E Johnson

Noninvasive diffuse optical monitoring of head and neck tumor blood flow and oxygenation during radiation delivery

This study explored using a novel diffuse correlation spectroscopy (DCS) flow-oximeter to noninvasively monitor blood flow and oxygenation changes in head and neck tumors during radiation delivery. A fiber-optic probe connected to the DCS flow-oximeter was placed on the surface of the radiologically/clinically involved cervical lymph node. The DCS flow-oximeter in the treatment room was remotely operated by a computer in the control room. From the early measurements, abnormal signals were observed when the optical device was placed in close proximity to the radiation beams. Through phantom tests, the artifacts were shown to be caused by scattered x rays and consequentially avoided by moving the optical device away from the x-ray beams. Eleven patients with head and neck tumors were continually measured once a week over a treatment period of seven weeks, although there were some missing data due to the patient related events. Large inter-patient variations in tumor hemodynamic responses were observed during radiation delivery. A significant increase in tumor blood flow was observed at the first week of treatment, which may be a physiologic response to hypoxia created by radiation oxygen consumption. Only small and insignificant changes were found in tumor blood oxygenation, suggesting that oxygen utilizations in tumors during the short period of fractional radiation deliveries were either minimal or balanced by other effects such as blood flow regulation. Further investigations in a large patient population are needed to correlate the individual hemodynamic responses with the clinical outcomes for determining the prognostic value of optical measurements.

Dosimetric characteristics of a newly designed grid block for megavoltage photon radiation and its therapeutic advantage using a linear quadratic model

Grid radiation therapy with megavoltage x-ray beam has been proven to be an effective technique for management of large, bulky malignant tumors. The clinical advantage of GRID therapy, combined with conventional radiation therapy, has been demonstrated using a prototype GRID block [Mohiuddin, Curtis, Grizos, and Komarnicky, Cancer 66, 114-118 (1990)]. Recently, a new GRID block design with improved dosimetric properties has become commercially available from Radiation Product Design, Inc. (Albertive, MN). This GRID collimator consists of an array of focused apertures in a cerrobend block arranged in a hexagonal pattern having a circular cross-section with a diameter and center-to-center spacing of 14.3 and 21.1 mm, respectively, in the plane of isocenter. In this project, dosimetric characteristics of the newly redesigned GRID block have been investigated for a Varian 21EX linear accelerator (Varian Associates, Palo Alto, CA). These determinations were performed using radiographic films, thermoluminescent dosimeters in Solid Water phantom materials, and an ionization chamber in water. The output factor, percentage depth dose, beam profiles, and isodose distributions of the GRID radiation as a function of field size and beam energy have been measured using both 6 and 18 MV x-ray beams. In addition, the therapeutic advantage obtained from this treatment modality with the new GRID block design for a high, single fraction of dose has been calculated using the linear quadratic model with alpha/beta ratios for typical tumor and normal cells. These biological characteristics of the new GRID block design will also be presented.

Dosimetric evaluation of parallel opposed spatially fractionated radiation therapy of deep‐seated bulky tumors

Application of a single fraction of parallel opposed GRID beams as a means of increasing the efficiency of radiation delivery to deep-seated tumors has been investigated. This evaluation was performed by measurement of dosimetric characteristics of the GRID radiation field in parallel opposed and single beam geometry. The limitations of the parallel opposed technique in terms of field size and tumor thickness have been evaluated for the conditions of acceptable spatial modulation. The results of this investigation have demonstrated an increase in therapeutic advantage for the parallel opposed technique over the single beam method when treating a deep seated tumor.

Diffuse optical measurements of head and neck tumor hemodynamics for early prediction of chemoradiation therapy outcomes

Abstract. This study used a hybrid near-infrared diffuse optical instrument to monitor tumor hemodynamic responses to chemoradiation therapy for early prediction of treatment outcomes in patients with head and neck cancer. Forty-seven patients were measured once per week to evaluate the hemodynamic status of clinically involved cervical lymph nodes as surrogates for the primary tumor response. Patients were classified into two groups: complete response (CR) (n=29) and incomplete response (IR) (n=18). Tumor hemodynamic responses were found to be associated with clinical outcomes (CR/IR), wherein the associations differed depending on human papillomavirus (HPV-16) status. In HPV-16 positive patients, significantly lower levels in tumor oxygenated hemoglobin concentration ([HbO2]) at weeks 1 to 3, total hemoglobin concentration at week 3, and blood oxygen saturation (StO2) at week 3 were found in the IR group. In HPV-16 negative patients, significantly higher levels in tumor blood flow index and reduced scattering coefficient (μs′) at week 3 were observed in the IR group. These hemodynamic parameters exhibited significantly high accuracy for early prediction of clinical outcomes, within the first three weeks of therapy, with the areas under the receiver operating characteristic curves (AUCs) ranging from 0.83 to 0.96.

