T Jenkins

SOUNDS OF SEX AND DEATH IN THE SEA: BOTTLENOSE DOLPHIN WHISTLES SUPPRESS MATING CHORUSES OF SILVER PERCH

ABSTRACT Prey often exhibit avoidance behaviors when predators are present. We observed diminished loudness of mating choruses of male silver perch Bairdiella chrysoura in spawning areas when vocalizing bottlenose dolphins Tursiops truncatus, which hunt fish acoustically, were present. Experimental playback of bottlenose dolphin sounds revealed that male silver perch mating calls were reduced by an average of 9 dB. This “acoustical avoidance” behavior, demonstrated previously for interactions involving bats hunting insects and frogs, may also be a common phenomenon in acoustically mediated predator-prey interactions in the sea.

Toward endobronchial Ir-192 high-dose-rate brachytherapy therapeutic optimization

A number of patients with lung cancer receive either palliative or curative high-dose-rate (HDR) endobronchial brachytherapy. Up to a third of patients treated with endobronchial HDR die from hemoptysis. Rather than accept hemoptysis as an expected potential consequence of HDR, we have calculated the radial dose distribution for an Ir-192 HDR source, rigorously examined the dose and prescription points recommended by the American Brachytherapy Society (ABS), and performed a radiobiological-based analysis. The radial dose rate of a commercially available Ir-192 source was calculated with a Monte Carlo simulation. Based on the linear quadratic model, the estimated palliative, curative and blood vessel rupture radii from the center of an Ir-192 source were obtained for the ABS recommendations and a series of customized HDR prescriptions. The estimated radius at risk for blood vessel perforation for the ABS recommendations ranges from 7 to 9 mm. An optimized prescription may in some situations reduce this radius to 4 mm. The estimated blood perforation radius is generally smaller than the palliative radius. Optimized and individualized endobronchial HDR prescriptions are currently feasible based on our current understanding of tumor and normal tissue radiobiology. Individualized prescriptions could minimize complications such as fatal hemoptysis without sacrificing efficacy. Fiducial stents, HDR catheter centering or spacers and the use of CT imaging to better assess the relationship between the catheter and blood vessels promise to be useful strategies for increasing the therapeutic index of this treatment modality. Prospective trials employing treatment optimization algorithms are needed.

Targeting the adventitia with intracoronary beta-radiation: comparison of two dose prescriptions and the role of centering coronary arteries

PURPOSE To compare by intravascular ultrasound (IVUS) the efficacy of delivering the prescribed dose to the adventitia between two commonly used dose prescriptions for intracoronary radiotherapy. METHODS AND MATERIALS In 59 human postangioplasty coronary vessels, one IVUS cross-section (1 mm thick) with the highest plaque burden was used for creating dose-volume histograms with different hypothetical positions of the source. RESULTS On average, prescription to 1 mm beyond lumen surface resulted in delivery of the prescribed dose (20 Gy +/- 20%) to a higher fraction of adventitial volume than with the prescription to 2 mm from the source, with source placed in vessel center, lumen center, or in the IVUS catheter position. Source placement in the lumen center resulted in a low dose heterogeneity to the adventitia and the least dose heterogeneity to the intima. CONCLUSIONS Prescription to 1 mm beyond lumen surface appeared more effective in delivering the prescribed dose to the adventitia than the American Association of Physicists in Medicine (AAPM) recommended prescription to 2 mm from the source center. Moreover, centering the source in the lumen provides the better balance of effective adventitial targeting and intimal dose homogeneity. Modification of the current AAPM recommendation for dose prescription for intracoronary radiotherapy should be considered.

