Claire Dempsey

A phantom assessment of achievable contouring concordance across multiple treatment planning systems

In this paper, the highest level of inter- and intra-observer conformity achievable with different treatment planning systems (TPSs), contouring tools, shapes, and sites have been established for metrics including the Dice similarity coefficient (DICE) and Hausdorff Distance. High conformity values, e.g. DICE(Breast_Shape)=0.99±0.01, were achieved. Decreasing image resolution decreased contouring conformity.

Optimal single 3T MR imaging sequence for HDR brachytherapy of cervical cancer

Purpose The superior image quality of 3 tesla (3T) magnetic resonance (MR) imaging in cervical cancer offers the potential to use a single image set for brachytherapy. This study aimed to determine a suitable single sequence for contouring tumour and organs at risk, applicator reconstruction, and treatment planning. Material and methods A 3T (Skyra, Siemens Healthcare AG, Germany) MR imaging system with an 18 channel body matrix coil generated HDR cervical cancer brachytherapy planning images on 20 cases using plastic-based treatment applicators. Seven different T2-weighted Turbo Spin Echo (TSE) sequences including both 3D and contiguous 2D scans based on sagittal, axial (transverse), and oblique planes were analysed. Each image set was assessed for total scanning time and usefulness in tumour localization via inter- and intra-observer analysis of high-risk clinical target volume (HR CTV) contouring. Applicator reconstruction in the treatment planning system was also considered. Results The intra-observer difference in HR CTV volumes between 2D and 3D axial-based image sets was low with an average difference of 3.1% for each observer. 2D and 3D sagittal image sets had the highest intra- and inter observer differences (over 15%). A 2D axial ‘double oblique’ sequence was found to produce the best intra- (average difference of 0.6%) and inter-observer (mean SD of 9.2%) consistency and greatest conformity (average 0.80). Conclusions There was little difference between 2D and 3D-based scanning sequences; however the increased scanning time of 3D sequences have potential to introduce greater patient motion artifacts. A contiguous 2D sequence based on an axial T2-weighted turbo-spin-echo (TSE) sequence orientated in all planes of the treatment applicator provided consistent tumour delineation whilst allowing applicator reconstruction and treatment planning.

Dosimetric comparison of optimization methods for multichannel intracavitary brachytherapy for superficial vaginal tumors

PURPOSE Multichannel vaginal applicators allow treatment of a more conformal volume compared with a single, central vaginal channel. There are several optimization methods available for use with multichannel applicators, but no previous comparison of these has been performed in the treatment of superficial vaginal tumors. Accordingly, a feasibility study was completed to compare inverse planning by simulated annealing (IPSA), dose point optimization (DPO), and graphical optimization for high-dose-rate brachytherapy using a multichannel, intracavitary vaginal cylinder. METHODS AND MATERIALS This comparative study used CT data sets from five patients with superficial vaginal recurrences of endometrial cancer treated with multichannel intracavitary high-dose-rate brachytherapy. Treatment plans were generated using DPO, graphical optimization, surface optimization with IPSA (surf IPSA), and two plans using volume optimization with IPSA. The plans were evaluated for target coverage, conformal index, dose homogeneity index, and dose to organs at risk. RESULTS Best target coverage was achieved by volume optimization with IPSA 2 and surf IPSA with mean V100 values of 93.89% and 91.87%, respectively. Doses for the most exposed 2-cm(3) of the bladder (bladder D2cc) was within tolerance for all optimization methods. Rectal D2cc was above tolerance for one DPO plan. All volume optimization with IPSA plans resulted in higher vaginal mucosa doses for all patients. Greatest homogeneity within the target volume was seen with surf IPSA and DPO. Highest conformal indices were seen with surf IPSA and graphical optimization. CONCLUSIONS Optimization with surf IPSA was user friendly for the generation of treatment plans and achieved good target coverage, conformity, and homogeneity with acceptable doses to organs at risk.

Lessons learned from a HDR brachytherapy well ionisation chamber calibration error

The outcomes of a recent brachytherapy well-type ionization chamber calibration error are given in the hope that other brachytherapy treatment centres may better understand the importance of each entry stated in a well chamber calibration certificate. A Nucletron Source Dosimetry System (SDS) PTW well-type ionization chamber was sent for a biennial calibration in September 2010. Upon calibration of the chamber, it was discovered that the previous calibration (in July 2008) contained a +2.6% error in the chamber calibration coefficient. Investigation of the information on the 2008 well chamber calibration certificate indicated the source of the error, which could or should have been detected by both the calibration laboratory and/or the radiation therapy department upon return of the chamber. Consideration must be given to all values and conditions given on the calibration certificate when accepting a ionization chamber back from a calibration laboratory. The issue of whether the source strength from the source calibration certificate or the measured source strength from the calibrated ionization chamber should be entered into the treatment unit is also raised.