SU‐E‐T‐82: Evaluation of Two Detector Arrays for Treatment Gantry Angle Specific IMRT Quality Assurance

Purpose: To evaluate the dosimetric characteristics of two commercially available detector arrays, the MapCHECK2 diode array and the PTW Octavius® II ionchamber array, and to assess their use with patient‐specific step‐and‐shoot IMRT QA delivered from treatment specific gantry angles. Methods: Detector properties were compared in their response to varying dose rates, field sizes, and gantry angles. Ten patient‐specific IMRTQuality Assurance (QA) plans were analyzed. The selected patient files were separated into high and low degrees of modulation, e.g. Head and Neck vs. Prostate. Delivered doses to PTW and MapCHECK2 were evaluated with 5% 3mm and Van Dyk Gamma analysis criteria. Results: For both arrays, our findings showed a 1% difference in the measurement for the 100MU/min to 600 MU/min dose rate change for the 10 × 10 cm2field size. A pearson product‐moment correlation test calculated to evaluate eachdetectors deviation from TPS for increasing field size resulted correlation coefficients of −0.8 and 0.01 for PTW and MapCHECK2 respectively, exhibiting PTWs field size dependence which increases by 5% for a 10×10 o 1×1 field. The average 5 × 5 cm2 measurement deviation from the TPS over all angles resulted in 2% and 4% absolute error for PTW and MapCHECK2 respectively, with MapCHECK2 erring up to 36% only at the lateral beams angles 90° and 270° (±10°). For the other angles the absolute error decreased to 1% for MapCHECK2. All 10 patient‐specific IMRT QAs passed the gamma criteria on both detector arrays.Conclusions: BothMapCHECK2 and PTW detector arrays can be used for treatment angle specific IMRT QA. Care must be taken for gantry angles within 10° of the detector plane of MapCHECK2. This is less pronounced for rotational delivery where a smaller portion of the beams will contribute to the total dose from these directions.

SU‐FF‐T‐357: Physical and Biological Characteristics of Megavoltage Grid Radiation Therapy

Purpose:Dosimetric characteristics of a newly designed GRID block for megavoltage radiotherapy were investigated using TLD, film, and ionization chamber.Therapeutic advantage of this treatment modality was determined using a linear quadratic model in terms of treatmentdose, GRID hole size, center‐to‐center spacing between the holes, and sensitivity of tissues to radiation.Method and Materials: Recently a newly redesigned GRID block has been introduced by Radiation Product Design, Inc. for different linear accelerators.Dosimetric characteristics (i.e. GRID output, dose profiles, and %DD) of three GRID blocks with different hole size and spacing were experimentally investigated. These GRID blocks were designed for a Varian 21EX linear accelerator. The measurements were performed using radiographic films and thermoluminescent dosimeters(TLDs) in Solid Water™ phantom materials and an ionization chamber in water. The measured dose profiles were then utilized to evaluate the therapeutic advantage of the GRID therapy as a function of dose, grid hole size and spacing, and also radiosensitivity of tissues with a linear‐quadratic (LQ) model. Therapeutic advantage of these GRID blocks was determined for a single fraction dose of up to 30 Gy. Results:Dosimetric characteristics of the GRID field were determined as a function of field size, beam energy, and Grid geometry. The results had shown that the therapeutic advantage of GRID increases by increasing the hole size, decreasing the spacing between the holes, and increasing the treatmentdose. Moreover, the advantage of the GRID therapy was more pronounced for the radioresistant cells than radiosensitive cells. Conclusion: GRID radiotherapy exhibited a significant therapeutic advantage over the open field radiotherapy when the tumor cells were more radioresistant than normal cells. In addition, the therapeutic advantage of the GRID therapy is dependent on the treatmentdose, GRID hole size and spacing between the holes.

SU-F-T-424: Mitigation of Increased Surface Dose When Treating Through A Carbon Fiber Couch Top

PURPOSE To study the effect of the Varian carbon fiber couch top on surface dose for patients being treated using single PA beams in the supine position and to identify simple methods for surface dose reduction. METHODS Measurements of surface dose were obtained in Solid Water phantoms using both a parallel plate ionization chamber (PTW Advanced Markus) and EBT2 Radiochromic films for both 6 and 10MV photons. All measurements were referenced to a depth considered a typical for PA Spine fields. Techniques used to reduce the surface dose included introducing an air standoff using Styrofoam sheets to suspend the phantom surface above the couch top and by adding a thin high Z scattering foil on the table surface. Surface doses were evaluated for typical field sizes, standoff heights, and various scattering materials. Comparisons were made to the surface dose obtainable when treating through a Varian Mylar covered tennis racket style couch top. RESULTS Dependence on typical spine field sizes was relatively minor. Dependence on air gap was much more significant. Surface doses decreased exponentially with increases in air standoff distance. Surface doses were reduced by approximately 50% for an air gap of 10cm and 40% for a 15cm air gap. Surface doses were reduced by an additional 15% by the addition of a 1mm Tin scattering foil. CONCLUSION Using simple techniques, it is possible to reduce the surface dose when treating single PA fields through the Varian carbon fiber couch top. Surface doses can be reduced to levels observed when treating though transparent Mylar tops by adding about 15 cm of air gap. Further reductions are possible by adding thin scattering foils, such as Tin or Lead, on the couch surface. This is a low cost approach to reduce surface dose when using the Varian carbon fiber couch top.

