X-ray film has been used for the dosimetry of intensity modulated radiation therapy (IMRT). However, the over-response of the film to low-energy photons is a significant problem in photon beam dosimetry, especially in regions outside penumbra. In IMRT, the radiation field consists of multiple small fields and their outside-penumbra regions; thus, the film dosimetry, for it involves the source of over-response in its radiation field. In this study we aim to verify and possibly improve film dosimetry for IMRT. Two types of modulated beams were constructed by combining five to seven different static radiation fields using 6 MV x rays. For verifying film dosimetry, x-ray films and an ion chamber were used to measure dose profiles at various depths in a phantom. The film setups include both parallel and perpendicular arrangements against the beam incident direction. In addition, to reduce an over-response, we placed 0.01 in. (0.25 mm) thick lead filters on both sides of the film. Compared with ion-chamber measurement, measured dose profiles showed the film over-response at outside-penumbra and low-dose regions. The error increased with depths and approached 15% as a maximum for the field size of 15 cm x 15 cm at 10 cm depth. The use of filters reduced the error down to 3%. In this study we demonstrated that film dosimetry for IMRT involves sources of error due to its over-response to low-energy photons, with the error most transparent in the low-dose region. The use of filters could enhance the accuracy in film dosimetry for IMRT. In this regard, the use of an optimal filter condition is recommended.

Film Dosimetry for Intensity Modulated Radiation Therapy: Dosimetric Evaluation.

In this project, we investigated the use of an electronic portal imaging device (EPID), together with the treatment planning system (TPS) and MLC log files, to determine the delivered doses to the patient and evaluate the agreement between the treatment plan and the delivered dose distribution. The QA analysis results are presented for 15 VMAT patients using the EPID measurements, the ScandiDos Delta4 dosimeter, and the beam fluence calculated from the multileaf collimator (MLC) log file. EPID fluence images were acquired in continuous acquisition mode for each of the patients and they were processed through an in-house MATLAB program to create an opening density matrix (ODM), which was used as the input fluence for the dose calculation in the TPS (Pinnacle3). The EPID used in this study was the aSi1000 Varian on a Novalis TX linac equipped with high-definition MLC. The actual MLC positions and gantry angles were retrieved from the MLC log files and the data were used to calculate the delivered dose distributions in Pinnacle. The resulting dose distributions were then compared against the corresponding planned dose distributions using the 3D gamma index with 3 mm/3% passing criteria. The ScandiDos Delta4 phantom was also used to measure a 2D dose distribution for all the 15 patients and a 2D gamma was calculated for each patient using the Delta4 software. The average 3D gamma using the EPID images was 96.1% ± 2.2%. The average 3D gamma using the log files was 98.7% ± 0.5%. The average 2D gamma from the Delta4 was 98.1% ± 2.1%. Our results indicate that the use of the EPID, combined with MLC log files and a TPS, is a viable method for QA of VMAT plans.

Anatomy-based, patient-specific VMAT QA using EPID or MLC log files

Purpose: To retrospectively compare the potential dosimetric advantages of a multichannel vaginal applicator vs. a single channel one in intracavitary vaginal high-dose-rate (HDR) brachytherapy after hysterectomy, and evaluate the dosimetric advantage of fractional re-planning. Material and methods: We randomly selected 12 patients with endometrial carcinoma, who received adjuvant vaginal cuff HDR brachytherapy using a multichannel applicator. For each brachytherapy fraction, two inverse treatment plans (for central channel and multichannel loadings) were performed and compared. The advantage of fractional re-planning was also investigated. Results: Dose-volume-histogram (DVH) analysis showed limited, but statistically significant difference (p = 0.007) regarding clinical-target-volume dose coverage between single and multichannel approaches. For the organs-at-risk rectum and bladder, the use of multichannel applicator demonstrated a noticeable dose reduction, when compared to single channel, but statistically significant for rectum only (p = 0.0001). For D 2cc of rectum, an average fractional dose of 6.1 ± 0.7 Gy resulted for single channel vs. 5.1 ± 0.6 Gy for multichannel. For D 2cc of bladder, an average fractional dose of 5 ± 0.9 Gy occurred for single channel vs. 4.9 ± 0.8 Gy for multichannel. The dosimetric benefit of fractional re-planning was demonstrated: DVH analysis showed large, but not statistically significant differences between first fraction plan and fractional re-planning, due to large inter-fraction variations for rectum and bladder positioning and filling. Conclusions: Vaginal HDR brachytherapy using a multichannel vaginal applicator and inverse planning provides dosimetric advantages over single channel cylinder, by reducing the dose to organs at risk without compromising the target volume coverage, but at the expense of an increased vaginal mucosa dose. Due to large inter-fraction dose variations, we recommend individual fraction treatment plan optimization. J Contemp Brachytherapy 2014; 6, 4: 362–370 DOI: 10.5114/jcb.2014.47816

