Dosimetric accuracy of three dose calculation algorithms for radiation therapy of in situ non-small cell lung carcinoma.

Background: Study determines differences in calculated dose distributions for non-small cell lung carcinoma (NSC LC) patients. NSC LC cases were investigated, being the most common lung cancer treated by radiotherapy in our clinical practice. Materials and methods: A retrospective study of 15 NSCLC patient dose distributions originally calculated using standard superposition (SS) and recalculated using collapsed cone (CC ) and Monte Carlo (MC) based algorithm expressed as dose to medium in medium (MCDm) and dose to water in medium (MCDw,) was performed so that prescribed dose covers at least 99% of the gross target volume (GTV). Statistical analysis was performed for differences of conformity index (CI), heterogeneity index (HI), gradient index (GI), dose delivered to 2% of the volume (D2%), mean dose (Dmean) and percentage of volumes covered by prescribed dose (V70Gy). For organs at risk (OARs), Dmean and percentage of volume receiving 20 Gy and 5Gy (V20Gy, V5Gy) were analysed. Results: Statistically significant difference for GTVs was observed between MCDw and SS algorithm in mean dose only. For planning target volumes (PTVs), statistically significant differences were observed in prescribed dose coverage for CC, MCDm and MCDw. The differences in mean CI value for the CC algorithm and mean HI value for MCDm and MCDw were statistically significant. There is a statistically significant difference in the number of MUs for MCDm and MCDw compared to SS. Conclusion: All investigated algorithms succeed in managing the restrictive conditions of the clinical goals. This study shows the drawbacks of the CC algorithm compared to other algorithms used.

National reference levels of CT procedures dedicated for treatment planning in radiation oncology.

OBJECTIVE: To present results of the first national survey on reference levels of CT imaging performed for the treatment planning purposes in radiation oncology in Croatia. METHODS: Data for CT protocols of five anatomical regions including head, head and neck, pelvis, breast, and thorax were collected at eight radiation oncology departments in Croatia. Data included volume CT dose index (CTDIvol), dose-length product (DLP), scan length and set of acquisition and reconstruction parameters. Data on a total of 600 patients were collected. Median values of scan length, DLP and CTDIvol were calculated for each acquisition protocol. Third quartiles of the median CTDIvol and DLP values were proposed as the national radiotherapy planning reference levels (RPRL). RESULTS: The largest CoV were assessed for RT Breast (63.8% for CTDIvol), RT Thorax (79.7% for DLP) and RT H&N (21.2% for scan length). RT Head had the lowest CoV for CTDIvol (1,9%) and DLP (17,2%), while RT Breast had the lowest coefficient of variation for scan length (12.8%). Proposed national RPRLs are: for RT Head CTDIvol16cm = 62 mGy and DLP16cm = 1738 mGy.cm; for RT H&N CTDIvol16cm = 35 mGy and DLP16cm = 1444 mGy.cm; for RT Breast CTDIvol32cm = 16 mGy and DLP32cm = 731 mGy.cm; for RT Thorax CTDIvol32cm = 17 mGy and DLP32cm = 865 mGy.cm; for RT Pelvis CTDIvol32cm = 20 mGy and DLP32cm = 1133 mGy.cm. CONCLUSIONS: Results of this study show variations in CT imaging for treatment planning practice at the national level which call for optimization of procedures.

Determination of physiotherapy ultrasound beam quality parameters from images derived using thermochromic material.

The evaluation of the performance of nine physiotherapy ultrasound transducers used clinically was performed in the hospital environment using an acoustically absorbing thermocromic tile developed at the National Physical Laboratory (UK). The method consists of exposing an acoustic absorber tile, part of which contains a thermochromic pigment, to the ultrasonic beam, thereby forming an image of the intensity profile of the transducer. Images acquired using thermochromic materials were postprocessed in order to estimate effective radiating area (ERA) and beam nonuniformity ratio (BNR) for ultrasound transducers operating within the frequency range from 1.0 to 3.3 MHz, and nominal applied intensities in the range of 1-2W/cm2. Results of our measurements have shown that thermocromic tile can be used for quality control of ultrasound transducers in the hospital environment. Experimental results show that proposed method can be used to distinguish highly non - uniform ultrasound beams with high value of BNR. Influence of exposure duration on obtained ERA and BNR values was also analysed. Our results show that values for ERA increase with insonation time, while BNR values decrease. In order to compare our results with theory we have estimated temperature rise in thermochromic material experimentally and compare it with theoretical prediction.

Evaluation of two-dimensional dose distributions for pre-treatment patient-specific IMRT dosimetry.

Background The accuracy of dose calculation is crucial for success of the radiotherapy treatment. One of the methods that represent the current standard for patient-specific dosimetry is the evaluation of dose distributions measured with an ionization chamber array inside a homogeneous phantom using gamma method. Nevertheless, this method does not replicate the realistic conditions present when a patient is undergoing therapy. Therefore, to more accurately evaluate the treatment planning system (TPS) capabilities, gamma passing rates were examined for beams of different complexity passing through inhomogeneous phantoms. Materials and methods The research was performed using Siemens Oncor Expression linear accelerator, Siemens Somatom Open CT simulator and Elekta Monaco TPS. A 2D detector array was used to evaluate dose distribution accuracy in homogeneous, semi-anthropomorphic and anthropomorphic phantoms. Validation was based on gamma analysis with 3%/3mm and 2%/2mm criteria, respectively. Results Passing rates of the complex dose distributions degrade depending on the thickness of non-water equivalent material. They also depend on dose reporting mode used. It is observed that the passing rate decreases with plan complexity. Comparison of the data for all set-ups of semi-anthropomorphic and anthropomorphic phantoms shows that passing rates are higher in the anthropomorphic phantom. Conclusions Presented results raise a question of possible limits of dose distribution verification in assessment of plan delivery quality. Consequently, good results obtained using standard patient specific dosimetry methodology do not guarantee the accuracy of delivered dose distribution in real clinical cases.

