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.

/pdf/performance-assessment-of-the-bebig-multisource-high-dose-3ozyaslfp1.pdf

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.

Many modern linear accelerators are equipped with asymmetric collimators or jaws that can be moved independently. Asymmetric jaws have got many clinical applications in radiation therapy. In the present study, the dosimetric characteristics of asymmetric collimators from our linear accelerator with 6 and 18 MV X-rays were carried out. The field size factors (FSF) and half value layer (HVL) were measured in a water phantom using 0.6 cc Farmer chamber for symmetric and asymmetric fields for both 6 and 18 MV X-rays. Measurements of beam penumbra, percentage depth dose (PDD), cross beam profiles and calculated isodose curves were measured by RFA 300 for both asymmetric and symmetric fields. The FSF were found to agree with in 3% for symmetric and asymmetric fields. The HVL in water was found to be 15.8 cm and 14.4 cm for 6 MV photons and 26 cm and 22.9 cm for 18 MV photons at the central axis and at 20 cm off the central axis. At 30 cm depth the percentage depth dose for symmetric and asymmetric fields were found to differ as high as 6% for 6 MV and 4% for 18 MV fields. No observable difference in penumbra was noticed for symmetric and asymmetric fields of same dimensions. The constrictions of isodose curves at the edge nearer to central axis were noticed for asymmetrically placed fields. The observed differences could be due to the passage of primary beam through differential thickness of the flattening filter which alters the beam quality.

Dosimetric investigation of dual energy photon beams with assymmetric collimator jaws

Background and Objective: Radiation induced toxicities in heart and lungs are diminishes the survival rate of cancer patients. The purpose of this study is to evaluate the dosimetric parameters of hybrid plans for chest wall irradiation of left breast carcinoma patients using Monaco treatment planning system and compare with the volumetric-modulated arc therapy (VMAT) treatment plans. Materials and Methods: Fifteen carcinoma left breast patients were randomly selected to evaluate the advantage of hybrid plan over VMAT. Hybrid plans were generated with 70% and 30% dose contribution from forward intensity-modulated radiotherapy and VMAT, respectively, whereas VMAT had been done with full prescription. Conformity and homogeneity indices were evaluated for target coverage between hybrid and VMAT plans. Results: Hybrid plan has proved its superiority over VMAT in terms of better organ at risk sparing and lesser low dose spillage and at the same time providing comparable target coverage. In low-dose spectrum, VMAT showed higher dose–volume than the hybrid plan. The maximum variation was found to be 44.75% at 7 Gy and the minimum dose difference was observed at 1 Gy (6.02%). Conclusion: This study suggests that the hybrid plan could be a better option for left-sided chest wall irradiation in regular clinical practice.

Dosimetric evaluation of hybrid and volumetric-modulated arc therapy plan for left-sided chest wall irradiation in MONACO treatment planning system

Peripheral dose (PD) or the dose outside the geometrical boundaries of the radiation field is of clinical importance when anatomical structures with low dose tolerances might be involved(1). It is the aim of this study is to estimate the PD on linear accelerators on different wedge systems without multileaf collimator (MLC). Measurements were performed on a dual energy linear accelerator equipped with tertiary MLC and enhanced dynamic wedge (EDW). Measurements were made using an ionization chamber embedded in a Radiation Field Analyser (RFA-300) with the secondary collimator and MLC setting of 5x5, 10x10, 15x15, and 20x20 cm2, and with the MLC fully retracted. The effects of SSD on PD were measured at three SSDs of 90, 100, and 110 cm for the irradiation fields of 5x5, 10x10, 15x15, and 20x20 cm2 and the effects of the three different wedges (Upper wedge, Lower Wedge and Enhanced Dynamic Wedge) on PD were measured for 45° wedges with field size of 15x15 cm2. Data were taken from 3 cm to 24 cm away from the field edge. Results show that due to tertiary MLC, PD can be reduced by means of a factor of two to three at certain distance from the edge of the field compared with TG-36 data. In between the wedges, the PD was less for the EDW when compared with the upper and lower physical wedges. We conclude that the reduction in PD is significant in reducing or eliminating the need for external peripheral shielding to reduce the dose on affected critical organs.

Peripheral dose from a dual energy linear accelerator equipped with tertiary multileaf collimators and enhanced dynamic wedge.

