Evaluation of LiF:Mg,Ti (TLD-100) for Intraoperative Electron Radiation Therapy Quality Assurance

Background

Purpose of the present work was to investigate thermoluminescent dosimeters (TLDs) response to intraoperative electron radiation therapy (IOERT) beams. In an IOERT treatment, a large single radiation dose is delivered with a high dose-per-pulse electron beam (2–12 cGy/pulse) during surgery. To verify and to record the delivered dose, in vivo dosimetry is a mandatory procedure for quality assurance. The TLDs feature many advantages such as a small detector size and close tissue equivalence that make them attractive for IOERT as in vivo dosimeters.

Methods

LiF:Mg,Ti dosimeters (TLD-100) were irradiated with different IOERT electron beam energies (5, 7 and 9 MeV) and with a 6 MV conventional photon beam. For each energy, the TLDs were irradiated in the dose range of 0–10 Gy in step of 2Gy. Regression analysis was performed to establish the response variation of thermoluminescent signals with dose and energy.

Results

The TLD-100 dose-response curves were obtained. In the dose range of 0–10 Gy, the calibration curve was confirmed to be linear for the conventional photon beam. In the same dose region, the quadratic model performs better than the linear model when high dose-per-pulse electron beams were used (F test; p<0.05).

Conclusions

This study demonstrates that the TLD dose response, for doses ≤10Gy, has a parabolic behavior in high dose-per-pulse electron beams. TLD-100 can be useful detectors for IOERT patient dosimetry if a proper calibration is provided.     

DNA strand breaks based on Monte Carlo simulation in and around the Lipiodol with flattening filter and flattening filter-free photon beams

BackgroundThe current study aims to investigate the DNA strand breaks based on the Monte Carlo simulation within and around the Lipiodol with flattening filter (FF) and flattening filter-free (FFF) photon beams.Materials and methodsThe dose-mean lineal energy (y_D) and DNA single- and double strand breaks (DSB/SSB) based on spatial patterns of inelastic interactions were calculated using the Monte Carlo code: particle and heavy ion transport system (PHITS). The ratios of dose using standard radiation (200 kVX) to the dose of test radiation (FF and FFF of 6 MV X-ray (6MVX) and 10 MVX beams) to produce the same biological effects was defined as RBE_DSB. The RBE_DSB within the Lipiodol and in the build-up and build-down regions was evaluated.ResultsThe RBE_DSB values with the Lipiodol was larger than that without the Lipiodol at the depth of 4.9 cm by 4.2% and 2.5% for 6 MVX FFF and FF beams, and 3.3% and 2.5% for 10 MVX FFF and FF beams. The RBE_DSB values with the Lipiodol was larger than that without the Lipiodol at the depth of 6.5 cm by 2.9% and 2.4% for 6 MVX FFF and FF beams, and 1.9% and 1.4% for 10 MVX FFF and FF beams. In the build-down region at the depth of 8.1 cm, the RBE_DSB values with the Lipiodol was smaller than that without the Lipiodol by 4.2% and 2.9% for 6 MVX FFF and FF beams, and 1.4% and 0.1% for 10 MVX FFF and FF beams.ConclusionsThe current study simulated the DNA strand break except for the physical dose difference. The lower and FFF beam occurred the higher biological effect.     

Monte Carlo comparison of superficial dose between flattening filter free and flattened beams

This study investigates the superficial dose from FFF beams in comparison with the conventional flattened ones using a Monte Carlo (MC) method. Published phase-space files which incorporated real geometry of a TrueBeam accelerator were used for the dose calculation in phantom and clinical cases. The photon fluence on the central axis is 3 times that of a flattened beam for a 6 MV FFF beam and 5 times for a 10 MV beam. The mean energy across the field in air at the phantom surface is 0.92–0.95 MeV for the 6 MV FFF beam and 1.18–1.30 MeV for the corresponding flattened beam. At 10 MV, the values are 1.52–1.72 and 2.15–2.87 MeV for the FFF and flattened beams, respectively. The phantom dose at the depth of 1 mm in the 6 MV FFF beam is 6% ± 2.5% (of the maximum dose) higher compared to the flattened beam for a 25 × 25 cm² field and 14.6% ± 1.9% for the 2 × 2 cm² field. For the 10 MV beam, the corresponding differences are 3.4% ± 1.5% and 10.7% ± 0.6%. The skin dose difference at selected points on the patient's surface between the plans using FFF and flattened beams in the head-and-neck case was 6.5% ± 2.3% (1SD), and for the breast case it was 6.4% ± 2.3%. The Monte Carlo simulations showed that due to the lower mean energy in the FFF beam, the clinical superficial dose is higher without the flattening filter compared to the flattened beam.     

