T.J. Jordan

The IPEM code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV based on an absorbed dose to water calibration

This report contains the recommendations of the Electron Dosimetry Working Party of the UK Institute of Physics and Engineering in Medicine (IPEM). The recommendations consist of a code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV. The code is based on the absorbed dose to water calibration service for electron beams provided by the UK standards laboratory, the National Physical Laboratory (NPL). This supplies direct N(D,w) calibration factors, traceable to a calorimetric primary standard, at specified reference depths over a range of electron energies up to approximately 20 MeV. Electron beam quality is specified in terms of R(50,D), the depth in water along the beam central axis at which the dose is 50% of the maximum. The reference depth for any given beam at the NPL for chamber calibration and also for measurements for calibration of clinical beams is 0.6R(50.D) - 0.1 cm in water. Designated chambers are graphite-walled Farmer-type cylindrical chambers and the NACP- and Roos-type parallel-plate chambers. The practical code provides methods to determine the absorbed dose to water under reference conditions and also guidance on methods to transfer this dose to non-reference points and to other irradiation conditions. It also gives procedures and data for extending up to higher energies above the range where direct calibration factors are currently available. The practical procedures are supplemented by comprehensive appendices giving discussion of the background to the formalism and the sources and values of any data required. The electron dosimetry code improves consistency with the similar UK approach to megavoltage photon dosimetry, in use since 1990. It provides reduced uncertainties, approaching 1% standard uncertainty in optimal conditions, and a simpler formalism than previous air kerma calibration based recommendations for electron dosimetry.

Clinical validation and benchmarking of knowledge-based IMRT and VMAT treatment planning in pelvic anatomy

PURPOSE The aim of this work was to determine whether a commercial knowledge-based treatment planning (KBP) module can efficiently produce IMRT and VMAT plans in the pelvic region (prostate & cervical cancer), and to assess sensitivity of plan quality to training data and model parameters. METHODS Initial benchmarking of KBP was performed using prostate cancer cases. Structures and dose distributions from 40 patients previously treated using a 5-field IMRT technique were used for model training. Two types of model were created: one excluded statistical outliers (as identified by RapidPlan guidelines) and the other had no exclusions. A separate model for cervix uteri cancer cases was subsequently developed using 37 clinical patients treated for cervical cancer using RapidArc™ VMAT, with no exclusions. The resulting models were then used to generate plans for ten patients from each patient group who had not been included in the modelling process. Comparisons of generated RapidPlans with the corresponding clinical plans were carried out to indicate the required modifications to the models. Model parameters were then iteratively adjusted until plan quality converged with that obtained by experienced planners without KBP. RESULTS Initial automated model generation settings led to poor conformity, coverage and efficiency compared to clinical plans. Therefore a number of changes to the initial KBP models were required. Before model optimisation, it was found that the PTV coverage was slightly reduced in the superior and inferior directions for RapidPlan compared with clinical plans and therefore PTV parameters were adjusted to improve coverage. OAR doses were similar for both RapidPlan and clinical plans (p>0.05). Excluding outliers had little effect on plan quality (p≫0.05). Manually fixing key optimisation objectives enabled production of clinically acceptable treatment plans without further planner intervention for 9 of 10 prostate test patients and all 10 cervix test patients. CONCLUSIONS The Varian RapidPlan™ system was able to produce IMRT & VMAT treatment plans in the pelvis, in a single optimisation, that had comparable sparing and comparable or better conformity than the original clinically acceptable plans. The system allows for better consistency and efficiency in the treatment planning process and has therefore been adopted clinically within our institute with over 100 patients treated.

A critical evaluation of the PTW 2D‐ARRAY seven29 and OCTAVIUS II phantom for IMRT and VMAT verification

Quality assurance (QA) for intensity‐ and volumetric‐modulated radiotherapy (IMRT and VMAT) has evolved substantially. In recent years, various commercial 2D and 3D ionization chamber or diode detector arrays have become available, allowing for absolute verification with near real time results, allowing for streamlined QA. However, detector arrays are limited by their resolution, giving rise to concerns about their sensitivity to errors. Understanding the limitations of these devices is therefore critical. In this study, the sensitivity and resolution of the PTW 2D‐ARRAY seven29 and OCTAVIUS II phantom combination was comprehensively characterized for use in dynamic sliding window IMRT and RapidArc verification. Measurement comparisons were made between single acquisition and a multiple merged acquisition techniques to improve the effective resolution of the 2D‐ARRAY, as well as comparisons against GAFCHROMIC EBT2 film and electronic portal imaging dosimetry (EPID). The sensitivity and resolution of the 2D‐ARRAY was tested using two gantry angle 0° modulated test fields. Deliberate multileaf collimator (MLC) errors of 1, 2, and 5 mm and collimator rotation errors were inserted into IMRT and RapidArc plans for pelvis and head & neck sites, to test sensitivity to errors. The radiobiological impact of these errors was assessed to determine the gamma index passing criteria to be used with the 2D‐ARRAY to detect clinically relevant errors. For gamma index distributions, it was found that the 2D‐ARRAY in single acquisition mode was comparable to multiple acquisition modes, as well as film and EPID. It was found that the commonly used gamma index criteria of 3% dose difference or 3 mm distance to agreement may potentially mask clinically relevant errors. Gamma index criteria of 3%/2 mm with a passing threshold of 98%, or 2%/2 mm with a passing threshold of 95%, were found to be more sensitive. We suggest that the gamma index passing thresholds may be used for guidance, but also should be combined with a visual inspection of the gamma index distribution and calculation of the dose difference to assess whether there may be a clinical impact in failed regions. PACS numbers: 87.55.Qr, 87.56.Fc

