Mammo Yewondwossen

A gated deep inspiration breath-hold radiation therapy technique using a linear position transducer

For patients with thoracic and abdominal lesions, respiration‐induced internal organ motion and deformations during radiation therapy are limiting factors for the administration of high radiation dose. To increase the dose to the tumor and to reduce margins, tumor movement during treatment must be minimized. Currently, several types of breath‐synchronized systems are in use. These systems include respiratory gating, deep inspiration breath‐hold, active breathing control, and voluntary breath‐hold. We used a linear position transducer (LPT) to monitor changes in a patients abdominal cross‐sectional area. The LPT tracks changes in body circumference during the respiratory cycle using a strap connected to the LPT and wrapped around the patients torso. The LPT signal is monitored by a computer that provides a real‐time plot of the patients breathing pattern. In our technique, we use a CT study with multiple gated acquisitions. The Philips Medical Systems Q series CT imaging system is capable of operating in conjunction with a contrast injector. This allows a patient performing the deep inspiration breath‐hold maneuver to send a signal to trigger the CT scanner acquisitions. The LPT system, when interfaced to a LINAC, allows treatment to be delivered only during deep inspiration breath‐hold periods. Treatment stops automatically if the lung volume drops from a preset value. The whole treatment can be accomplished with 1 to 3 breath‐holds. This technique has been used successfully to combine automatically gated radiation delivery with the deep inspiration breath‐hold technique. This improves the accuracy of treatment for moving tumors, providing better target coverage, sparing more healthy tissue, and saving machine time. PACS numbers: 87.53.2j, 87.57.‐s

Low Z target switching to increase tumor endothelial cell dose enhancement during gold nanoparticle‐aided radiation therapy

PURPOSE Previous studies have introduced gold nanoparticles as vascular-disrupting agents during radiation therapy. Crucial to this concept is the low energy photon content of the therapy radiation beam. The authors introduce a new mode of delivery including a linear accelerator target that can toggle between low Z and high Z targets during beam delivery. In this study, the authors examine the potential increase in tumor blood vessel endothelial cell radiation dose enhancement with the low Z target. METHODS The authors use Monte Carlo methods to simulate delivery of three different clinical photon beams: (1) a 6 MV standard (Cu/W) beam, (2) a 6 MV flattening filter free (Cu/W), and (3) a 6 MV (carbon) beam. The photon energy spectra for each scenario are generated for depths in tissue-equivalent material: 2, 10, and 20 cm. The endothelial dose enhancement for each target and depth is calculated using a previously published analytic method. RESULTS It is found that the carbon target increases the proportion of low energy (<150 keV) photons at 10 cm depth to 28% from 8% for the 6 MV standard (Cu/W) beam. This nearly quadrupling of the low energy photon content incident on a gold nanoparticle results in 7.7 times the endothelial dose enhancement as a 6 MV standard (Cu/W) beam at this depth. Increased surface dose from the low Z target can be mitigated by well-spaced beam arrangements. CONCLUSIONS By using the fast-switching target, one can modulate the photon beam during delivery, producing a customized photon energy spectrum for each specific situation.

Sci-Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques - 07: Transitioning from extended-distance total body irradiation to optimized VMAT total marrow irradiation

Purpose: TMI targets only the bone marrow, with the intent of sparing normal tissues. The NSCC has recently implemented a TMI protocol which includes VMAT fields to treat the bone marrow from head to mid-thigh and extended SSD POP fields to treat the lower legs. This work describes the commissioning and initial clinical results of the first reported VMAT TMI treatments in Canada. Methods: Detailed CT simulation, imaging, planning and treatment procedures were developed by a multi-disciplinary team. Patients have 1 cm of bolus over the lower legs and 0.5 cm of bolus around the lower arms. The PTV includes all bone, except mandible, facial bones and hands, with the objective of V(12 Gy) > 90%. Detailed analysis of the influence of field overlap was performed to determine optimal field placement and image-guidance tolerances. Results: PTV coverage was achieved for all cases as V(12 Gy) ranged from 90.4–96.3%. The minimum dose to the PTV, D(99%), ranged from 91.4–97.87% and V(90%Rx=10.8 Gy) ranged from 99.1–100.0%. The lungs, liver and heart had an average Dmean of (7.8±0.3)Gy/(65±2)%, (7.6±0.7)Gy/(63±5)%, and (6.8±0.4)Gy/(56±4)% respectively. Conclusions: Commissioning required input and collaboration from all team members. Transitioning from TBI to TMI requires additional time for contouring, treatment planning, QA, and treatment. Patient benefit can however be seen in the quality of OAR sparing.

