GafChromic® RTQA film for routine quality assurance of high-energy photon beams

High Cytotoxic Effect by Combining Copper-64 with a NOTA-Terpyridine Platinum Conjugate

Terpyridine platinum (TP)-based chemotherapeutic agents target three-dimensional structures on DNA known as G-quadruplexes. We report the rational design and synthesis of a TP conjugate combined with copper-64 (⁶⁴Cu), the decay characteristics of which include emission of β^- and Auger electrons for radiotherapy and β⁺ particles for positron emission tomography (PET) imaging. The present experimental studies show that the novel [⁶⁴Cu]Cu-1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)-TP is stable, permitting selective killing of cancer cells. The antitumor activity of [⁶⁴Cu]Cu-NOTA-TP at high apparent molar activity is in the low nanomolar range and 27,800-fold greater than that of ^natCu-NOTA-TP at 24 h post treatment. These results suggest that this combination of a cytotoxic TP agent with ⁶⁴Cu has considerable potential for cancer treatment and PET imaging.

Custom-designed Small Animal focal iRradiation Jig (SARJ): design, manufacture and dosimetric evaluation

OBJECTIVE

Preclinical animal models allow testing and refinement of novel therapeutic strategies. The most common preclinical animal irradiators are fixed source cabinet irradiators, which are vastly inferior to clinical linear accelerators capable of delivering highly conformal and precise treatments. The purpose of this study was to design, manufacture and test an irradiation jig (small animal focal irradiation jig, SARJ) that would enable focal irradiation of subcutaneous tumours in a standard fixed source cabinet irradiator.

METHODS AND MATERIALS

A lead shielded SARJ was designed to rotate animal holders about the longitudinal axis and slide vertically from the base plate. Radiation dosimetry was undertaken using the built-in ion chamber and GAFChromic RTQA2 and EBT-XD films. Treatment effectiveness was determined by irradiating mice with subcutaneous melanoma lesions using a dose of 36 Gy in three fractions (12 Gy x 3) over three consecutive days.

RESULTS

The SARJ was tested for X-ray shielding effectiveness, verification of dose rate, total dose delivered to tumour and dose uniformity. Accurate and uniform delivery of X-ray dose was achieved. X-ray doses were limited to the tumour site when animal holders were rotated around their longitudinal axis to 15o and 195o, allowing sequential dose delivery using parallel-opposed tangential beams. Irradiation of subcutaneous melanoma tumour established on the flanks of mice showed regression.

CONCLUSION

SARJ enabled delivery of tangential parallel-opposed radiation beams to subcutaneous tumours in up to five mice simultaneously. SARJ allowed high throughput testing of clinically relevant dose delivery using a standard cabinet-style fixed source irradiator.

ADVANCES IN KNOWLEDGE

A custom designed jig has been manufactured to fit into conventional cabinet irradiators and is dosimetrically validated to deliver clinically relevant dose distributions to subcutaneous tumours in mice for preclinical studies.

Implementation of a program for quality assurance on leaf positioning accuracy using Gafchromic(®) RTQA2 films

In radiotherapy treatments the correct dose delivery to the target volume and the consequent conservation of healthy tissues is affected by multileaf collimator (MLC) leaf positioning accuracy and reproducibility, mostly in intensity-modulated radiation therapy (IMRT): For this reason a quality assurance (QA) program is necessary to ensure the best treatment possible to each patient. The aim of this study is the implementation of a method using Gafchromic(®) RTQA 2 films to perform routine QA on the MLC, both for qualitative and quantitative analysis. A flatbed document scanner (Epson 10000XL) was used in conjunction with radiochromic detector; a scanning protocol was firstly defined to improve readout accuracy. RTQA2 films were irradiated with 6 MV X-rays at different dose levels to obtain calibration curve. To evaluate the leaf positioning accuracy in different conditions, a rhomboidal shape and a field consisting in three rectangular segments were selected. The images quantitative analysis was handled with a program developed in MATLAB to evaluate the differences between expected and measured leaves positions. The reproducibility and global uncertainty of the method were estimated to be equal to 0.5% and 0.6 mm, respectively. Moreover, a qualitative test was performed: A garden picket fence field, consisting in multiple segments 2 × 22 cm(2), was realized setting known leaves shifts to test the method sensitivity. The picket fence test shows that the method is able to detect displacements equal to 1 mm. The results suggest that Gafchromic(®) RTQA2 films represent a reliable tool to perform MLC routine QA.

Paraspinal volumetric modulated arc therapy

OBJECTIVES

The processes involved in the treatment of paraspinal tumours by volumetric modulated arc therapy (VMAT) are described here by means of an illustrative case.

