SU-E-T-399: Dosimetric and Geometric Evaluation of a Novel Stereotactic Radiotherapy Device for Breast Cancer: The GammaPod.

PURPOSE: A dedicated stereotactic irradiation device, the GammaPodTM, was developed to treat early stage breast cancer. This study presents the first description of the dosimetric and geometric characteristics from the prototype unit. METHODS: The GammaPod stereotactic radiotherapy device is an assembly of a hemi-spherical source carrier containing 36 Co-60 sources, a tungsten collimator, a dynamically controlled treatment table and a breast immobilization cup embedded with a stereotactic coordinate system. The source carrier and the variable-size collimator rotate synchronously to form 36 non-coplanar, concentric arcs focused at the isocenter. The treatment table enables motion in three dimensions facilitating continuous dose painting in comparison to a sphere packing approach. Geometric and dosimetric evaluations and a method for absorbed dose calibration are provided. Dosimetric verifications of the dynamically delivered plans are performed for eight patients in hypothetical pre-op, post-op and dose painting treatment scenarios. RESULTS: Loaded with a cumulative activity of 4320 Ci, the GammaPod unit delivers 5.31 Gy/min at the isocenter. Due to non-coplanar beam arrangement and dynamic dose shaping features, the GammaPod delivers uniform doses to the targets with excellent conformity. The spatial accuracy of the device is less than 1 mm. Single shot profiles with the 25 mm collimator are measured with radiochromic film and found to be in good agreement with respect to the MC based calculations (congruence of FWHM less than 1 mm). Dosimetric verifications corresponding to all treatment plans corresponding to three target scenarios for each of the eight patients demonstrated Gamma index pass rates greater than 97%. CONCLUSIONS: The first description of the dosimetric and geometric evaluation of the GammaPod was performed. The observed level of agreement between the treatment planning system calculations and dosimetric measurements has confirmed that the system can deliver highly complex treatment plans with remarkable geometric and dosimetric accuracy. C Yu and J Zhang have commercial affiliations with Xcision Medical Systems.

SU-E-J-207: Compensation of Target Distortion of Pancreatic Tumor in Free-Breathing CT Using 4D Contour Propagation.

PURPOSE: Due to lack of soft-tissue contrast, target distortion for the upper-abdomen targets such as pancreatic tumors is complicated and requiring sufficient remedy. By applying automatic contour propagation, the authors use the information obtained from 4D CT to test if the deformable image registration compensates the respiration-induced distortion of pancreatic tumor in free breathing (FB) CT images. METHODS: Ten patients with unresected pancreatic cancer treated with either preoperative or definitive chemoradiation were studied. Pancreas GTVs were delineated on the FB CT. Using deformable image registration, the FB GTV contours were propagated to each phase of the 4D CT images taken right after the FB CT, and were compared with the FB GTV to see difference in tumor volume and tumor size along individual dimensions. A one-dimensional tumor motion in proportion to cos4(ωt) was simulated to calculate the probability distribution function for different magnitude of distortions during FB CT scans, and a binary classification test was conducted to analyze the observed results. RESULTS: The probability distribution function predicted that four out of the ten cases would have substantial target distortion given the variation in target motion amplitudes. Three of these four cases show substantial difference in the superior-inferior size of FB GTV compared to the average 4D GTV, taking into account the uncertainties caused by motions perpendicular to the scanning axis and resolution of the CT scanner. The binary classification test yielded a precision of 75% and an accuracy of 90%. CONCLUSIONS: Pancreatic GTV distorted due to respiration-induced tumor motion is effectively compensated by contour propagation from free-breathing CT to 4D CT using DIR. Union of GTVs of all breathing phases or IGTV can be genreated from 4D set of GTVs propagated from that of free breathing. This study is partially supported by NIH grant 1R01CA133539-01A2. I do not have conflict of interest.

A simulation study of irregular respiratory motion and its dosimetric impact on lung tumors.

This study is aimed at providing a dosimetric evaluation of the irregular motion of lung tumors due to variations in patients' respiration. Twenty-three lung cancer patients are retrospectively enrolled in this study. The motion of the patient clinical target volume is simulated and two types of irregularities are defined: characteristic and uncharacteristic motions. Characteristic irregularities are representative of random fluctuations in the observed target motion. Uncharacteristic irregular motion is classified as systematic errors in determination of the target motion during the planning session. Respiratory traces from measurement of patient abdominal motion are also used for the target motion simulations. Characteristic irregular motion was observed to cause minimal changes in target dosimetry with the largest effect of 2.5% ± 0.9% (1σ) reduction in the minimum target dose (D(min)) observed for targets that move 2 cm on average and exhibiting 50% amplitude variations within a session. However, uncharacteristic irregular motion introduced more drastic changes in the clinical target volume (CTV) dose; 4.1% ± 1.7% reduction for 1 cm motion and 9.6% ± 1.7% drop for 2 cm. In simulations with patients' abdominal motion, corresponding changes in target dosimetry were observed to be negligible (<0.1%). Only uncharacteristic irregular motion was identified as a clinically significant source of dosimetric uncertainty.

The impact of temporal inaccuracies on 4DCT image quality.

Accurate delineation of target volumes is one of the critical components contributing to the success of image-guided radiotherapy treatments and several imaging modalities are employed to increase the accuracy in target identification. Four-dimensional (4D) techniques are incorporated into existing radiation imaging techniques like computed tomography (CT) to account for the mobility of the target volumes. However, these methods in some cases introduce further inaccuracies in the target delineation when further quality assurance measures are not implemented. A source of commonly observed inaccuracy is the misidentification of the respiration cycles and resulting respiration phase assignments used in the construction of the 4D patient model. The aim of this work is to emphasize the importance of optimal respiration phase assignment during the 4DCT image acquisition process and to perform a quantitative assessment of the effect of inaccurate phase assignments on the overall image quality. The accuracy of the phase assignment was assessed by comparison with an independent calculation of the respiration phases. Misplaced phase assignments manifest themselves as deformations and artifacts in reconstructed images. These effects are quantified as volumetric discrepancies in the localization of target objects represented by spherical phantoms. Measurements are performed using a fully programmable motion phantom designed and built at Mayo Clinic (Rochester, MN). Implementation of a case based independent check and correction procedure is also demonstrated with emphasis on the use of this procedure in the clinical environment. Review of clinical 4D scans performed in this institution showed discrepancies in the phase assignments in about 40% of the cases when compared to our independent calculations. It is concluded that for improved image reconstruction, an independent check of the sorting procedure should be performed for each clinical 4DCT case.