SU‐FF‐T‐147: Improved Calibration Method of EDR Films for IMRT‐QA

Purpose: Due to film processing variation and the optical density not being linear with the dose, a full calibration has to be obtained for every IMRT‐QA with film. We are proposing the application of a linearization method and the use of a single dose calibration EDR film for relative and absolute dose measurement in IMRT‐QA. Method and Materials: The linearization method was studied for the EDR films using 6, 10 and 15 MV photon beams. The films were developed in a Konica film processor, then scanned with a Vidar VXR‐16 scanner and analyzed using RIT‐114 version 4.1. A standard 30‐point calibration curve was obtained with the 6 MV beam. Using the method developed in a previous paper, a curve fitting was obtained for a sigmoid expression modulated by a 3rd degree polynomial: ApparentDose = b (1+ a 1 x + a 2 x 2 + a 3 x 3 ) [ log ( m )− log ( m − x )] ; where x is the net optical density, m is the net saturation density, b, a1, a2 and a3 are parameters of the model. Results: A value of 1870.1945 was obtained for b that is a parameter related to the dose unit. The net saturation density was 3.5587 for our system. The parameters a1, a2 and a3 are, respectively, −0.4551, 0.1167, −0.0134. For every IMRT‐QA a spreadsheet is used to obtain a 70 point calibration curve for RIT, using only one exposed film developed together with the composed and enface films, obtaining <5% uncertainty. Conclusion: Some softwares are available to make film dosimetry in IMRT QA less cumbersome, however daily calibration still remains a time consuming procedure. Absolute dosimetry requires a full calibration every time film is used. The linearization method presented here is being used in our department for isodose comparison and for absolute measurement at calculation point. The overall time is reduced while obtaining uncertainty comparable to the existing methods.

SU‐FF‐J‐19: Prototype Fixture for IGRT Coordinate Registration and 4D CT Reconstruction

Purpose: The potential use of in‐room CT technology for target localization requires accurate methods for mapping spatial and temporal data between the CT and linac coordinate spaces. This can be done by absolute calibration of the imaging device prior to scanning or by capturing known fiducial patterns within the image data. We have designed a hybrid technique for in‐room CTs that defines a unified coordinate system referenced to the treatment table. It is based on patterns encoded in the unused portion of CTimages below the treatment table surface. These patterns have two primary functions: 1) to facilitate the registration of image coordinates for image‐guided radiation therapy; and 2) to encode an external respiratory signal for 4D CTreconstruction. Methods and Materials: A fixture was made from a 6 mm thick acrylic plate embedded with small aluminum rods. The rods formed a pattern used for spatial calibration. In addition, a long “V” shaped piece of extruded aluminum was mounted on the fixture between bearings and attached to a motor. The motor was computer controlled such that the “V” could point in any direction to encode a respiratory signal. The entire fixture was mounted beneath the “tennis racket” portion of a carbon fiber treatment table. Results:CTimages clearly showed the spatial patterns and respiratory signal encoded by the fixture. Dynamic respiratory signals were imaged with minimal artifacts at angular velocities up to six rpm with 0.8s CT gantry rotation. Custom software was able to decode the image metadata and use it for IGRT coordinate registration and 4D CTreconstruction. Attenuation of 6 MV radiation by the fixture was measured to be < 3%. Conclusions: The prototype met all design goals and facilitated IGRT coordinate registration and 4D CTreconstruction.Conflict of Interest: Research sponsored by Siemens corporation.

SU-FF-J-20: An Evaluation of User Variability for Image-Guided Radiation Therapy (IGRT) Shift Determination

Purpose: To evaluate the Image‐Guided(IGRT) shift variability calculated by different users using the same patient data and same IGRT process. Method and Materials:IGRT was performed using the Siemens Primatom. This system consists of a Siemens Primus Linear Accelerator and a Siemens Emotion diagnostic CT on rails. Patients have a daily, pretreatment CT scan taken that is transferred to the Siemens Coherence Workstation where the daily CT is fused with the original treatment planningCT. The fusion is performed so that the daily internal organ or critical structure position can be determined and the organ/structure can be aligned to give the same position relative to the isocenter that was determined from the treatment plan. A shift in the x, y and z directions are made to facilitate this alignment. Four users retrospectively calculated the shifts required for an IGRT patient using the same CT data set and the same IGRT process. The treatment area for the patient was the prostate gland and a total of 39 daily shifts were performed. Results: The maximum variation on any one day was 0.90cm in the right/left direction, 1.10cm in the superior/inferior direction and 0.93cm in the anterior/posterior direction. The averages of the maximum daily variations were 0.39cm, 0.54cm and 0.55cm respectively. Conclusion: There are multiple systems that will perform IGRT but the advantage to using CT‐on‐rails systems is the high soft tissue resolution you get from the diagnostic CT sets. The higher resolution allows the user much more information that can be included in the shift evaluation. Our results show the user variability to be acceptable but when implementing this type of IGRT, user variability must be considered.