Quality control aspects of the Philips multileaf collimator

BACKGROUND AND PURPOSE Linear accelerators equipped with multileaf collimators (MLCs) are becoming more common and are widely available from most commercial manufacturers. There is a need to ensure they retain their commissioning specification using a preventative maintenance and quality control (QC) programme. This paper considers the design criteria of the Philips MLC which are important to the production of a comprehensive quality control programme. MATERIALS AND METHODS The specific QC problems related to MLCs are identified as the positional accuracy of the leaves and their relationship to the back-up collimators, leakage considerations, the relationship of X-ray to light field and the influence of gravity on the positioning and leakage characteristics of the leaves. These problems are considered in relation to the general design considerations of the MLC, and methods of performing routine quality control checks are discussed. RESULTS AND CONCLUSIONS The introduction of MLCs into clinical use results in new QC procedures being developed but it can be concluded that for the Philips MLC only an extra 30 min of QC time is needed per month and that its use has added little to the general down-time of this department.

Lack of late skin necrosis in man after high-dose irradiation using small field sizes: experiences of grid therapy

Out of a total of 437 patients with superior vena caval syndrome or advanced malignancy, given single-dose grid radiotherapy, four survived to 7 years. The dose to the skin under each of the 77 holes in the grid was approximately 58 Gy. The lack of skin necrosis in the total of 308 skin circles of 1 cm diameter among these survivors, compared with known necrosis rates in larger irradiated areas, implies that there is a marked field-size effect for late necrosis in small areas of irradiated skin.

The design and evaluation of a phantom for the audit of the treatment chain for prostate radiotherapy

BACKGROUND AND PURPOSE A phantom has been designed and built for a multi-institutional technique audit of the planning and delivery for radiotherapy to the prostate. The phantom was designed to test both the geometric and dosimetric accuracy of each aspect of the process. MATERIALS AND METHODS The phantom consists of two curved water filled perspex tanks either side of a central block of solid water equivalent material. There are two options for the central section; a target defining block and a dose measurement block. The target defining block uses air holes to define a 3-D target volume for imaging via a CT scanner or a simulator. These holes can subsequently be filled with steel pins to allow megavoltage imaging. The dose measurement block allows thimble chamber measurements to be made at pre-selected points in a 5x5mm array. Five dose measurement points, typical for a prostate planning target volume (PTV) were selected. Initial evaluation of the phantom was performed by auditing the prostate radiotherapy planning and treatment chain at one institution. RESULTS Agreement between the phantom and planned geometry confirmed that the stages of image acquisition, transfer and manipulation were accurately performed. Agreement within 0.5% was found between phantom and water tank measurements for dose calibration at a reference point. The measured dose delivered was within 2% of the dose calculated by the planning computer for all of the selected measurement points. The target volume marked by the steel pins was visible using electronic portal imaging. CONCLUSIONS The phantom is a useful tool for the technique audit of prostate radiotherapy.

Volumetric-modulated arc therapy (RapidArc) vs. conventional fixed-field intensity-modulated radiotherapy for 18F-FDG-PET-guided dose escalation in oropharyngeal cancer: A planning study

UNLABELLED Fluorine-18-fluorodeoxyglucose-positron emission tomography (¹⁸F-FDG-PET)-guided focal dose escalation in oropharyngeal cancer may potentially improve local control. We evaluated the feasibility of this approach using volumetric-modulated arc therapy (RapidArc) and compared these plans with fixed-field intensity-modulated radiotherapy (IMRT) focal dose escalation plans. MATERIALS AND METHODS An initial study of 20 patients compared RapidArc with fixed-field IMRT using standard dose prescriptions. From this cohort, 10 were included in a dose escalation planning study. Dose escalation was applied to ¹⁸F-FDG-PET-positive regions in the primary tumor at dose levels of 5% (DL1), 10% (DL2), and 15% (DL3) above standard radical dose (65 Gy in 30 fractions). Fixed-field IMRT and double-arc RapidArc plans were generated for each dataset. Dose-volume histograms were used for plan evaluation and comparison. The Paddick conformity index (CI(Paddick)) and monitor units (MU) for each plan were recorded and compared. Both IMRT and RapidArc produced clinically acceptable plans and achieved planning objectives for target volumes. Dose conformity was significantly better in the RapidArc plans, with lower CI(Paddick) scores in both primary (PTV1) and elective (PTV2) planning target volumes (largest difference in PTV1 at DL3; 0.81 ± 0.03 [RapidArc] vs. 0.77 ± 0.07 [IMRT], p = 0.04). Maximum dose constraints for spinal cord and brainstem were not exceeded in both RapidArc and IMRT plans, but mean doses were higher with RapidArc (by 2.7 ± 1 Gy for spinal cord and 1.9 ± 1 Gy for brainstem). Contralateral parotid mean dose was lower with RapidArc, which was statistically significant at DL1 (29.0 vs. 29.9 Gy, p = 0.01) and DL2 (29.3 vs. 30.3 Gy, p = 0.03). MU were reduced by 39.8-49.2% with RapidArc (largest difference at DL3, 641 ± 94 vs. 1261 ± 118, p < 0.01). ¹⁸F-FDG-PET-guided focal dose escalation in oropharyngeal cancer is feasible with RapidArc. Compared with conventional fixed-field IMRT, RapidArc can achieve better dose conformity, improve contralateral parotid sparing, and uses fewer MU.

