An approach to assess real workload of medical linear accelerators in Sri Lanka

Due to the introduction of new radiotherapy treatment techniques and the increased number of cancer patients, there is a higher possibility of redundant workload on radiotherapy machines. Therefore, this study aims to assess the workload in three linear accelerator units. An 8-week survey was conducted in the selected three units, during which all treatment parameters were accumulated. The highest number of treatment sessions per week and highest weekly workload of dose at the isocenter were reported as 465 ± 24 and 1295 ± 93 Gy, respectively in the Varian unit at Apeksha Hospital. The cardinal angle was identified as the 00-300 range in all selected units. The modulation factors for the intensity modulated radiotherapy techniques showed significant variations among the three units. The estimated real weekly workload is quite high compared to previous studies. This study recommends conducting at least a one-year survey, considering all parameters, to obtain more reliable results.

On methods for radiometric surveying in radiotherapy bunkers

Radiometric surveys in radiotherapy bunkers have been carried out in Brazil for many years, both by the same radiotherapy facility for verification of shielding as by the regulatory agency for licensing and control purposes. In recent years, the Intensity Modulated Radiation Therapy (IMRT) technique has been gradually incorporated into many facilities. Therefore, it has been necessary to consider the increased leakage component that has an important impact on the secondary walls. For that, a radiometric survey method has been used that considers an increased 'time of beam-on' for the secondary walls. In this work we discuss two methods of doing this: the first considers that this 'time of beam-on' affects the sum of the two components, leakage and scattered. In another method it is considered that only the leakage component is affected by this extended 'time of beam-on'. We compare the methods and show that for secondary walls withU= 1 the first method overestimates dose rates by important percentages and for secondary walls withU< 1 it can both overestimate or underestimate the dose rates, depending on the parameters of the project. An optimized procedure is proposed, according to the use factor (U) of the secondary wall to be measured.

The need to update NCRP 151 data for 10 MV linear accelerator bunker shielding based on new measurements and Monte Carlo simulations

Linear accelerator bunker shielding protocols such as NCRP 151 have previously been tested against a large sample of measured data from real bunkers and machines but differences in per-energy concrete penetration (TVLs) for 10 MV has not yet been resolved. These differences are likely due to historical beam data and can potentially result in over-exposure of radiation workers and the public. This study examines a cohort of clinical linac bunker survey measurements and compares them to popular shielding protocols. Differences were investigated using contemporary beam data for both Monte Carlo simulation and in analytical equations. For primary barriers, NCRP 151 underestimates the dose rate through concrete by on average a factor of 2 with secondary barriers and maze entrance doses having much better agreement. Use of contemporary beam data in Monte Carlo simulation and an analytical equation yielded TVL values much closer to the measured values compared to NCRP 151. The TVL data in NCRP 151 is outdated and needs to be updated based upon the energy spectra of modern linear accelerators. Until then, physicists should use the TVL values shown in this study instead.

Measuring foetal dose from tomotherapy treatments

INTRODUCTION

Treating pregnant women in the radiotherapy clinic is a rare occurrence. When it does occur, it is vital that the dose received by the developing embryo or foetus is understood as fully as possible. This study presents the first investigation of foetal doses delivered during helical tomotherapy treatments. Six treatment plans were delivered to an anthropomorphic phantom using a tomotherapy machine. These included treatments of the brain, unilateral and bilateral head-and-neck, chest wall, and upper lung. Measurements of foetal dose were made with an ionisation chamber positioned at various locations longitudinally within the phantom to simulate a variety of patient anatomies. All measurements were below the established limit of 100 mGy for a high risk of damage during the first trimester. The largest dose encountered was 75 mGy (0.125% of prescription dose). The majority of treatments with measurement positions less than 30 cm fell into the range of uncertain risk (50 - 100 mGy). All treatments with measurement positions beyond 30 cm fell into the low risk category (< 50 mGy). For the cases in this study, tomotherapy resulted in foetal doses that are at least on par with, if not significantly lower than, similar 3D conformal or intensity-modulated treatments delivered with other devices. Recommendations were also provided for estimating foetal doses from tomotherapy plans.

