The Value of On-Site Proton Audits

Improved stoichiometric model for megavoltage computed tomography number conversion and dose calculation within the TomoTherapy delivery system

PURPOSE

This study investigated a novel stoichiometric computed tomography (CT) number calibration (SCC) model for megavoltage CT (MVCT). This model was specifically designed to convert CT numbers to mass densities (MDs) for dose calculations when using the TomoTherapy MVCT radiotherapy delivery system.

MATERIALS AND METHODS

The MVCT-SCC model extended the conventional SCC model originally developed for kilovoltage CT by incorporating additional parameters to account for pair production and water-equivalent CT number corrections, which are specific to MVCT applications. The accuracy of the model was evaluated through a benchmark test that involved comparing its performance to those of three conventional models using a Catphan700 phantom equipped with 10 density plugs and air. The MVCT numbers derived from the MVCT-SCC and conventional models were assessed by comparing them to the actual measured values using the root mean square error (RMSE) in Hounsfield units (HU). Additionally, potential percentage estimated mean dose error (PMDE) discrepancies were quantified based on the tolerances specified in the CT-MD conversion table and the observed RMSE values.

RESULTS

The RMSE for the MVCT numbers calculated with the MVCT-SCC model was significantly lower, at 10.8 HU, compared to values of >35.3 HU for the conventional models. Furthermore, the PMDE when using the MVCT-SCC model was within 1 %, which was significantly better than the differences of >2 % observed with the conventional models.

CONCLUSION

The results of the benchmark test confirmed that the MVCT-SCC model significantly enhanced the accuracy of the calculated HU numbers and dose estimates compared to conventional models.

Characterization of LiF TLD-100 in carbon ion beams for remote audits

BACKGROUND

A passive dosimeter framework for the measurement of dose in carbon ion beams has yet to be characterized or implemented for regular use.

PURPOSE

This work determined the dose calculation correction factors for absorbed dose in thermoluminescent dosimeters (TLDs) in a therapeutic carbon ion beam. TLD could be a useful tool for remote audits, particularly in the context of clinical trials as new protocols are developed for carbon ion radiotherapy.

METHODS

TLD-100 were irradiated in a carbon ion beam at the Centro Nazionale di Adroterapia Oncologica (CNAO) in Pavia, Italy. The dose correction factors for linearity, fading, and beam quality were characterized. Fading was characterized from 5 to 100 days post-irradiation. For linearity, the TLDs were irradiated to absorbed doses ranging from 1 to 15 Gy in both the entrance of a high-energy pristine carbon ion peak and the center of a 2 cm spread-out Bragg peak. For beam quality, the TLD was irradiated to the same absorbed dose (3 Gy) in several pristine carbon ion Bragg peaks, as well as in several spread-out Bragg peaks. Each correction factor was calculated and compared to photon correction factors. The correction factors were also compared between high and low dose-averaged linear energy transfer (LETD) in the carbon ion beams. The absorbed dose was compared between ion chamber and TLD-100 in the several tissue substitute phantom materials, applying the carbon ion TLD correction factors.

RESULTS

There was no statistically significant difference in the TLD fading correction factor between photons, low LETD carbon ion beams, or high LETD carbon ion beams. The TLD linearity correction factor did differ between photons, low LETD carbon ions, and high LETD carbon ions. The beam quality correction factor was large and changed linearly with LETD. The overall uncertainty of the carbon ion absorbed dose calculation was 3.9% at the one-sigma level, driven largely by a 3.5% uncertainty in the beam quality correction. TLD measurements were within 1.2% of ion chamber measurements in the phantom material for polyethylene, solid water (Gammex and Sun Nuclear), acrylic, blue water, and techtron HPV. TLD measurements in balsa wood were within 3.0% and cork was 6.6% low compared to ion chamber.

CONCLUSION

TLD-100 can be used for passive dosimetry in a therapeutic carbon ion beam. Importantly, the linearity and beam quality correction factors are both different from photon therapy, and dependent on LETD of the carbon ion beam. This opens the possibility of TLD use for carbon ion output audits, phantom audits, and in vivo dose measurements, but may be subject to more complicated management and slightly larger uncertainties than are achieved in photon beams.

