Application of failure mode and effects analysis to validate a novel hybrid Linac QC program that integrates automated and conventional QC testing.

A hybrid quality control (QC) program was developed that integrates automated and conventional Linac QC, realizing the benefits of both automated and conventional QC, increasing efficiency and maintaining independent measurement methods. Failure mode and effects analysis (FMEA) was then applied in order to validate the program prior to clinical implementation. The hybrid QC program consists of automated QC with machine performance check and DailyQA3 array on the TrueBeam Linac, and Delta4 volumetric modulated arc therapy (VMAT) standard plan measurements, alongside conventional monthly QC at a reduced frequency. The FMEA followed the method outlined in TG-100. Process maps were created for each treatment type at our center: VMAT, stereotactic body radiotherapy (SBRT), conformal, and palliative. Possible failure modes were established by evaluating each stage in the process map. The FMEA followed semiquantitative methods, using data from our QC records from eight Linacs over 3 years for the occurrence estimates, and simulation of failure modes in the treatment planning system, with scoring surveys for severity and detectability. The risk priority number (RPN) was calculated from the product of the occurrence, severity, and detectability scores and then normalized to the maximum and ranked to determine the most critical failure modes. The highest normalized RPN values (100, 90) were found to be for MLC position dynamic for both VMAT and SBRT treatments. The next highest score was 35 for beam position for SBRT, and the majority of scores were less than 20. Overall, these RPN scores for the hybrid Linac QC program indicated that it would be acceptable, but the high RPN score associated with the dynamic MLC failure mode indicates that it would be valuable to perform more rigorous testing of the MLC. The FMEA proved to be a useful tool in validating hybrid QC.

Implementation of a comprehensive set of optimised CBCT protocols and validation through imaging quality and dose audit.

OBJECTIVES: Cone-beam computed tomography (CBCT) for radiotherapy treatment verification has increased in frequency; therefore, it is crucial to optimise image quality and radiation dose to patients. The aim of this study was to implement optimised CBCT protocols for the Varian TrueBeams for most tumour sites in adult patients. METHODS: A combination of patient size-specific CBCT protocols from the literature and developed in-house was used. Scans taken before and after optimisation were compared by senior radiographers and physicists to evaluate how changes affected image quality and clinical usability for online image registration. The change in dose for each new CBCT protocol was compared to the Varian default. A clinical audit was performed following implementation to evaluate the changes in imaging dose for all patients receiving a CBCT during that period. RESULTS: Ten CBCT protocols were introduced including head and neck and patient-size-specific thorax and pelvis/abdomen protocols. Scans from 102 patients with images before and after optimisation were assessed, none of the scans showed image quality changes compromising clinical usability and for some image quality was improved. Between November 2020 and June 2021, 1185 patients had CBCTs using the new protocols. The imaging dose was reduced for 52% of patients, remained the same for 37% and increased for 12%. CONCLUSIONS: This study showed that substantial dose reductions and image quality improvements can be achieved with simple changes in the default settings of the Varian TrueBeam CBCT without affecting the radiographers' confidence in online image registration. ADVANCES IN KNOWLEDGE: This study represents a comprehensive assessment and optimisation of CBCT protocols for most sites, validated on a large cohort of patients.

Correlation of hypoxia PET tracer uptake with hypoxic radioresistance in cancer cells: PET biomarkers of resistance to stereotactic radiation therapy?