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Single versus multichannel applicator in high-dose-rate vaginal brachytherapy optimized by inverse treatment planning.

A comprehensive system characterisation was performed of the Eckert & Ziegler BEBIG GmbH MultiSource® High Dose Rate (HDR) brachytherapy treatment unit with an 192Ir source. The unit is relatively new to the UK market, with the first installation in the country having been made in the summer of 2009. A detailed commissioning programme was devised and is reported including checks of the fundamental parameters of source positioning, dwell timing, transit doses and absolute dosimetry of the source. Well chamber measurements, autoradiography and video camera analysis techniques were all employed. The absolute dosimetry was verified by the National Physical Laboratory, UK, and compared to a measurement based on a calibration from PTB, Germany, and the supplied source certificate, as well as an independent assessment by a visiting UK centre. The use of the 'Krieger' dosimetry phantom has also been evaluated. Users of the BEBIG HDR system should take care to avoid any significant bend in the transfer tube, as this will lead to positioning errors of the source, of up to 1.0 mm for slight bends, 2.0 mm for moderate bends and 5.0 mm for extreme curvature (depending on applicators and transfer tube used) for the situations reported in this study. The reason for these errors and the potential clinical impact are discussed. Users should also note the methodology employed by the system for correction of transit doses, and that no correction is made for the initial and final transit doses. The results of this investigation found that the uncorrected transit doses lead to small errors in the delivered dose at the first dwell position, of up to 2.5 cGy at 2 cm (5.6 cGy at 1 cm) from a 10 Ci source, but the transit dose correction for other dwells was accurate within 0.2 cGy. The unit has been mechanically reliable, and source positioning accuracy and dwell timing have been reproducible, with overall performance similar to other existing HDR equipment. The unit is capable of high quality brachytherapy treatment delivery, taking the above factors into account.

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Performance assessment of the BEBIG MultiSource? high dose rate brachytherapy treatment unit

Recent studies have demonstrated that per-beam planar intensity-modulated radiation therapy (IMRT) quality assurance (QA) passing rates may not predict clinically relevant patient dose errors. This work is to evaluate the effect of dose variations introduced in dynamic multi-leaf collimator (DMLC) modeling and delivery processes on clinically relevant metrics for IMRT. Ten head and neck (HN) IMRT plans were randomly selected for this study. The conventional per-beam IMRT QA was performed for each plan by 2 different methods: (1) with gantry angle of 0 (gantry pointing downward) for all IMRT fields and (2) with gantry at specific angles as designed in the IMRT plan. For each patient, a batch analysis was done for each scenario and then imported to the 3DVH (Sun Nuclear Corp.) for processing. A "corrected DVH" was generated and compared to the DVH from the treatment plan. Their differences represented errors introduced from the combination of the treatment planning system (TPS) dose calculation algorithm and beam-delivery. The dose metrics from the two scenarios were compared with the corresponding calculated doses, and then their differences were analyzed. Although all per-beam planar IMRT QA had high Gamma passing rates 99.3 ± 1.3% (92.3-100%) for "2%/3 mm" criteria, there were significant errors in some of the calculated clinical dose metrics. Such as, for all the plans studied, there were as much as 3.2%, 5.7%, 5.6%, 2.3%, 4.1%, and 23.8% errors found in max cord dose, max brainstem dose, mean parotid dose, larynx dose, oral cavity dose, and PTV(D95) dose, respectively. The differences in errors for clinical metrics obtained between the two scenarios (zero gantry angle vs. true gantry angles) can also be significant: max cord dose (2.9% vs. 0.2%), max brainstem dose (3.8% vs. 0.4%), mean parotid dose (2.3% vs. 4.5%), mean larynx dose (3.9% vs. 2.0%), mean oral cavity dose (1.6% vs. 3.9%), and PTV(D95) dose (-0.4% vs. -2.6%). However, in the two scenarios, a strong and clear correlation between the dose differences for each of the organ structures was observed. This study confirms that conventional IMRT QA performance metrics are not predictive of dose errors in PTV and organs-at-risk. The clinically-relevant-dose QA has allowed us to predict the patient dose-volume relationships.