Research Trends in the Study of the Relative Biological Effectiveness: A Bibliometric Study

The relative biological effectiveness is a mathematical quantity first defined in the 1950s. This has resulted in more than 4,000 scientific papers published to date. Yet defining the correct value of the RBE to use in clinical practice remains a challenge. A scientific analysis in the radiation research literature can provide an understanding of how this mathematical quantity has evolved. The purpose of this study is to investigate documents published since 1950 using bibliometric indicators and network visualization. This analysis seeks to provide an assessment of global research activities, key themes, and RBE research within the radiation-related field. It strives to highlight top-performing authors, organizations, and nations that have made major contributions to this research domain, as well as their interactions. The Scopus Collection was searched for articles and reviews pertaining to RBE in radiation research from 1950 through 2023. Scopus and Bibiometrix analytic tools were used to investigate the most productive countries, researchers, collaboration networks, journals, along with the citation analysis of references and keywords. A total of 4,632 documents were retrieved produced by authors originating from 71 countries. Publication trends could be separated in 20-year groupings beginning with slow accrual from 1950 to 1970, an early rise from 1970-1990, followed by a sharp increase in the years 1990s-2010s that matches the development of charged particle therapy in clinics worldwide and opened discussion on the true value of the RBE in proton beam therapy. Since the 2010s, a steady 200 papers, on average, have been published per year. The United States produced the most publications overall (N = 1,378) and Radiation Research was the most likely journal to have published articles related to the RBE (606 publications during this period). J. Debus was the most prolific author (112 contributions, with 2,900 citations). The RBE has captured the research interest of over 7,000 authors in the past decade alone. This study supports that notion that the growth of the body of evidence surrounding the RBE, which started 75 years ago, is far from reaching its end. Applications to medicine have continuously dominated the field, with physics competing with Biochemistry, Genetics and Molecular Biology for second place over the decades. Future research can be predicted to continue.

Technical note: Radiological clinical equivalence for phantom materials in carbon ion therapy

PURPOSE

As carbon ion radiotherapy increases in use, there are limited phantom materials for heterogeneous or anthropomorphic phantom measurements. This work characterized the radiological clinical equivalence of several phantom materials in a therapeutic carbon ion beam.

METHODS

Eight materials were tested for radiological material-equivalence in a carbon ion beam. The materials were computed tomography (CT)-scanned to obtain Hounsfield unit (HU) values, then irradiated in a monoenergetic carbon ion beam to determine relative linear stopping power (RLSP). The corresponding HU and RLSP for each phantom material were compared to clinical carbon ion calibration curves. For absorbed dose comparison, ion chamber measurements were made in the center of a carbon ion spread-out Bragg peak (SOBP) in water and in the phantom material, evaluating whether the material perturbed the absorbed dose measurement beyond what was predicted by the HU-RLSP relationship.

RESULTS

Polyethylene, solid water (Gammex and Sun Nuclear), Blue Water (Standard Imaging), and Techtron HPV had measured RLSP values that agreed within ±4.2% of RLSP values predicted by the clinical calibration curve. Measured RLSP for acrylic was 7.2% different from predicted. The agreement for balsa wood and cork varied between samples. Ion chamber measurements in the phantom materials were within 0.1% of ion chamber measurements in water for most materials (solid water, Blue Water, polyethylene, and acrylic), and within 1.9% for the rest of the materials (balsa wood, cork, and Techtron HPV).

CONCLUSIONS

Several phantom materials (Blue Water, polyethylene, solid water [Gammex and Sun Nuclear], and Techtron HPV) are suitable for heterogeneous phantom measurements for carbon ion therapy. Low density materials should be carefully characterized due to inconsistencies between samples.

Prioritizing clinical trial quality assurance for photons and protons: A failure modes and effects analysis (FMEA) comparison

BACKGROUND AND PURPOSE

The Global Clinical Trials RTQA Harmonization Group (GHG) set out to evaluate and prioritize clinical trial quality assurance.

METHODS

The GHG compiled a list of radiotherapy quality assurance (QA) tests performed for proton and photon therapy clinical trials. These tests were compared between modalities to assess whether there was a need for different types of assessments per modality. A failure modes and effects analysis (FMEA) was performed to assess the risk of each QA failure.

RESULTS

The risk analysis showed that proton and photon therapy shared four out of five of their highest-risk failures (end-to-end anthropomorphic phantom test, phantom tests using respiratory motion, pre-treatment patient plan review of contouring/outlining, and on-treatment/post-treatment patient plan review of dosimetric coverage). While similar trends were observed, proton therapy had higher risk failures, driven by higher severity scores. A sub-analysis of occurrence × severity scores identified high-risk scores to prioritize for improvements in RTQA detectability. A novel severity scaler was introduced to account for the number of patients affected by each failure. This scaler did not substantially alter the ranking of tests, but it elevated the QA program evaluation to the top 20th percentile. This is the first FMEA performed for clinical trial quality assurance.

CONCLUSION

The identification of high-risk errors associated with clinical trials is valuable to prioritize and reduce errors in radiotherapy and improve the quality of trial data and outcomes, and can be applied to optimize clinical radiotherapy QA.