SU-F-T-607: Study of Dose Falloff Slope in RapidArc Planning of Lung SBRT.

PURPOSE SBRT of lung tumor results in steep dose fall-off outside the PTV to limit the normal tissue dose. RTOG report 0915 recommended metrics for plan quality measure, CI, D2cm, R50% and HDloc were evaluated. This study was aimed at ascertaining moderate to low dose spillage using 5 mm ring structures outside the PTV. METHODS In this retrospective study, 41 clinical RapidArc lung SBRT plans (Eclipse TPS ver 8.9) previously treated at our institution in a TrueBeam STx linac were analyzed. The delivered plans were re-planed using either 2 or 3 complete or partial arcs in coplanar or non-coplanar arrangement with upto ±15 degree couch angle. Seven concentric 5 mm wide rings were created outside the PTV for evaluation of dose fall-off based on mean, maximum doses as a function of distance. Mean and maximum percent dose fall-off in the rings (% dose/mm) were compared between the delivered and research plans. The differences in the mean and maximum percent doses between coplanar and non-coplanar arc plans were studied for any significant differences. RESULTS Mean dose falloff in the rings outside the PTV followed an exponential curve. For all the patients, interestingly the slope of maximum dose falloff increased at the intersection of 1st and 2nd ring and monotonically fell at outer rings. Altering coplanar to non-coplanar arc arrangements had a minimal impact on the CI as well as D2cm, R50% values (p-value > 0.05). CONCLUSION Slope of dose falloff from the PTV surface could serve as an additional metric. Coplanar arc plan had slightly steep dose falloff outside the PTV than non-coplanar arcs, but the differences are statistically insignificant.

SU-E-T-78: A Study of Dose Falloff Gradient in RapidArc Planning of Lung SBRT

Purpose: Rapid dose falloff beyond PTV is an important criterion for normal tissue sparing in SBRT. RTOG protocols use D2cm and R50% for plan quality evaluation. This study is aimed at analyzing the dose falloff gradient beyond the PTV extending into normal tissue structures and to ascertain the impact of PTV geometry and location on the dose falloff gradient in RapidArc planning of lung SBRT Methods: In this retrospective study, we analyzed 39 clinical RapidArc lung SBRT treatment plans that met RTOG-0915 criteria. Planning was done on Eclipse 8.9 for delivery on either Novalis NTx or TrueBeam STx equipped with HD MLCs. PTV volumes ranged between 5.3 and 113 cc (2.2 to 6 cm sphere equivalent diameter respectively) and their geographic locations were distributed in both lungs. 6X, 6X-FFF, 10X, and 10X-FFF energies were used for planning. All of these SBRT plans were planned using either 2 or 3 full or hemi arcs, with moderate couch kicks. Dose falloff gradients were obtained by generating 7 concentric 5 mm rings beyond PTV surface. Mean dose in each ring is used to evaluate percentage dose falloff gradient as a function of distance from the PTV surface. Results: The mean percentage dose falloff beyond PTV surface in all plans followed an exponential decay and the data was modeled with double exponential decay fit. Photon energy selection in the plan had a minimal impact on the mean percentage dose fall off beyond PTV surface. Conclusion: Dose falloff beyond PTV surface as a function of distance can be ascertained by the use of the double exponential decay fit coefficients in RapidArc planning of lung SBRT. This will help also in plan quality evaluation in addition to D2cm and R50% defined by RTOG.

SU‐GG‐J‐37: Development of a Quantitative Target Localizing and Tracking Method for IGRT

Purpose: To develop a quantitative target localizing and tracking method based on the contrast‐to‐noise ratio (CNR) that is derived from the images.Methods and materials: The kV, MV, CT and CBCTimages for Catphan504, a phantom including high contrast sensitometric objects with different attenuations and CT numbers were included in this study. Contrast‐to‐noise ratio is defined by CNR = (T‐B)/σ, where T is the target and B the background in the surrounding area and determined by the corresponding pixel values; σ is the noise caused by photon scatters and related to the regional standard deviation. A region of interest (ROI) was selected to cover the target, which measured T, and a relative larger ROI encompassing the target and surrounding tissue was defined as the background which provided B. CNRs were calculated for the targets in CT and other images used for target localization, and the correlations between them can be used to localize and track the target. Results: The CNR obtained from CT was 2.91 for air, 2.37 for LDPE, and 2.16 for Delrin, while corresponding CNRs from CBCT were 2.91, 2.36 for, and 2.14, respectively, which were comparable. For a sagittal image,CBCT provided better image quality (CNR 1.18) than kV imaging (1.02) and the quality of MV image (0.37) was poor. The CT and CBCTimages for a lung patient were used to test the method. The CNR for the tumor was 0.564 for CT and 0.567 for CBCT, which were comparable. The CNR for the tissue adjacent to the tumor was 0.418 for CT and 0.421 for CBCT. This showed that the target can be distinguished and tracked with CNR.Conclusions:CNR was used to quantify the visibility of the targets in multi‐modality images and can be potentially used for automated target localizing and real‐time tracking.