Methodology for commissioning a brachytherapy treatment planning system in the era of 3D planning

To describe the steps undertaken to commission a 3D high dose rate (HDR) brachytherapy treatment planning system (TPS). Emphasis was placed on validating previously published recommendations, in addition to checking 3D parameters such as treatment optimization and dose volume histogram (DVH) analysis. Commissioning was performed of the brachytherapy module of the Nucletron Oncentra MasterPlan treatment planning system (version 3.2). Commissioning test results were compared to an independent external beam TPS (Varian Eclipse v 8.6) and the previously commissioned Nucletron Plato (v 14.3.7) brachytherapy treatment planning system, with point doses also independently verified using the brachytherapy module in RadCalc (v 6.0) independent point dose calculation software. Tests were divided into eight categories: (i) Image import accuracy, (ii) Reconstruction accuracy, (iii) Source configuration data check, (iv) Dose calculation accuracy, (v) Treatment optimization validation, (vi) DVH reproducibility, (vii) Treatment export check and (viii) Printout consistency. Point dose agreement between Oncentra, Plato and RadCalc was better than 5% with source data and dose calculation protocols following the American Association of Physicists in Medicine (AAPM) guidelines. Testing of image accuracy (import and reconstruction), along with validation of automated treatment optimization and DVH analysis generated a more comprehensive set of testing procedures than previously listed in published recommendations.

Medical physics workforce modelling: do we need what we want?

“Please Sir, I want some more.” While Oliver Twist [1] was talking about a basic necessity of life with this request, many medical physicists are left crying these words when they start thinking about medical physics staffing numbers in departments across Australasia. As part of ACPSEM departmental accreditation for TEAP purposes, the F2000 model [2] has been the benchmark for suitable staffing levels. But is this model appropriate for departments in 2018 (and into the future) and how have Australasian departments changed over the past decade, particularly in light of TEAP graduates entering the profession, with TEAP now 15 years old? How do we, as a profession, decide what staffing numbers a department truly “needs” against the numbers we “want” to have? In recent years, several groups have tried to measure and define workload versus equipment versus staffing models for radiation therapy services [3–6], yet there seems to be no universal consensus on what this ratio should be. This is difficult given the different roles and definitions of a medical physicist in an individual department as well as whether ‘technicians’ or ‘physics assistants’ are employed. Unfortunately, health economics makes things even more complicated and the increasing prevalence of private practise radiation therapy departments globally can skew survey numbers against us. A thorough analysis of Australasian medical physicists’ versus equipment versus technique versus patient numbers versus operating hours versus radiation incidents (both major and minor) is well beyond the scope of this editorial but would certainly make for interesting reading and perhaps great ammunition for those seeking to improve their staffing budget. In my role as ACPSEM ROMP training coordinator, I have been heavily involved in departmental accreditation since 2014 and have seen the good, the bad and the scary. Initial analysis of data from ACPSEM departmental accreditation records spanning 2014–2018 shows that all departments are below F2000 levels, some I would call dangerously so, and that there are clear differences between public, private, metropolitan and remote facilities. Previous medical physics workforce surveys have been conducted in 2006, 2009 and 2012 by Howell Round [7–9]. These surveys were very comprehensive, covering not only radiation oncology but also diagnostic imaging and nuclear medicine. Ideally, it would be fantastic to be able to continue gathering this important workforce data and I would gladly help anyone who wishes to volunteer for this massive task. In 2006, there was a shortfall of qualified physicists of 32% in Australia and 15% in New Zealand based on the simplified F2000 ratio of 1.7 qualified physicists per linac [7]. In 2009 this shortfall grew to 35% in Australia and 23% in New Zealand [8]. In 2012, the shortfall in Australia grew further to 37% but fell in New Zealand to 13% [9]. In Australia, between 2006 and 2012 there appeared to be more additional linacs installed (67) than additional qualified medical physicists employed (64). Using a sample dataset of ACPSEM accredited departments, it appears that these shortfalls have been improving. Based purely on linac * Claire Dempsey claire.dempsey@calvarymater.org.au

Reply to letter to the editor: Medical physics workforce modelling: do we need what we want?