Planning Study of Flattening Filter Free Beams for Volumetric Modulated Arc Therapy in Squamous Cell Carcinoma of the Scalp

Purpose

Flattening filter free (FFF) beams show the potential for a higher dose rate and lower peripheral dose. We investigated the planning study of FFF beams with their role for volumetric modulated arc therapy (VMAT) in squamous cell carcinoma of the scalp.

Methods and Materials

One patient with squamous cell carcinoma which had involvement of entire scalp was subjected to VMAT using TrueBeam linear accelerator. As it was a rare skin malignancy, CT data of 7 patients with brain tumors were also included in this study, and their entire scalps were outlined as target volumes. Three VMAT plans were employed with RapidArc form: two half-field full-arcs VMAT using 6 MV standard beams (HFF-VMAT-FF), eight half-field quarter-arcs VMAT using 6 MV standard beams (HFQ-VMAT-FF), and HFQ-VMAT using FFF beams (HFQ-VMAT-FFF). Prescribed dose was 25×2 Gy (50 Gy). Plan quality and efficiency were assessed for all plans.

Results

There were no statistically significant differences among the three VMAT plans in target volume coverage, conformity, and homogeneity. For HFQ-VMAT-FF plans, there was a significant decrease by 12.6% in the mean dose to the brain compared with HFF-VMAT-FF. By the use of FFF beams, the mean dose to brain in HFQ-VMAT-FFF plans was further decreased by 7.4% compared with HFQ-VMAT-FF. Beam delivery times were similar for each technique.

Conclusions

The HFQ-VMAT-FF plans showed the superiority in dose distributions compared with HFF-VMAT-FF. HFQ-VMAT-FFF plans might provide further normal tissue sparing, particularly in the brain, showing their potential for radiation therapy in squamous cell carcinoma of the scalp.     

Energy spectrum and dose enhancement due to the depth of the Lipiodol position using flattened and unflattened beams

Aim

Lipiodol was used for stereotactic body radiotherapy combining trans arterial chemoembolization. Lipiodol used for tumour seeking in trans arterial chemoembolization remains in stereotactic body radiation therapy. In our previous study, we reported the dose enhancement effect in Lipiodol with 10× flattening-filter-free (FFF). The objective of our study was to evaluate the dose enhancement and energy spectrum of photons and electrons due to the Lipiodol depth with flattened (FF) and FFF beams.

Characterization of a microDiamond detector in high-dose-per-pulse electron beams for intra operative radiation therapy

PurposeTo characterize a synthetic diamond dosimeter (PTW Freiburg microDiamond 60019) in high dose-per-pulse electron beams produced by an Intra Operative Radiation Therapy (IORT) dedicated accelerator.MethodsThe dosimetric properties of the microDiamond were assessed under 6, 8 and 9 MeV electron beams by a NOVAC11 mobile accelerator (Sordina IORT Technologies S.p.A.).The characterization was carried out with dose-per-pulse ranging from 26 to 105 mGy per pulse. The microDiamond performance was compared with an Advanced Markus ionization chamber and a PTW silicon diode E in terms of dose linearity, percentage depth dose (PDD) curves, beam profiles and output factors.ResultsA good linearity of the microDiamond response was verified in the dose range from 0.2 Gy to 28 Gy. A sensitivity of 1.29 nC/Gy was measured under IORT electron beams, resulting within 1% with respect to the one obtained in reference condition under ⁶⁰Co gamma irradiation. PDD measurements were found in agreement with the ones by the reference dosimeters, with differences in R₅₀ values below 0.3 mm. Profile measurements evidenced a high spatial resolution of the microDiamond, slightly worse than the one of the silicon diode. The penumbra widths measured by the microDiamond resulted approximately 0.5 mm larger than the ones by the Silicon diode. Output factors measured by the microDiamond were found within 2% with those obtained by the Advanced Markus down to 3 cm diameter field sizes.ConclusionsThe microDiamond dosimeter was demonstrated to be suitable for precise dosimetry in IORT applications under high dose-per-pulse conditions.     