Feasibility of using glass-bead thermoluminescent dosimeters for radiotherapy treatment plan verification

OBJECTIVE To investigate the feasibility of using glass beads as novel thermoluminescent dosemeters (TLDs) for radiotherapy treatment plan verification. METHODS Commercially available glass beads with a size of 1-mm thickness and 2-mm diameter were characterized as TLDs. Five clinical treatment plans including a conventional larynx, a conformal prostate, an intensity-modulated radiotherapy (IMRT) prostate and two stereotactic body radiation therapy (SBRT) lung plans were transferred onto a CT scan of a water-equivalent phantom (Solid Water(®), Gammex, Middleton, WI) and the dose distribution recalculated. The number of monitor units was maintained from the clinical plan and delivered accordingly. The doses determined by the glass beads were compared with those measured by a graphite-walled ionization chamber, and the respective expected doses were determined by the treatment-planning system (TPS) calculation. RESULTS The mean percentage difference between measured dose with the glass beads and TPS was found to be 0.3%, -0.1%, 0.4%, 1.8% and 1.7% for the conventional larynx, conformal prostate, IMRT prostate and each of the SBRT delivery techniques, respectively. The percentage difference between measured dose with the ionization chamber and glass bead was found to be -1.2%, -1.4%, -0.1%, -0.9% and 2.4% for the above-mentioned plans, respectively. The results of measured doses with the glass beads and ionization chamber in comparison with expected doses from the TPS were analysed using a two-sided paired t-test, and there was no significant difference at p < 0.05. CONCLUSION It is feasible to use glass-bead TLDs as dosemeters in a range of clinical plan verifications. ADVANCES IN KNOWLEDGE Commercial glass beads are utilized as low-cost novel TLDs for treatment-plan verification.

EP-1432: Experiences in model creation using Varian Rapid PlanTM for 5 field IMRT prostate treatments

lymph nodes and a boost volume including the prostate and seminal vesicles; TPs were generated in simultaneous boost technique. For AP, a progressive engine is used where the user defines prioritized optimization goals for PTV-coverage and dose thresholds and priorities for each organ at risk (OAR). The AP engine automatically creates objectives and required optimization aid structures (OAS), and multiple optimization loops iteratively reformulate and adjust the optimization objectives to meet the goals and further lower dose to OAR with minimal compromise to the target coverage. For manual planning, additional OAS have to be generated by the planner, objectives and priorities have to be adjusted manually for each optimization loop. For plan comparison, various dose and dose volume metrics (Dmed, D98%, D2% V95% for target volumes, D2%, Dmed and Vx% for OARs) as well as homogeneity index (HI = (D2%-D98%)/ D50%) and conformity index (CIPaddick = TV2PI/(PI*TV)) were evaluated. Efficiency of the plan optimization procedure was estimated by means of total time required to create a TP. Results: PTV coverage V95% was 93.5±3.5% and 97.9±1.3% and boost coverage was 95.5±2.0% and 98.3±1.7% for MP and AP, respectively. Homogeneity index for the PTV was 0.14±0.02 and 0.12±0.02 and for the boost it was 0.11±0.02 and 0.07±0.02 for MP and AP, respectively. CI was 13% and 16% higher in manual plans compared to automatic plans for PTV and boost, respectively. Dmed and D2% for bladder and femoral heads showed no particular differences between manual and automatic plans. However, considerable deviations in Dmed were found for the rectum (27.8±4.7Gy vs 33.3±5.8Gy for MP and AP, respectively) and intestine (25.2±7.5Gy and 22.8±8.2Gy for MP and AP, respectively). Further, VTissue30% representing tissue outside the target volumes received 36% more dose in AP compared to MP. The time to create a treatment plan was <1 hour for MP and >2 hours for AP. Conclusions: Automatically generated TPs improve target coverage and homogeneity at the cost of slightly decreased conformity when compared to manual TPs. OAR sparing is mostly comparable, higher dose contribution to normal tissue outside the PTV was found for AP. Since higher low dose volume was detected in normal tissue for AP plans, each TP needs to be evaluated by an experienced planner and adapted when necessary. Prioritized optimization goals in AP need to be carefully established and the overall time required to create a plan remains to be optimized.