I thank Professors Van Dyk and Battista for their commentary on this editorial [1, 2]. I completely agree that activitybased modelling is paramount to determine staffing needs of the current and future workforce and that equipmentbased models do not consider the wider roles of a medical physicist nor that different staffing levels may be appropriate for different departments. Analysis of F2000 values with department specific data indicates that departments with essentially the same equipment profiles have widely varying F2000 staff calculations. For example: departments with 6 linacs and an HDR service in Australasia had F2000 values ranging from 9.5 to 21.4 qualified physicists. In reality, the departments at the extremes of this range had 6 and 8.4 qualified medical physicists respectively. The editorial was written to highlight how staffing levels (both calculated via F2000 [3] and actual) have changed overall in Australasia over the last decade. This was done by comparing medical physicist numbers with the simplified F2000 model that had been previously used for workforce analysis, and is still the standard for state and federal governments of Australia. It was an “apples to apples” approach that was intended to start the discussion across Australasian departments, as it is clear that the simplified model is not suitable, and even the full F2000 model is in need of review. I also thank Professors Van Dyk and Battista for providing an excellent set of additional references that should be included as part of an Australasian-based workforce modelling evaluation. With thorough examination of models from other countries it is hoped that we can generate a realistic and locally-focussed interpretation that can be provided to administrators and governments in order to update their understanding of medical physics staffing standards. This recognition will be a much needed leap forward for the profession in Australasia, both now and into the future.

Electron beam energy QA — a note on measurement tolerances

Monthly QA is recommended to verify the constancy of high‐energy electron beams generated for clinical use by linear accelerators. The tolerances are defined as 2%/2 mm in beam penetration according to AAPM task group report 142. The practical implementation is typically achieved by measuring the ratio of readings at two different depths, preferably near the depth of maximum dose and at the depth corresponding to half the dose maximum. Based on beam commissioning data, we show that the relationship between the ranges of energy ratios for different electron energies is highly nonlinear. We provide a formalism that translates measurement deviations in the reference ratios into change in beam penetration for electron energies for six Elekta (6‐18 MeV) and eight Varian (6‐22 MeV) electron beams. Experimental checks were conducted for each Elekta energy to compare calculated values with measurements, and it was shown that they are in agreement. For example, for a 6 MeV beam a deviation in the measured ionization ratio of ±15% might still be acceptable (i.e., be within ±2 mm), whereas for an 18 MeV beam the corresponding tolerance might be ±6%. These values strongly depend on the initial ratio chosen. In summary, the relationship between differences of the ionization ratio and the corresponding beam energy are derived. The findings can be translated into acceptable tolerance values for monthly QA of electron beam energies. PACS number(s): 87.55, 87.56Monthly QA is recommended to verify the constancy of high-energy electron beams generated for clinical use by linear accelerators. The tolerances are defined as 2%/2 mm in beam penetration according to AAPM task group report 142. The practical implementation is typically achieved by measuring the ratio of readings at two different depths, preferably near the depth of maximum dose and at the depth corresponding to half the dose maximum. Based on beam commissioning data, we show that the relationship between the ranges of energy ratios for different electron energies is highly nonlinear. We provide a formalism that translates measurement deviations in the reference ratios into change in beam penetration for electron energies for six Elekta (6-18 MeV) and eight Varian (6-22 MeV) electron beams. Experimental checks were conducted for each Elekta energy to compare calculated values with measurements, and it was shown that they are in agreement. For example, for a 6 MeV beam a deviation in the measured ionization ratio of ±15% might still be acceptable (i.e., be within ±2 mm), whereas for an 18 MeV beam the corresponding tolerance might be ±6%. These values strongly depend on the initial ratio chosen. In summary, the relationship between differences of the ionization ratio and the corresponding beam energy are derived. The findings can be translated into acceptable tolerance values for monthly QA of electron beam energies. PACS number(s): 87.55, 87.56.

EP-1262: Four field radiotherapy, IMRT or VMAT in cervix cancer: when do the benefits of advanced planning become redundant?