Analysis of physical parameters and determination of inflection point for Flattening Filter Free beams in medical linear accelerator

Background

Medical Linear accelerators manufactured without flattening filters are increasing popular in recent days. The removal of flattening filter results in increased dose rate, reduced mean energy, reduction in head leakage and lateral scattering, which have shown advantageous when used for special treatment procedures.

Aim

This study aims to analyze physical parameters of FFF beams and to determine the inflection point for standardizing the beam flatness and penumbra.

Materials and methods

The beam profiles and depth dose patterns were measured using Radiation Field Analyzer (RFA) with 0.13 cc cylindrical ion chamber. The beam energy characteristics, head scatter factor (Sc) were obtained for 6FFF and 10FFF beams and compared with 6 MV and 10 MV photons, respectively. The symmetry and stability of unflattened regions were also analyzed. In addition, the study proposes a simple physical concept for obtaining inflection point for FFF beams and results were compared using the Akima spline interpolation method. The inflection point was used to determine the field size and penumbra of FFF beams.

Results

The Sc varied from 0.922 to 1.044 for 6FFF and from 0.913 to 1.044 for 10FFF with field sizes from 3 cm × 3 cm to 40 cm × 40 cm which is much less than FF beams. The obtained value of field size and penumbra for both simple physical concept and Akima spline interpolation methods is within the ±1.0 mm for the field size and ±2 mm penumbra. The results indicate that FFF beams reduce Sc compared with FF beams due to the absence of a flattening filter.

Conclusion

The proposed simple method to find field size and penumbra using inflection point can be accepted as it is closely approximated to mathematical results. Stability of these parameters was ascertained by repeated measurements and the study indicates good stability for FFF beam similar to that of FF beams.     

Software simulation of tumour motion dose effects during flattened and unflattened ITV-based VMAT lung SBRT

PurposeRestricted studies comparing different dose rate parameters are available while ITV-based VMAT lung SBRT planning leads to perform the analysis of the most suitable parameters of the external beams used. The special emphasis was placed on the impact of dose rate on dose distribution variations in target volumes due to interplay effects.MethodsFour VMAT plans were calculated for 15 lung tumours using 6 MV photon beam quality (flattening filter FF vs. flattening filter free FFF beams) and maximum dose rate of 600 MU/min, 1000 MU/min and 1400 MU/min. Three kinds of motion simulations were performed finally giving 180 plans with perturbed dose distributions.Results6FFF-1400 MUs/min plans were characterized by the shortest beam on time (1.8 ± 0.2 min). Analysing the performed motion simulation results, the mean dose (Dmean) is not a sensitive parameter to related interplay effects. Looking for local maximum and local minimum doses, some discrepancies were found, but their significance was presented for individual patients, not for the whole cohort. The same was observed for other verified dose metrics.ConclusionsGenerally, the evaluation of VMAT robustness between FF and FFF concepts against interplay effect showed a negligible effect of simulated motion influence on tumour coverage among different photon beam quality parameters. Due to the lack of FFF beams, smaller radiotherapy centres are able to perform ITV-based VMAT lung SBRT treatment in a safe way. Radiotherapy department having FFF beams could perform safe, fast and efficient ITV-based VMAT lung SBRT without a concern about significance of interplay effects.     