METHODS

Az single anticlockwise arc from gantry angle 179° to 181° was constructed using SmartArc (Philips Radiation Oncology Systems, Fitchburg, WI) with control points spaced at 2°. The dose prescription was 60 Gy in 30 fractions to cover the planning target volume (PTV) as uniformly as possible while sparing the 0.3-cm planning risk volume (PRV) around the spinal cord. The plan was verified before treatment using a diode array phantom and radiochromic film. Treatment delivery was on a Synergy linear accelerator with a beam modulator head (Elekta Ltd, Crawley, UK).

RESULTS

Homogeneous dose coverage of the PTV was achieved with a D(2%) of 62.0 Gy and D(98%) of 55.6 Gy. Maximum spinal cord dose was 49.9 Gy to 0.1 cm(3) and maximum dose to the spinal cord PRV was 55.4 Gy to 0.1 cm(3). At pre-treatment verification, the percentage of the high-dose region receiving a dose within 3% and 3 mm of the planned dose was 98.8% with the diode array and 93.4% with film. Delivery time was 2 min 15 s and the course of treatment was successfully completed.

CONCLUSIONS

VMAT was successfully planned, verified and delivered for this challenging tumour site. VMAT provides a very suitable method of treating complex paraspinal tumours, offering a high-quality conformal dose distribution with a short delivery time.

Dose profile measurement using an imaging plate: Evaluation of filters using Monte Carlo simulation of 4 MV x-rays

Computed radiography (CR) is gradually replacing film. The application of CR for two-dimensional profiles and off-axis ratio (OAR) measurement using an imaging plate (IP) in a CR system is currently under discussion. However, a well known problem for IPs in dosimetry is that they use high atomic number (Z) materials, such as Ba, which have an energy dependency in a photon interaction. Although there are some reports that it is possible to compensate for the energy dependency with metal filters, the appropriate thicknesses of these filters and where they should be located have not been investigated. The purpose of this study is to find the most suitable filter for use with an IP as a dosimetric tool. Monte Carlo simulation (Geant4 8.1) was used to determine the filter to minimize the measurement error in OAR measurements of 4 MV x-rays. In this simulation, the material and thickness of the filter and distance between the IP and the filter were varied to determine most suitable filter conditions that gave the best fit to the MC calculated OAR in water. With regard to changing the filter material, we found that using higher Z and higher density material increased the effectiveness of the filter. Also, increasing the distance between the filter and the IP reduced the effectiveness, whereas increasing the thickness of the filter increased the effectiveness. The result of this study showed that the most appropriate filter conditions consistent with the calculated OAR in water were the ones with the IP sandwiched between two 2 mm thick lead filters at a distance of 5 mm from the IP or the IP sandwiched directly between two 1 mm lead filters. Using these filters, we measured the OAR at 10 cm depth with 100 cm source-to-surface distance and surface 10x10 cm(2) field size. The results of this measurement represented that it is possible to achieve measurements with less than within 2.0% and 2.0% in the field and with less than 1.1% and 0.6% out of the field by using 2 and 1 mm lead filters, respectively.

Quantifying the effect of respiratory motion on lung tumour dosimetry with the aid of a breathing phantom with deforming lungs

The contribution of organ and tumour motion to the degradation of planned dose distributions during radiotherapy to the breathing lung has been experimentally investigated and quantified. An anthropomorphic, tissue-equivalent breathing phantom with deformable lungs has been built, in which the lung tumour can be driven in any arbitrary 3D trajectory. The trajectory is programmed into a motion controller connected to a high-precision moving platform that is connected to the tumour. The motion controller is connected to the accelerator's dose counter and the speed of motion is scaled to the dose rate. This ensures consistent delivery despite variation in either the dose rate or inter-segment timing. For this study, the phantom was made to breathe by a set of periodic equations representing respiratory motion by an asymmetric, trigonometric function. Several motion amplitudes were selected to be applied in the primary axis of motion. Five three-dimensional, geometrically conformal (3DCRT) fractions with different starting phases (spaced uniformly in the breathing cycle) were delivered to the phantom and compared to a delivery where the phantom was static at the end-expiration position. A set of intensity-modulated radiotherapy plans (IMRT) was subsequently delivered in the same manner. Bigger amplitudes of motion resulted in a higher degree of dose blurring. Severe underdosages were observed when deliberately selecting the PTV wrongly, their extent being correlated with the degree of margin error. IMRT motion-averaged dose distributions exhibited areas of high dose in the gross tumour volume (GTV) which were not present in the static irradiations, arising from booster segments that the optimizer was creating to achieve planning target volume (PTV) homogeneity during the inverse-planning process. 3DCRT, on the other hand, did not demonstrate such effects. It has been concluded that care should be taken to control the delivered fluence when delivering IMRT to the breathing lung, even when the PTV margin has been adequately chosen to include the extent of the breathing motion.