SU‐E‐T‐371: Measurement of Transit Dose of An Ir‐192 HDR Brachytherapy Stepping Source Using a 2D‐Array of Ion Chambers

Purpose: The MatriXX Evolution 2D ionization chamber array is one of the 2D ionization chamber arrays developed by IBA dosimetry (IBA Dosimetry, Germany) for megavoltage real‐time absolute 2D dosimetry, verification of intensity‐modulated radiation therapy(IMRT) and volumetric modulated arc therapy (VMAT). We have recently shown that this 2D ionization chamber array can be used for HDR brachytherapy Ir‐192 beam dose verification. The aim of this work is to investigate the use of the movie mode of the MatriXX Evolution for measurement of transit dose of the Ir‐192 source of a Nucletron Micro‐Selectron HDR brachytherapy unit. Methods: The MatriXX Evolution measurement can be performed in different measurement modes depending on the measurement situation. For megavoltage dynamic field measurement the 2D ionization chamber array is equipped with movie mode to perform measurements continuously. During this measurement each single shot is saved and can be viewed, played as a movie, or analyzed afterwards. The integration time for each sample or movie image can be varied by the user with the shortest sampling time being 20 ms. This dynamic mode measurement of the MatriXX Evolution was used to measure the entrance, exit and the inter‐dwell transit dose of the HDR source. Results: At a fixed point of measurement, the transit dose depends on the radioactive source strength and velocity along the trajectory. The transit dosemeasured with MatriXX Evolution move mode has been found to be consistent with that already presented by other dosimeters. Conclusions: The feasibility of using a 2D ionization chamber move mode in determining the transit dose of HDR stepping source is explored in the present work. Results of this investigation show that 2D ionization chamber move mode can be used to measure transit dose.

WE‐DE‐BRA‐08: A Linear Accelerator Target Allowing Rapid Switching Between Treatment and High‐Contrast Imaging Modes

PURPOSE During radiotherapy treatment, lung tumors can display substantial respiratory motion. This motion usually necessitates enlarged treatment margins to provide full tumour coverage. Unfortunately, these margins limit the dose that can be prescribed for tumour control and cause complications to normal tissue. Options for real-time methods of direct detection of tumour position, and particularly those that obviate the need for inserted fiducial markers, are limited. We propose a method of tumor tracking without implanted fiducial markers using a novel fast switching-target that toggles between a FFF copper/tungsten therapy mode and a FFF low-Z target mode for imaging. In this work we demonstrate proof-of-concept of this new technology. METHODS The prototype includes two targets: i) a FFF copper/tungsten target equivalent to that in the Varian 2100 EX 6 MV, and ii) a low-Z (carbon) target with a thickness of 110% of continuous slowing down approximation range (CSDA) at 7 MeV. The two targets can be exchanged with a custom made linear slide and motor-driven actuator. The usefulness of the switching-target concept is demonstrated through experimental BEV Planar images acquired with continual treatment and imaging at a user-defined period. RESULTS The prototype switching-target demonstrates that two recent advances in linac technology (FFF target for therapy and low-Z target) can be combined with synergy. The switching-target approach offers the capacity for rapid switching between treatment and high-contrast imaging modes, allowing intrafractional tracking, as demonstrated in this work with dynamic breathing phantom. By using a single beam-line, the design is streamlined and may obviate the need for an auxiliary imaging system (e.g., kV OBI.) CONCLUSION: This switching-target approach is a feasible combination of two current advances in linac technology (FFF target for therapy and a FFF low-Z target) allowing new options in on-line IGRT.