https://journals.sagepub.com/doi/pdf/10.7785/tcrt.2012.500353

Using a novel dose QA tool to quantify the impact of systematic errors otherwise undetected by conventional QA methods: clinical head and neck case studies.

Implementation of Intensity Modulation Radiotherapy (IMRT) and patient dose verification was carried out with film and I'mariXX using linear accelerator with 120-leaf Millennium dynamic multileaf collimator (dMLC) The basic mechanical and electrical commissioning and quality assurance tests of linear accelerator were carried out The leaf position accuracy and leaf position repeatability checks were performed for static MLC positions Picket fence test and garden fence test were performed to check the stability of the dMLC and the reproducibility of the gap between leaves The radiation checks were performed to verify the position accuracy of MLCs in the collimator system The dMLC dosimetric checks like output stability, average leaf transmission and dosimetric leaf separation were also investigated The variation of output with gravitation at different gantry angles was found to be within 09 % The measured average leaf transmission for 6 MV was 16 % and 18% for 18 MV beam The dosimetric leaf separation was found to be 22 mm and 23 mm for 6 MV and 18 MV beams In order to check the consistency of the stability and the precision of the dMLC, it is necessary to carryout regular weekly and monthly checks The dynalog files analysis for Garden fence, leaf gap width and step wedge test patterns carried out weekly were in good agreement Pretreatment verification was performed for 50 patients with ion chamber and I'mariXX device The variations of calculated absolute dose for all treatment fields with the ion chamber measurement were within the acceptable criterion Treatment Planning System (TPS) calculated dose distribution pattern was comparable with the I'mariXX measured dose distribution pattern Out of 50 patients for which the comparison was made, 36 patients were agreed with the gamma pixel match of >95% and 14 patients were with the gamma pixel match of 90-95% with the criteria of 3% delta dose (DD) and 3 mm distance-to-agreement (DTA) Commissioning and quality assurance of dMLC for IMRT application requires considerable time and effort Many dosimetric characteristics need to be assessed carefully failing which the delivered dose will be significantly different from the planned dose In addition to the issues discussed above we feel that individual MU check is necessary before the treatment is delivered

IMRT implementation and patient specific dose verification with film and ion chamber array detectors

