Investigation of the relative surface dose from Lipowitz-metal tissue compensators for 24- and 6-MV photon beams

A ring-based compensator IMRT system optimized for low- and middle-income countries: Design and treatment planning study

PURPOSE

We propose a novel compensator-based IMRT system designed to provide a simple, reliable, and cost-effective adjunct technology, with the goal of expanding global access to advanced radiotherapy techniques. The system would employ easily reusable tungsten bead compensators that operate independent of a gantry (e.g., mounted in a ring around the patient). Thereby the system can be retrofitted to existing linac and cobalt teletherapy units. This study explores the quality of treatment plans from the proposed system and the dependence on associated design parameters.

METHODS

We considered ⁶⁰ Co-based plans as the most challenging scenario for dosimetry and benchmarked them against clinical MLC-based plans delivered on a linac. Treatment planning was performed in the Pinnacle treatment planning system with commissioning based on Monte Carlo simulations of compensated beams. ⁶⁰ Co-compensator IMRT plans were generated for five patients with head-and-neck cancer and five with gynecological cancer and compared to respective IMRT plans using a 6 MV linac beam with an MLC. The dependence of dosimetric endpoints on compensator resolution, thickness, position, and number of beams was assessed. Dosimetric accuracy was validated by Monte Carlo simulations of dose distribution in a water phantom from beams with the IMRT plan compensators.

RESULTS

The ⁶⁰ Co-compensator plans had on average equivalent PTV coverage and somewhat inferior OAR sparing compared to the 6 MV-MLC plans, but the differences in dosimetric endpoints were clinically acceptable. Calculated treatment times for head-and-neck plans were 7.6 ± 2.0 min vs 3.9 ± 0.8 min (6 MV-MLC vs ⁶⁰ Co-compensator) and for gynecological plans were 8.7 ± 3.1 min vs 4.3 ± 0.4 min. Plan quality was insensitive to most design parameters over much of the ranges studied, with no degradation found when the compensator resolution was finer than 6 mm, maximum thickness at least 2 tenth-value-layers, and more than five beams were used. Source-to-compensator distances of 53 and 63 cm resulted in very similar plan quality. Monte Carlo simulations suggest no increase in surface dose for the geometries considered here. Simulated dosimetric validation tests had median gamma pass rates of 97.6% for criteria of 3% (global)/3 mm with a 10% threshold.

CONCLUSIONS

The novel ring-compensator IMRT system can produce plans of comparable quality to standard 6 MV-MLC systems. Even when ⁶⁰ Co beams are used the plan quality is acceptable and treatment times are substantially reduced. ⁶⁰ Co-compensator IMRT plans are adequately modeled in an existing commercial treatment planning system. These results motivate further development of this low-cost adaptable technology with translation through clinical trials and deployment to expand the reach of IMRT in low- and middle-income countries.

DETERMINATION OF TRANSFER FUNCTIONS OF MCP-200 ALLOY USING 6 MV PHOTON BEAM FOR BEAM INTENSITY MODULATION

Transfer functions of MCP-200 alloy were determined for beam intensity modulation. 6 MV Photon beam, obtained from linear accelerator, is attenuated by blocks of MCP-200 alloy with different thicknesses placed at a distance of one meter from the source. The attenuated beam was allowed to fall on photographic film and optical densities were measured. Films were scanned and profiles were obtained for different thicknesses of the alloy and in the absence of any attenuator. Transfer functions were obtained theoretically as well as analytically by fitting the experimental results into theoretical data. The whole work was completed using optimum dose units and found to be 30 cGy.

LIMITATIONS OF GADOLINIUM ALLOYS IN CANCER TREATMENT AND RADIATION SHIELDING

Characteristic emission from gadolinium-doped amorphous aluminum nitride thin films deposited on Si (111) substrate are studied, with cathodoluminescence and photoluminescence, for their suitability as tissue compensators in radiation treatment planning. These films are deposited by reactive sputtering at liquid nitrogen temperature, using 100–200 W rf power, 5–8 mTorr nitrogen, and a metal target of aluminum and gadolinium. Thermal annealing was performed at a temperature of 900°C. A sharp ultraviolet peak is observed at 314 nm, corresponding to 6 P 7/2 → 8 S 7/2 transition. The ultraviolet emission is intense enough to harm human tissues if it is used as a tissue compensator. Thermal annealing further enhances the intensity of ultraviolet emission, indicating that its longer use as tissue compensator will provide more harm and damage to the human body.

Inverse radiosurgery treatment planning through deconvolution and constrained optimization

An inverse radiosurgery treatment planning approach is presented which calculates conformal dose distributions for small, irregularly shaped targets. Two general approaches have been suggested for solution of the inverse radiotherapy problem: explicit and implicit. Explicit methods are typically fast, but generally require geometric and/or dosimetric simplifications. Implicit methods have also been used, but are computationally expensive because they require iterative manipulation of each beam's individual elements. The method presented here incorporates an integrated approach in order to efficiently solve the inverse problem without requiring simplifications which may affect the accuracy of the final result. A deconvolution algorithm (explicit approach) is utilized to determine the intensity modulation function for multiple user-selected beam's eye views of the desired dose distribution. A simulated annealing algorithm (implicit approach) then optimizes each beam's macroscopic weight. Additionally, this method is fully three-dimensional and accurately models phantom scatter by incorporating Monte Carlo generated energy deposition kernels into the dosimetry process. Several small target structure examples are presented and applicability of this methodology to larger targets for general radiotherapy cases is addressed.

A reticle retrofit and dosimetric consideration for a linear accelerator

PURPOSE

An imperfect reticle system in an accelerator causes uncertainties in source-skin distance (SSD), off-axis distance (OAD), isocenter, and so forth. A reticle was designed and fabricated, and its implications on x-ray and electron beam dosimetry were investigated.

METHODS AND MATERIALS

A new reticle frame was dimensioned to fit snugly in the accelerator. The frame was fabricated to carry a pair of adjustable cross wires and to allow the machine operation in the photon and electron modes. The impact of the cross wires on 6 MV photon and 5-10 MeV electron beam parameters such as dose rate (Gy/monitor unit), beam uniformity, surface dose, and so forth, were studied using suitable ion chambers and phantoms.

RESULTS

The retrofitted system offered long-term mechanical stability leading to precise SSD, OAD, and isocenter measurements. Changes introduced by the cross wires on the 6 MV photon and 5-10 MeV electron beams are presented.

CONCLUSION

Long-term stability of a reticle in an accelerator is important for an accurate patient setup and for making reliable dosimetric measurements. Beam characteristrics have to be studied whenever modifications on a reticle system are made.

Secondary electron scatter in radiotherapy: implications for treatment fields in proximity to the lens of the eye and the testes

For external beam photon radiotherapy treatments, secondary electrons pose unique shielding problems. Dose due to these electrons can be deposited under shielding blocks and also outside of collimated beam edges. The sources of contamination electrons are many, but the dominant one is identified as the plastic accessory tray commonly used to support shielding blocks. Because these electrons only deposit dose superficially, the critical structures that have been established which are in range of them are the lens of the eye and the testes. The present work outlines a technique that involves a low atomic number absorber placed directly over either of these structures (outside of the primary beam) to provide the necessary shielding. Use of the technique will ensure that critical structures in proximity to treatment portals have the dose to them minimized as much as possible.