Purpose/Objective: To analyse 4 field conventional radiotherapy (4FRT), intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) in the definitive management of cervix cancer and to assess whether any dosimetric differences persist with the use of larger geometric PTV margins to account for daily variability in uterine position. Materials and Methods: CT datasets were obtained for 20 consecutive patients with intact cervix cancer previously treated with definitive external beam radiotherapy (with or without chemotherapy). The clinical target volume was contoured to encompass gross tumour and potential microscopic disease including the remaining cervix, uterus, upper vagina, parametrium, adnexa and regional lymph nodes. The planning target volume (PTV) was generated with uterine margins of 1.5cm, 2cm, 2.5cm, 3cm and 4cm used to account for internal motion of the uterus, generating five PTVs for each patient: PTV1.5, PTV2, PTV2.5, PTV3 and PTV4. The rectum, bladder, bowel and femoral heads were contoured as organs at risk (OARs). For each patient, a 4FRT plan was generated based on conventional field borders ensuring coverage of PTV1.5. IMRT and VMAT plans were generated for each PTV, except when the PTV extended outside 4FRT fields. Prescription dose was 45Gy in 25 fractions. Plans were evaluated for target coverage, conformity, dose homogeneity and dose to OARs. Planning time, total monitor units and estimated delivery time were also evaluated. Results: The median patient age was 56 years (range: 27-84 years). Stage 1, 2, 3 and 4A disease was seen in 1, 12, 4 and 3 patients respectively with nodal involvement in 13 patients. PTV2.5, PTV3 and PTV4 extended outside the 4FRT fields in 9 (45%), 13 (65%) and 20 (100%) patients respectively. Target coverage was excellent for the 4FRT, IMRT and VMAT plans generated, with no significant difference between techniques, however, IMRT and VMAT plans were associated with a reduction in dose to OARs compared to 4FRT. Mean monitor units was lowest with 4FRT, followed by VMAT then IMRT. Conclusions: IMRT and VMAT are associated with dosimetric advantages over 4FRT when margins less than 3cm are used, with exceptional target coverage and superior OAR sparing. Uterine margins of 3cm or greater commonly resulted in the PTV extending outside conventional radiotherapy fields, demonstrating no benefit for these advanced planning techniques in patients with large variation in uterine position. Furthermore, accurate uterine localisation with image guidance is critical when considering implementation of IMRT or VMAT for cervix cancer treatment. Prospective studies are also needed to verify that these newer techniques reduce the rates of acute and late toxicities without compromising long-term disease control. EP-1263 A new look at the tumor lesion of the vagina through the prism of MRI: diagnosis and monitoring of brachytherapy N.V. Nudnov, J.M. Kreynina, S.P. Aksenova Russian Scientific Center of Roentgenoradiology, Deputy Director of Research, Moskow, Russian Federation Russian Scientific Center of Roentgenoradiology, Brachytherapy, Moskow, Russian Federation Russian Scientific Center of Roentgenoradiology, Radiology, Moskow, Russian Federation

SU‐E‐J‐213: An Evaluation of the Reproducibility of Radiotherapy Contouring Utilizing Multiple Institutions and Treatment Planning Systems

Purpose: Consistency of radiotherapy contours is required to ensure consistency of treatment and for this reason many studies have been undertaken and are expected in the future comparing contours of multiple observers or systems. This study was undertaken to determine the minimum uncertainty achievable when undertaking this type of investigation including multiple centres and treatment planning systems. Methods: A Computed Tomography (CT) scan was taken of a commercially available uniformity Phantom. This dataset was then imported into various contouring software programs including Pinnacle, Xio and Focal at the same institution and variations at different institutions. Contours of the perimeter of the phantom and a detailed cylinder inside the phantom were contoured using the same observer at provided window levels. The perimeter of the phantom was auto‐contoured using auto‐threshold. The inside circle was contoured manually. Contours were then exported from the treatment planning systems and into CERR for analysis. Results: A comparison of the phantom perimeter from Focal and Pinnacle at a single institution demonstrated a Concordance Index (CI) of 0.98, while the manually contoured cylinder has a CI of 0.77. When comparing between institutions the CI ranged from 0.75–0.85 for the cylinder. Variation in the phantom perimeter contours was mainly in the Z direction with 2 slices (0.4cm) not being contoured in Focal compared to Pinnacle. Maximum variation in the X and Y direction for the phantom perimeter was 0.098cm. The centre of mass of all phantom perimeter contours were within 0.10cm, with the largest variance between institutions occurring in the anterior‐posterior direction. Conclusion: The variation between auto‐contouring and manually contouring a high contrast object for different treatment planning systems has been established. As expected manually contouring produces greater variation than auto‐threshold contours between different treatment planning system. Funding from Cancer Australia and The National Breast Cancer Foundation, Project grant 1033237