Linear accelerator bunker shielding for stereotactic radiotherapy

Shielding protocols such as NCRP 151, IAEA SRS 47 and IPEM 47 are commonly used for the design of radiotherapy facilities. Some work has been accomplished in updating the basic formula with the advent of IMRT but little consideration has been made for unflattened beams and stereotactic techniques apart from for facilities housing devices like the CyberKnife. The inevitable scenario of a stereotactic-only linear accelerator was considered in this study. The necessary shielding requirements were determined based on stereotactic data from a year's worth treatments from one clinic as well as further measurements of leakage, scatter and use factors. These values were compared to recommendations in the literature. While tenth value layer amounts, workload and barrier widths could be kept at the status quo, major changes could be made to the parameters of leakage, scatter and use factors while still maintaining safety. Some differences could also be seen for the use of IMRT factors. Current shielding protocols were found to inadequately describe methodology for the shielding of a stereotactic-only radiotherapy linac bunker, producing overly-conservative wall thicknesses which is in disagreement with the principles of ICRP.

Conservatism in linear accelerator bunker shielding

Conservatism in the shielding of linear accelerator bunkers is engrained in the methodology of international protocols and guidelines. However, the degree to which this cautious and prudent approach is necessary should be judged against the International Committee of Radiation Protection's principles of exposure justification and optimisation. Radiation survey data from 75 concrete barriers was aggregated and compared to exposure predictions from three popular protocols in order to assess any conservatism in factors used to calculate scatter, leakage and beam penetration. These findings, in addition to a list of common conservative practices, were then used to tally the possible fiscal impact of an over-conservative approach to linear accelerator bunker shielding. While primary beam penetration was accurately predicted, stated conservatisms in scatter and leakage was found to be largely misplaced. An estimated total factor of conservatism calculated from a tally was found to be in agreement with literature values of radiotherapist occupational exposure. This factor amounted to a cost increase of 43% for a single bunker if all conservative assumptions were made. There are aspects of linear accelerator shielding design that have been shown to be overly conservative, beyond what is justifiable by the International Committee of Radiation Protection. Some adjustment to international protocol methodology may be required.

Results of a 10-year survey of workload for 10 treatment vaults at a high-throughput comprehensive cancer center

The workload for shielding purposes of modern linear accelerators (linacs) consists of primary and scatter radiation which depends on the dose delivered to isocenter (cGy) and leakage radiation which depends on the monitor units (MUs). In this study, we report on the workload for 10 treatment vaults in terms of dose to isocenter (cGy), monitor units delivered (MUs), number of treatment sessions (Txs), as well as, use factors (U) and modulation factors (CI) for different treatment techniques. The survey was performed for the years between 2006 and 2015 and included 16 treatment machines which represent different generations of Varian linear accelerators (6EX, 600C, 2100C, 2100EX, and TrueBeam) operating at different electron and x‐ray energies (6, 9, 12, 16 and 20 MeV electrons and, 6 and 15 MV x‐rays). An institutional review board (IRB) approval was acquired to perform this study. Data regarding patient workload, dose to isocenter, number of monitor units delivered, beam energies, gantry angles, and treatment techniques were exported from an ARIA treatment management system (Varian Medical Systems, Palo Alto, Ca.) into Excel spreadsheets and data analysis was performed in Matlab. The average (± std‐dev) number of treatment sessions, dose to isocenter, and number of monitor units delivered per week per machine in 2006 was 119 ± 39 Txs, (300 ± 116) × 10²cGys, and (78 ± 28) × 10³MUs respectively. In contrast, the workload in 2015 was 112 ± 40 Txs, (337 ± 124) × 10²cGys, and (111 ± 46) × 10³MUs. 60% of the workload (cGy) was delivered using 6 MV and 30% using 15 MV while the remaining 10% was delivered using electron beams. The modulation factors (MU/cGy) for IMRT and VMAT were 5.0 (± 3.4) and 4.6 (± 1.6) respectively. Use factors using 90° gantry angle intervals were equally distributed (~0.25) but varied considerably among different treatment techniques. The workload, in terms of dose to isocenter (cGy) and subsequently monitor units (MUs), has been steadily increasing over the past decade. This increase can be attributed to increased use of high dose hypo‐fractionated regimens (SBRT, SRS) and the increase in use of IMRT and VMAT, which require higher MUs per cGy as compared to more conventional treatment (3DCRT). Meanwhile, the patient workload in terms of treatment sessions per week remained relatively constant. The findings of this report show that variables used for shielding purposes still fall within the recommendation of NCRP Report 151.