Dose response of a radiophotoluminescent glass dosimeter for TomoTherapy,CyberKnife, and flattening-filter-free linear accelerator output measurements in dosimetry audit

PurposeWe experimentally determined the radiophotoluminescent glass dosimeter (RPLD) dose responses for TomoTherapy, CyberKnife, and flattening-filter-free (FFF) linear accelerator (linac) outputs for dosimetry audits in Japan.MethodsA custom-made solid phantom with a narrow central-axis spacing of three RPLD elements was used for output measurement to minimise the dose-gradient effect of the non-flattening filter beams. For RPLD dose estimation, we used the ISO 22127 formalism. Additional unit-specific correction factors were introduced and determined via the measured data. For TomoTherapy (7 units) and CyberKnife (4 units), the doses were measured under machine-specific reference fields. For FFF linac (5 units), in addition to the reference condition, we obtained the field-size effects for the range from 5×5 cm to 25×25 cm.ResultsThe correction factors were estimated as 1.008 and 0.999 for TomoTherapy and CyberKnife, respectively. For FFF linac, they ranged from 1.011 to 0.988 for 6 MV and from 1.011 to 0.997 for 10 MV as a function of the side length of the square field from 5 to 25 cm. The estimated uncertainties of the absorbed dose to water measured by RPLD for the units were 1.32%, 1.35%, and 1.30% for TomoTherapy, CyberKnife, and FFF linac, respectively. A summary of the dosimetry audits of these treatment units using the obtained correction factors is also presented. The average percentage differences between the measured and hospital-stated doses were <1% under all conditions.ConclusionRPLD can be successfully used as a dosimetry audit tool for modern treatment units.     

The effect of beam shape on physical parameters of head and neck simultaneous-integrated boost intensity-modulated radiation therapy

Aim

To evaluate the influence of the beam shape created by X-rays with “flat beams” and without “flattening-filter-free [FFF] beams” a flattening filter, and the isocenter locations for FFF beams on the treatment of a large irradiated volume for tumours.

Evaluation of Gafchromic® EBT3 films characteristics in therapy photon,electron and proton beams

PurposeTo evaluate the uncertainties and characteristics of radiochromic film-based dosimetry system using the EBT3 model Gafchromic^® film in therapy photon, electron and proton beams.Material and methodsEBT3 films were read using an EPSON Expression 10000XL/PRO scanner. They were irradiated in five beams, an Elekta SL25 6 MV and 18 MV photon beam, an IBA 100 MeV 5 × 5 cm² proton beam delivered by pencil-beam scanning, a 60 MeV fixed proton beam and an Elekta SL25 6 MeV electron beam. Reference dosimetry was performed using a FC65-G chamber (Elekta beam), a PPC05 (IBA beam) and both Markus 1916 and PPC40 Roos ion-chambers (60 MeV proton beam). Calibration curves of the radiochromic film dosimetry system were acquired and compared within a dose range of 0.4–10 Gy. An uncertainty budget was estimated on films irradiated by Elekta SL25 by measuring intra-film and inter-film reproducibility and uniformity; scanner uniformity and reproducibility; room light and film reading delay influences.ResultsThe global uncertainty on acquired optical densities was within 0.55% and could be reduced to 0.1% by placing films consistently at the center of the scanner. For all beam types, the calibration curves are within uncertainties of measured dose and optical densities. The total uncertainties on calibration curve due to film reading and fitting were within 1.5% for photon and proton beams. For electrons, the uncertainty was within 2% for dose superior to 0.8 Gy.ConclusionsThe low combined uncertainty observed and low beam and energy-dependence make EBT3 suitable for dosimetry in various applications.     

Absorption ratio of treatment couch and effect on surface and build-up region doses

Aim

In this study, at different fields, energies and gantry angles, treatment couch and rails dose absorption ratio and treatment couch effect on surface and build-up region doses were examined.

