameters recommended by TG-142 for multiple linear accelerators can be rapidly obtained and analyzed which can be used for evaluation of monthly checks.

SU-F-T-484: Initial Evaluation of a Novel 6D QA Phantom (HexaCheck) for Daily 6D Couch Correction Assessment

PURPOSEnTreatment couches with 6D correction capabilities have become a regular part of IGRT. While 6D couches have been shown to function mechanically well, phantoms for daily angular QA accuracy assessment have yet to be commercially available. Standard Imaging™ has developed a phantom designed for daily verification of pitch, roll, and yaw corrections. In this study, the mechanical integrity, reproducibility, and daily 6D correction stability of the phantom using both volumetric (Elekta- XVI kV-CBCT) and orthogonal kV (Brainlab ExacTrac) imaging was evaluated.nnnMETHODSnThe HexaCheck is a rotating base that works in conjunction with the MIMI imaging phantom. The MIMI is placed inside the HexaCheck and secured to prevent motion. The HexaCheck has screws in each rotational direction (pitch, roll, and yaw) along with a spring-loaded peg to lock each direction firmly. When a peg is released, the HexaCheck rotates 2.5° solely in that direction. Mechanical integrity was assessed using a level with 0.05° precision. For daily 6D corrections, a high resolution reference CT (0.59 × 0.59 × 1.25 mm3) of the HexaCheck was acquired. Ten (n=10) independent, daily acquisitions using both the ExacTrac with the 6D Brainlab™ couch and the Elekta XVI kV-CBCT with the HexaPOD™ couch were acquired to test detection and positioning accuracy.nnnRESULTSnPhysical measurements show that the HexaCheck rotations for pitch, roll, and yaw were 2.43±0.05 deg, 2.50±0.05 deg, and 2.62±0.05 deg, respectively. Maximum mean values for the imaging registration accuracy were 0.2° and 0.3° for ExacTrac and XVI, respectively. Maximum mean values for the couch positioning accuracy were 0.1° and 0.2° for the Brainlab couch and HexaPOD, respectively.nnnCONCLUSIONnThe HexaCheck performance yields consistent results for daily testing in all three rotational directions using both 2D and 3D imaging modalities and also appears to be a simple method for daily 3D rotational testing.

SU-G-201-07: Dosimetric Verification of a 3D Printed HDR Skin Brachytherapy Applicator

PURPOSEnThe use of radiation as a treatment modality for skin cancer has increased significantly over the last decade with standardized applicators. Utilizing 3D printing, the ability to make applicators specifically designed for each patients anatomy has become economically feasible. With this in mind it was the aim of this study to determine the dosimetric accuracy of a 3-D printed HDR brachytherapy applicator for the skin.nnnMETHODSnA CT reference image was used to generate a custom applicator based on an anthropomorphic head and neck phantom. To create the applicator a 1cm expansion anteriorly with 0.5cmX0.5cm trenches on the outer surface that were spaced 1cm sup-inf to accommodate standard 6F flexible catheters. The applicator was printed using PLA material using a printrbot simple printer. A treatment plan optimized to deliver a clinically representative volume was created in Oncentra and delivered with a nucletron afterloader. Measurements were made using TLDs and EBT3 gafchromic film that were placed between the applicator and the phantoms forehead. An additional piece of film was also used to qualitatively asses the dose distribution in the transverse plane. Using a standard vaginal cylinder and bolus, a standardized curve correlating TLD and film exposure-to-radiation dose was established by irradiating film to known doses (200,500,700 cGy) at a 3.5 cm radius distance.nnnRESULTSnEvaluated TLDs showed the absolute dose delivered to the skin surface using the 3-D printed bolus was 615cGy±6%, with a mean predicted TPS value in the measured area of 617.5±7%. Additionally, planar dose distributions had good qualitative agreement with calculated TPS isodoses.nnnCONCLUSIONnThis work demonstrates patient specific 3-D printed HDR brachytherapy applicators for skin cancer treatments are practical and accurate in TPS calculations but additional measurements are needed to verify additional sites and dose at depth.