Comparison of calculated dose distributions reported as dose-to-water and dose-to-medium for intensity-modulated radiotherapy of nasopharyngeal cancer patients.

Advanced dose calculation algorithms for radiation therapy treatment planning can report external beam photon dose 2-sided, in terms of dose-to-medium (Dm) and dose-to-water (Dw). The purpose of our study was to determinate the effect of Dw and Dm reporting modes built in Elekta Monaco treatment planning system on intensity-modulated radiotherapy dose distributions for patients with nasopharyngeal cancer. For 13 patients involved in this retrospective study, 2 plans were created: 1 using Dw and another according to Dm reporting mode. Treatment plans were normalized such that 100% planning target volume should be covered by 95% of prescribed dose. Dose-volume constraints were assigned according to international standards. The comparison between dose distributions was performed evaluating quantities important for respective volumes of interest. For target volumes, heterogeneity index and conformity index methodology were used along with the maximum dose concept. Also, for the comparisons over particular organ at risk, maximum dose or mean dose as well as dose-volume concepts were used. For all target volumes and majority of organs at risk, the differences between 2 reporting modes are statistically insignificant, but this is not the case for bony structured organs at risks: mandible and cochlea. It was observed that Dw is higher than Dm with mean difference of 9.91% (p = 0.000009) of the mandible volume covered with 70 Gy. The same trend was observed for left and right cochlea with difference in mean dose of 8.74% (p = 0.037) and 6.87% (p = 0.029), respectively. The comparative analysis of dosimetric parameters in this study shows that the selection of reporting modes in Monaco treatment planning system can produce dose differences up to 15% in high-density volumes such as mandible and cochlea, which might have clinical consequences.

Quantitative testing of physiotherapy ultrasound beam patterns within a clinical environment using a thermochromic tile.

The implementation of the non-standardized method developed at the National Physical Laboratory (UK) supporting the quality assurance of therapeutic ultrasonic beam parameters within a clinical environment is presented. The method consists of exposing an acoustic absorber tile, part of which contains a thermochromic pigment, to the ultrasonic beam, thereby forming an image of the two-dimensional intensity profile of the transducer. Nine different physiotherapy ultrasound treatment heads currently used clinically were tested using this method. Thermochromic images were postprocessed in order to estimate the Effective Radiating Area (ERA) for treatment heads operating within the frequency range from 1 MHz to 3.2 MHz, and nominal applied intensities in the range of 1-3 W/cm(2). Experimental results and comparisons with manufacturer specified values of ERA are presented. Differences in the experimentally derived results and the manufacturer values are typically well within 25%. The root-mean squared difference calculated over the nine treatment heads is 15.1%, with the thermochromic material tended to underestimate the ERA.

Survey of equipment quality control in radiotherapy centres in croatia: first results.

Implementation of advanced radiation therapy techniques in clinical practice can greatly improve tumour control and normal tissue sparing. An important part of this implementation is quality control (QC) of every part of the radiotherapy process, as it helps to detect errors and provides instant remedy. This increases the probability of successful radiation treatment and ensures patient radiation safety. Every radiotherapy quality assurance (QA) programme is based on quality control of radiotherapy equipment. The aim of our survey was to review QC practices in a number of radiotherapy centres in Croatia. As a first step, we defined a set of tests to check different parameters of linear accelerators and simulators in these centres. The tests were defined and performed according to protocols developed at two university hospitals. Test results varied largely between the centres. This calls for harmonisation of QC protocols.

Dosimetric verification of compensated beams using radiographic film.

INTRODUCTION: External photon beam modulation using compensators in order to achieve a desired dose distribution when brachytherapy treatment is followed by external beam radiation is a well-established technique. A compensator modulates the central part of the beam, and the dose beneath the thickest part of the compensator is delivered mostly by scattered, low energy photons. A two-dimensional detector with a good spatial resolution is needed for the verification of those beams. In this work, the influence of different types of detectors on the measured modulated dose distributions was examined. MATERIALS AND METHODS: Dosimetric verification was performed using X-Omat V, Eastman Kodak radiographic films at different depths in a solid water phantom. The film measurements were compared with those made by ionization chambers. Photon beams were also modelled using EGSnrc Monte Carlo algorithm to explain the measured results. RESULTS: Monte Carlo calculated over-response of the film under the thickest part of the compensator was over 15%, which was confirmed by measurements. The magnitude of over-response could be associated with changes in the spectra of photon energy in the beam. CONCLUSIONS: The radiographic film can be used for the dosimetry of compensated high energy photon beams, with limitations in volumes where photon spectra are hardly degraded.

Radiation therapy photon beams dose conformation according to dose distribution around intracavitary-applied brachytherapy sources.

Intracavitary application of brachytherapy sources followed by external beam radiation is essential for the local treatment of carcinoma of the cervix. Due to very high doses to the central portion of the target volume delivered by brachytherapy sources, this part of the target volume must be shielded while being irradiated by photon beams. Several shielding techniques are available, from rectangular block and standard cervix wedge to more precise, customized step wedge filters. Because the calculation of a step wedge filter's shape was usually based on effective attenuation coefficient, an approach that accounts, in a more precise way, for the scattered radiation, is suggested. The method was verified under simulated clinical conditions using film dosimetry. Measured data for various compensators were compared to the numerically determined sum of the dose distribution around brachytherapy sources and one of compensated beam. Improvements in total dose distribution are demonstrated, using our method. Agreement between calculation and measurements were within 3%. Sensitivity of the method on sources displacement during treatment has also been investigated.