Introduction: Radiotherapy is one of the major modality for cancer management playing curative, adjuvant, and palliative and sometimes has an alternative role to chemotherapy. Radiotherapy is practiced in two ways viz. External beam therapy and Brachytherapy. Electron beam therapy is widely used in the management of cancers. An electron beam is characterized by a finite range of penetration with a rapid dose fall off towards a slowly decaying x-ray background as the electrons traverse through tissues. The electron monte carlo (eMC) dose calculation algorithm for eclipse treatment system has been introduced by Varian Medical systems. The algorithm is commissioned and validated by comparing percentage depth dose (PDD) and gamma index.
Methods: Percentage depth dose curves were generated for all the energies for 4x4 cm2 and 10x10 cm2 field sizes. The depth of maximum dose (R100), therapeutic depth (R85), depth of 50% isodose (R50) and the relative surface (Ds) were compared with the measured and calculated PDD curves.
Results: The eMC calculated fluence and measured fluence were compared for all the energies and cones at nominal source to surface distance and extended distances. For 4x4 cm2 field size the maximum shift in R100 was 5 mm, R85 was 1.9 mm, R50 was 0.9 mm and the variation in the relative surface (DS) was about 25Gamma analysis shows excellent agreements with greater than 98% of the pixels passing the gamma requirements.
Conclusion: We have successfully commissioned and validated the electron monte carlo dose calculation at extended source to surface distance.

/pdf/commissioning-and-validation-of-the-electron-monte-carlo-3fzpbd7pj9.pdf

Commissioning and validation of the electron Monte Carlo dose calculation at extended source to surface distance from a medical linear accelerator

Accurate measurement of transit time of the HDR brachytherapy source of a remote after-loading unit is necessary to calculate the total radiation dose given to the treatment volume Presently, most of the HDR brachytherapy treatment planning systems neglect the transit time in the computation of dose The aim of this investigation is to use a well type ionization chamber to measure the transit time during the source movement between two dwell positions As well type ionization chamber and a precision electrometer (manufacturer CD instruments, Bangalore) were used to measure the charge generated during the movement of the Ir-192 source of a Gammamed HDR brachytherapy unit with an interstitial needle Effective transit time and effective speed were determined on the basis of methodology described by Sahoo [2] Corrections were done on the basis of relative sensitivity values for varaious dwell position in the ionization chamber In the present study the variation of effective speed with interdwell distance was minimal as compared with that of Sahoo [2] The effective transit times were 0129, 0182, 0301, 0402, 0701, and 0993 seconds for 1, 2, 4, 6, 8 and 10 cm interdwell separations respectively The effective transit times in the present study were higher than those of Sahoo [2] Software modification accounting for the dynamic dose should be incorporated into all HDR planning systems Such an improvement would enhance the safety and accuracy of HDR brachytherapy

/pdf/measurement-of-transit-time-of-a-gammamed-plus-remote-2u4w3w6afk.pdf

Measurement of Transit Time of a Gammamed-Plus Remote Afterloading High Dose Rate Brachytherapy Source

https://journals.viamedica.pl/rpor/article/download/73126/53216

Dosimetric evaluation of Gammamed High Dose Rate intraluminal brachytherapy applicators

A uniform dose to the target site is required with a knowledge of delivered dose, central axis depth dose and beam flatness for successful electron treatment at an extended source to surface distance (SSD). In an extended SSD treatment under dosage of the lateral tissue may occur due to reduced beam flatness. To study the changes in beam characteristics, the depth dose curves, beam flatness and isodose distributions were measured at different SSDs from 100 to 120 cm for clinically used field sizes from (4×4) to (25×25) cm 2 and beam energies ranging from 6 MeV to 20 MeV. Our results suggest that the change in depth dose is minimal except in the buildup region for most energy. In general surface dose is decreased as the SSD increased moderately. It was observed that the loss in beam flatness is significant for smaller fields, higher isodose lines, and lower energies. The penumbra enlarged and the uniformity index reduced with increasing SSD.

/pdf/electron-beam-characteristics-at-extended-source-to-surface-2yk16v3r8p.pdf

C. Varatharaj

Papers

IMRT implementation and patient specific dose verification with film and ion chamber array detectors

Commissioning and validation of the electron Monte Carlo dose calculation at extended source to surface distance from a medical linear accelerator

Measurement of Transit Time of a Gammamed-Plus Remote Afterloading High Dose Rate Brachytherapy Source

Dosimetric evaluation of Gammamed High Dose Rate intraluminal brachytherapy applicators

Electron beam characteristics at extended source to surface distance for a Clinac-DHX linear accelerator