Purpose: To implement the newly developed portal dosimetry software using independent dose prediction algorithm EPIDoseTM and evaluate this new tool for the pretreatment IMRT plan QA of whole pelvis with simultaneous integrated boost (WP‐SIB‐IMRT) of prostate cases by comparing with routine 2D array detector system (MapCHECKTM). Methods: All measurements were performed using 6 MV X‐ray beam at CLINAC 600C linear accelerator (Varian Associates, Palo Alto CA, USA) equipped with Millennium120 multileaf collimator(MLC). We have compared the measured doses between the two detectors (EPIDose and MapCHECK) and TPS calculated ones for every split field. In the current investigation gamma value distribution for 104 split fields were analyzed using gamma distributions. In particular, for each split field the following gamma distribution parameters were determined: mean gamma value, gamma delta value, defined as the previous mean gamma value + 1.5 SD and Percentage of points with gamma 2. Results: We have investigated 104 split fields using gamma distributions in terms of pre‐defined gamma frequency parameters. The mean gamma values are found to be 0.42 (SD:0.06) and 0.44 (SD: 0.06) for the EPIDose and MapCHECK, respectively. The average gamma delta for EPIDose and MapCHECK are found as 0.51 (SD:0.06) and 0.53 (SD: 0.07), respectively. Furthermore, the percentage of points with gamma 2 are 97.4%, 99.3% and 0.56%, respectively for EPIDose and 96.4%, 99.0% and 0.62% for MapCHECK. Conclusions: Based on our results obtained with EPIDose and strong agreement with MapCHECK, we may conclude that the EPIDose portal dosimetry system has been successfully implemented and validated with our routine 2D array detector. Project was supported by, INTERNATIONAL CENTRE for THEORETICAL PHYSICS

SU-E-T-366: Patient Specific IMRT Pre-Treatment Verification Using a Commercial Portal Dosimetry Software

INTRODUCTION The objective was to implement the DynaLog file-based automated FractionCHECKTM software which verifies the delivery accuracy of all patient treatments without any measuring devices and evaluate this tool for dynamic IMRT delivery verification of 40 cases. The present work was aimed to find the generalized results with user-defined tolerances of all treatment deliveries and asses the delivery results in terms of Pass/Warn/fail analysis. In addition to this, an in-depth analysis of mean RMS error of each BANK of the MLC's and mean 95th Percentile error of all the leaves and percentage of pixels passing Gamma between the planned and delivered fluences for all 40 dynamic IMRT cases. METHODS AND MATERIALS The DynaLog files were analyzed from all 40 IMRT deliveries which include ten plans each of head & neck, brain, cervix and oesophagus patients treated using 6 MV X-ray beam at DHX linear accelerator equipped with Millennium120 multileaf collimator (MLC). Before processing actual patient data, we have validated the FractionCHECKTM software using routine DMLC QA test fields such as X-wedge, Y-Wedge, Garden fence test and Pyramid Test. The mean RMS error and 95th percentile of each leaf was studied in four different sites of dynamic IMRT and fluence delivery accuracy at 1%, 1mm gamma criterion was determined over the entire course of 40 patients. RESULTS We have analyzed a total of 1134 fractions of dynamic IMRT cases of 40 patients in this study. Out of 25,166 log files evaluated for the entire course of these patients, 23,906 files have met pass criteria (Green), 1022 log files resulted in warning (Yellow), and 238 files have resulted in failure mode (RED) in the analysis. The overall mean RMS error for all 40 IMRT cases was found to be 0.34±0.08 mm, and the mean 95th percentile error of all leaves was 0.66 ± 0.18 mm, which was well below the recommended by TG142 as well with other published works. DISCUSSION AND CONCLUSION In a busy department where a large number of IMRT cases are being treated, the DynaLog analysis-based FractionCHECKTM software can be used as one of the efficient methods in verifying IMRT treatment deliveries. It can be implemented much quicker without long delay and can also be used for routine DMLC QA tests. We conclude that FractionCHECKTM is a fast, simple and powerful tool for routine QA of the IMRT fields and that based on our test field and patient data results, FractionCHECKTM could be used for treatment delivery assessment accuracy.

C. Varatharaj

Papers

Dosimetric investigation of dual energy photon beams with assymmetric collimator jaws

Dosimetric evaluation of hybrid and volumetric-modulated arc therapy plan for left-sided chest wall irradiation in MONACO treatment planning system

Peripheral dose from a dual energy linear accelerator equipped with tertiary multileaf collimators and enhanced dynamic wedge.

SU-E-T-366: Patient Specific IMRT Pre-Treatment Verification Using a Commercial Portal Dosimetry Software

Implementation and Evaluation of Phantomless Intensity Modulated Radiotherapy Delivery Verification Using FractionCHECK.