PURPOSE: The pO2 threshold of an ideal PET hypoxia tracer for radiotherapy planning in cancer would match those observed in clinically and biologically relevant processes such as radioresistance and HIF1α expression. To identify such tracers, we directly compared uptake in vitro of hypoxia PET tracers ([18F]FMISO, [64Cu]CuATSM, and analogues [64Cu]CuATS, [64Cu]CuATSE, [64Cu]CuCTS, [64Cu]CuDTS, [64Cu]CuDTSE, [64Cu]CuDTSM) with levels of radioresistance and HIF1α expression in cultured cancer cells under identical hypoxic conditions ranging from extreme hypoxia to normoxia. Pimonidazole uptake was also compared as a marker of hypoxia. METHODS: A custom-built hypoxia apparatus enabled all experiments to be performed under identical hypoxic conditions with constant measurement of pO2 in media using an OxyLab pO2™ probe. HCT116 human colonic carcinoma and MCF-7 human Caucasian breast adenocarcinoma cells were irradiated using a cobalt teletherapy unit. Clonogenic assays were used to assess survival. HIF1α expression was determined by western blotting, tracer uptake by gamma counting and pimonidazole binding by flow cytometry. RESULTS: Radioresistance, pimonidazole binding and HIF1α expression increased gradually as pO2 decreased between 25 mmHg and 0 mmHg. In contrast, all the PET hypoxia tracers showed a sharp increase in uptake only when pO2 levels fell below 1 mmHg. Above this threshold, tracer uptake was not elevated above that in normoxic cells. CONCLUSION: This study highlights an important mismatch in pO2 thresholds between these PET tracers and other markers of hypoxia: tracer uptake only occurred at oxygen levels that were well below levels that induced radioresistance, pimonidazole uptake and HIF1α expression. Although their pO2 thresholds do not match the threshold for resistance to conventionally fractionated radiotherapy (pO2 2.5-10 mmHg), their specificity for extreme hypoxia (pO2 ≪ 1 mmHg) suggests these PET tracers may be of particular use to predict outcomes in stereotactic radiation therapy where these maximally resistant cells play a key role in determining the biological effect.

Total skin electron beam therapy rationalization and utility of in vivo dosimetry in a high-volume centre.

OBJECTIVE: This paper reports on the rationalization of a substantial pool of in vivo dosimetry (IVD) data from patients treated with total skin electron beam therapy (TSEBT) and the application of this to verify the accurate delivery of TSEBT when changing linac manufacturer. METHODS: Thermoluminescent dosimeter IVD data from 149 patients were analyzed comparing the population mean and standard deviation for each site. The number of sites required to confirm the prescribed dose were reviewed considering both dosimetric and clinical relevance. The reduced sites were then used to assess the continued dosimetric accuracy on new equipment and the results were compared statistically using the Mann-Witney test. RESULTS: The trunk dose measurement points were reduced from nine to six and five extra trunk sites were identified and reviewed clinically prior to removal.Following change in manufacturer the trunk dose points showed no statistically significant change and confirmed that patients had received within 1.3% of the intended mean trunk dose using both delivery methods.A statistically significant change in 4 out of the 13 extra trunk sites was seen following the move to the new centre. However, all but one site showed a change of less than 1 standard deviation. CONCLUSION: The total number of measurement points per patient were reduced from 27 to 19 which constituted a 25% saving in preparation and read out.Accurate delivery of prescribed dose was confirmed following measurement point reduction for treatments delivered on linacs from two different manufacturers. ADVANCES IN KNOWLEDGE: Proven methodology for rationalization of IVD measurements for TSEBT.

Comparison of effective dose to children and adults from dual X-ray absorptiometry examinations.

Dual X-ray absorptiometry (DXA) is increasingly used to measure bone density in children. If the system software does not include pediatric scan modes, then child examinations must be performed using adult scan modes that give a higher radiation dose to children than adults. This report describes a study to compare the effective dose to children and adults from DXA scans performed on the Hologic Discovery and QDR4500 models. Depth dose measurements were made using thermoluminescent dosimeters in a Rando phantom and were mapped onto the Cristy mathematical phantoms representing a 5-, 10- and 15-year-old child and an adult, and effective dose (ED) was calculated using the ICRP Publication-60 tissue weighting factors. The ED for spine (hip) examinations performed with the Express mode using the default adult scan lengths were 16.1 (9.8), 11.1 (6.7), 5.6 (3.9) and 4.4 (3.1) microSv for a 5-, 10- and 15-year-old child and adult respectively. However, if care is taken to adjust scan lengths appropriately, the child doses were reduced to 9.1 (7.4), 7.1 (5.9) and 5.0 (3.7) microSv. ED figures for the Fast and Array modes were larger by factors of 1.5 and 3 respectively. EDs for whole body scans for a 5-, 10- and 15-year-old child and adult performed on the A-model (W-model) were 5.2 (10.5), 4.8 (9.6), 4.2 (8.4) and 4.2 (8.4) microSv. Using the infant whole body mode (only available on the A-model), they were 7.5 microSv for a 1-year-old and 8.9 microSv for a neonate. Although doses from child DXA examinations are low, it is still important to keep them as small as possible. DXA operators using Discovery systems can do this by using the Express scan mode, by setting appropriate values of the scan length before scan acquisition and by avoiding mistakes that lead to scans having to be unnecessarily repeated.