Surface dose measurements with GafChromic EBT film for 6 and 18 MV photon beams

The aim of this study was to determine the surface doses using GafChromic EBT films and compare them with plane-parallel ionization chamber measurements for 6 and 18 MV high energy photon beams. The measurements were made in a water equivalent solid phantom in the build-up region of the 6 and 18 MV photon beams at 100 cm SSD for various field sizes. Markus type plane-parallel ion chamber with fixed-separation between collecting electrodes was used to measure the percent depth doses. GafChromic EBT film measurements were performed both on the phantom surface and maximum dose depth at the same geometry with ion chamber measurements. The surface doses found using GafChromic EBT film were 15%, 20%, 29%and 39% ± 2% (1SD) for 6 MV photons, 6%, 11%, 23% and 32% ± 2% (1SD) for 18 MV photons at 5, 10, 20 and 30 cm² field sizes, respectively. GafChromic EBT film provides precise measurements for surface dose in the high energy photons. Agreement between film and plane-parallel chamber measurements was found to be within ±3% for 18 MV photon beams. There was 5% overestimate on the surface doses when compared with the plane-parallel chamber measurements for all field sizes in the 6 MV photon beams.     

Assessment of a conventional volumetric-modulated arc therapy knowledge-based planning model applied to the new Halcyon© O-ring linac in locoregional breast cancer radiotherapy

IntroductionThe aim of this study was to evaluate the performance of a knowledge-based planning (KBP) model for breast cancer trained on plans performed on a conventional linac with 6 MV FF (flattening filter) beams and volumetric-modulated arc therapy (VMAT) for plans performed on the new jawless Halcyon© system with 6 MV FFF (flattening filter-free) beams.Materials and methodsBased on the RapidPlan© (RP) KBP optimization engine, a DVH Estimation Model was first trained using 56 VMAT left-sided breast cancer treatment plans performed on a conventional linac, and validated on another 20 similar cases (without manual intervention). To determine the capacity of the model for Halcyon©, an additional cohort of 20 left-sided breast cancer plans was generated with RP and analyzed for both TrueBeam© and Halcyon© machines. Plan qualities between manual vs RP (followed by manual intervention) Halcyon© plans set were compared qualitatively by blinded review by radiation oncologists for 10 new independent plans.ResultsHalcyon© plans generated with the VMAT model trained with conventional linac plans showed comparable target dose distribution compared to TrueBeam© plans. Organ sparing was comparable between the 2 devices with a slight decrease in heart dose for Halcyon© plans. Nine out of ten automatically generated Halcyon© plans were preferentially chosen by the radiation oncologists over the manually generated Halcyon© plans.ConclusionA VMAT KBP model driven by plans performed on a conventional linac with 6 MV FF beams provides high quality plans performed with 6 MV FFF beams on the new Halcyon© linac.     

Influence of 6 MV and 20 MV X-radiation dose rate on in vitro survive of the K-562 cell line

BackgroundAnalysis of the survival rate of cells after irradiation with a specified dose of X-radiation might be one of the basic foundations for assessment of biological implications of ionizing radiation. Investigation of the influence of X-radiation dose rate on cells was carried out in vitro using the SF2 test.AimThe aim of this study was to investigate the influence of X-radiation dose rate on the surviving fraction of the K-562 cell line for two photon energies of 6 MV and 20 MV.Materials/MethodsTo measure the cells' reaction to X-radiation of variable dose rate human leukaemic K-562 cells were used. In order to fulfil the main aim of the study, the cell line was subjected to irradiation at two different dose rates. Total dose applied at once was 2 Gy. A quantitative evaluation of cell survival rate was carried out at every step of the experiment using a clonogenic assay.ResultsHigh dose rate at the energy of 6 MV decreased the percentage of surviving cells to 23%, while lower dose rate decreased it only to 36%. A similar effect is observed at the energy of 20MV-namely at the higher dose rate the percentage of surviving cells is 18%, whereas at the lower one it is only 34%.ConclusionsThe experiment has shown that when using a lower dose rate, the biological effect of ionizing radiation is less pronounced. However, at a higher dose rate higher radiosensitivity of cells is observed.     

