The Main Event
« Buyers, sellers under one roof | Main | The road to Boston »
Pick of the physics papers
Better late than never. In no particular order, here’s our top 10 listing of the best physics papers presented at the ASTRO scientific sessions.
• Investigations in the design of a novel linac-MRI system
Gino Fallone of Canada’s Cross Cancer Institute (Edmonton, AB) described how mounting a linac on a biplanar MRI system can enable real-time, 3D image guidance during radiotherapy. In the proposed design, the magnet and linac rotate together such that the radiation beam is always unobstructed and the linac remains stationary relative to the magnetic field, thus minimizing image distortions.
• Improved target localization with 3D cone beam CT on-board imaging in stereotactic body radiation therapy
For patients undergoing stereotactic body radiation therapy, cone-beam CT offers improved treatment set-up and target localization over that provided by 2D bony registration. That’s the conclusion of a study of 32 patients receiving 123 radiotherapy fractions, as presented by David Yoo of Duke University Medical Center (Durham, NC). Such advances in target tracking and immobilization during treatment allow for improved confidence in the accuracy and precision of stereotactic body radiation therapy delivery, said Yoo.
• Image guidance for external beam partial breast irradiation
Researchers at Washington University School of Medicine (St Louis, MO) have examined image-guidance methods for positioning patients receiving external-beam partial-breast irradiation. In a study of 26 subjects, laser alignment alone was seen to be sub-optimal for patient positioning, with significant discordance in set-up recommendations between systems using surface topography and those based on skeletal anatomy. As reported by Sasha Wahab, the team recommends the use of internal fiducial markers to validate set-up accuracy and notes that 3D surface mapping provides positioning accuracy to within 2 mm in any one dimension.
• Wide range and high spatial resolution in-vivo dosimetry using ultraminiature plastic scintillator with pulse counting method
A new ultraminiature dosimeter for in-vivo dosimetry was the subject of a presentation by Masayori Ishikawa from Hokkaido University Hospital in Japan. The dosimeter, which uses a plastic scintillator on the tip of a plastic optical fibre to detect incident radiation, was employed for dose measurement in the urethra of prostate cancer patients undergoing Ir-192 brachytherapy. The accuracy of the dosimeter was measured to be within 5% of the calculated dose distribution.
• Advanced mixed beam treatment techniques for breast and head and neck cancers
Mixed-beam radiation therapy (MBRT) combines intensity-modulated radiation therapy for lateral dose conformity with modulated electron radiation therapy for conformity in the depth direction. According to C M Charlie Ma from the Fox Chase Cancer Center (Philadelphia, PA), the technique provides excellent target coverage for treatment of breast and head-and-neck cancers. Ma detailed an advanced MBRT system comprising Monte Carlo-based treatment-planning software and multileaf collimators for photon and electron-beam modulation.
• Monitoring responses to treatment with high-frequency ultrasound in vivo: assessing response to radiation vs photodynamic therapy in melanoma xenograft tumours
High-frequency ultrasound could prove useful for determining a cancer patient’s response to a specific treatment modality, according to Behzad Banihashemi from Toronto Sunnybrook Cancer Centre (Toronto, ON). The research team used ultrasound imaging and backscatter analysis to evaluate the response to photodynamic therapy (PDT) and radiotherapy of melanoma tumours grown in mice. The researchers observed a close correlation between PDT-induced cell death and changes in ultrasound parameters, while radiotherapy did not cause any significant change in these parameters, consistent with the radioresistance of melanoma cells.
• Respiration motion prediction using time-delay kernel regression modelling
Researchers at the Thompson Cancer Center (Knoxville, TN) have developed a novel technique for dynamically predicting respiration motion and uncertainty up to 1.5 seconds in the future. A CCD-based respiratory-gating system measured 110 patient respiration cycles, which were used to construct a memory matrix for predicting future positions via time-delay kernel regression (TDKR). New respiration data are also continuously added to the model as acquired. Results indicate that TDKR modelling has similar predictive performance as previously studied parametric models, explained researcher Rebecca Seibert.
• Localization for MRI-based radiation treatment of intracranial lesions by using three-dimensional MRI and cone-beam CT matching
Zhiheng Wang from Duke University Medical Center (Durham, NC) presented the results of a study using MR images directly to match cone-beam CT (CBCT) images for localization in 10 patients being treated for intracranial lesions. The localization accuracy of this method was analysed by comparison with treatment-planning CT (CT-CBCT). The differences in couch shifts between the MRI-CBCT matching and CT-CBCT matching were within 2 mm in the lateral and vertical directions and within 3 mm in the longitudinal direction for all patients.
• Secondary particle energy deposition for proton and carbon therapy beams
Scientists at the University of Tennessee (Knoxville, TN) have evaluated the energy deposition from primary and secondary particles during proton and carbon-ion therapy. For proton beams, Monte Carlo simulations indicated that secondary particles contribute about 50% of the total energy deposited. For carbon beams, this contribution is less than 10%. A significant fraction of the fluence beyond the Bragg Peak for proton and heavy-ion therapy comes from secondary particles. Given that critical structures are often located near the Bragg peak, the biological effects of these secondary particles should be considered in treatment planning, reported researcher Youssef Charara.
• Orthogonal delivery to improve IMRT efficiency
Xin Dou from the University of Iowa (Iowa City, IA) proposed a method for improving IMRT delivery efficiency by splitting the prescribed dose distribution, or intensity map, into two orthogonal sub-intensity maps. More precisely, one sub-intensity map is delivered by moving the multileaf collimator (MLC) leaves horizontally and the other by moving the leaves vertically, with the MLC rotated by 90° between deliveries.