Time motion study to evaluate the impact of flattening filter free beam on overall treatment time for frameless intracranial radiosurgery using Varian TrueBeam® linear accelerator

BackgroundThe aim was to study the impact of the flattening filter free (FFF) beam on overall treatment time for frameless intracranial radiosurgery using TrueBeam^® LINAC.The development of frameless stereotactic radiosurgery (SRS) is possible due to the incorporation of image guidance in the delivery of treatment. It is important to analyze the cost and benefits of FFF beams for treating SRS by understanding the impact of FFF beams in reducing the treatment time.Materials and methodsDynamic conformal arc (DCA ) and volumetric arc therapy (VMAT) plans were generated using 6 MV with a flattening filter (FF) and FFF beams. Overall treatment time was divided into beam on time (BOT) and beam off time (BFT). Percentage beam on time reduction (PBOTR) and Percentage total time reduction (PTTR) factors were defined for the comparison.ResultsBOT reduction was observed to be significant for higher dose per fraction but subjected to the treatment technique and modulation differences. PBOTR values are much higher than PTTR values. The 39.9% of PBOTR resulted in only 8% PTTR for DCA and 65.3% resulted in 15.9% PTTR for VMAT.ConclusionMajor BFT was utilized for imaging and verification. FFF beam significantly reduced the beam on time and was found to be most effective if the fractional dose was as high as that for SRS. Newly defined PBOTR and PTTR factors are very useful indicators to evaluate the efficacy of FFF beams in terms of time reduction.     

Dosimetric impact of failing to apply correction factors to ion recombination in percentage depth dose measurements and the volume-averaging effect in flattening filter-free beams

PurposeTo examine whether it is essential to apply correction factors for ion recombination (k_S) to percentage depth dose (PDD) measurements and to the volume-averaging effect (k_vol) to ensure accurate absolute dose calibration for flattening filter-free (FFF) beams for the most commonly used ionization chambers.MethodsWe surveyed medical physicists worldwide (n = 159) to identify the five most common ionization chamber combinations used for absolute and relative reference dosimetry of FFF beams. We then assessed the overall absolute dose calibration error for FFF beams of the Artiste Siemens and TrueBeam Varian linear accelerators resulting from failing to apply correction factors k_S in the PDD(10) and the volume-averaging effect (k_vol) to such chamber combinations.ResultsAll the chamber combinations examined—the Farmer PTW 30013 ionization chamber used for absolute dosimetry, and the PTW 31010, PTW 30013, IBA CC04, IBA CC13, and PTW 31021 ionization chambers used for PDD curves measurements—showed non-negligible errors (≥0.5%). The largest error (1.6%) was found for the combination of the Farmer PTW 30013 chamber with the IBA CC13 chamber, which was the most widely used chamber combination in our survey.ConclusionsBased on our findings, we strongly recommend assessing the impact of failing to apply correction factors k_S in the PDD(10) and k_vol prior to using any chamber type for FFF beam reference dosimetry purposes.     

Dose rate effects of low-LET ionizing radiation on fish cells

Radiobiological responses of a highly clonogenic fish cell line, eelB, to low-LET ionizing radiation and effects of dose rates were studied. In acute exposure to 0.1–12 Gy of gamma rays, eelB’s cell survival curve displayed a linear–quadratic (LQ) relationship. In the LQ model, α, β, and α/β ratio were 0.0024, 0.037, and 0.065, respectively; for the first time that these values were reported for fish cells. In the multi-target model, n, D _o, and D _q values were determined to be 4.42, 2.16, and 3.21 Gy, respectively, and were the smallest among fish cell lines being examined to date. The mitochondrial potential response to gamma radiation in eelB cells was at least biphasic: mitochondria hyperpolarized 2 h and then depolarized 5 h post-irradiation. Upon receiving gamma rays with a total dose of 5 Gy, dose rates (ranging between 83 and 1366 mGy/min) had different effects on the clonogenic survival but not the mitochondrial potential. The clonogenic survival was significantly higher at the lowest dose rate of 83 mGy/min than at the other higher dose rates. Upon continuous irradiation with beta particles from tritium at 0.5, 5, 50, and 500 mGy/day for 7 days, mitochondria significantly depolarized at the three higher dose rates. Clearly, dose rates had differential effects on the clonogenic survival of and mitochondrial membrane potential in fish cells.