Beam-on Time Research Articles

Dosimetric Parameters in Hypofractionated Stereotactic Radiotherapy for Brain Metastases: Do Flattening Filter-Free Beams Bring Benefits? A Preliminary Study.

This study aimed to compare the dosimetric results of flattening filter-free (FFF) vs. flattened (FF) treatment plans for fractionated stereotactic radiotherapy (fSRT), with the goal to highlight potential advantages of FFF beams. A group of 18 patients with brain metastases treated with fSRT (30 Gy delivered in 5 fractions) were included. The dosimetric parameters evaluated were: (1) physical dosimetric parameters (number of monitor units (MUs), conformity index (CI), dose gradient index (DGI), beam on time (BOT)); (2) clinical dosimetric parameters pertaining to target volume (PTV) and organs at risk (OARs). Two treatment plans were performed for all patients: one used 6 MV FFF beams and the other used 6 MV flattened beams. A slight increase in MUs was observed for the FFF mode (+23.3 MUs). The CI showed a difference of -2.7% for the FF plans (p = 0.28), correlated with a poorer coverage of the PTV. DGI values reported in terms of PTV are in line with international recommendations and showed a +1.9% difference for FFF plans. An average BOT of 90.3 s was reported for FFF plans, which was 2.3 times shorter than that required for FF plans delivery (p ≤ 0.001). A slight decrease of PTV coverage (-1.26%, p = 0.036) for FF plans can be considered relevant, but no other significant differences were observed between the two optimizations. No statistically significant benefit of using FFF beams to reduce V20 for normal brain could be demonstrated. These dosimetric results encourage the implementation of fSRT with standard flattened beams in centers where FFF linacs are not available.

Dosimetric accuracy of the Convolution algorithm for Leksell Gamma Plan radiosurgery treatment planning: Evaluation in the presence of clinically relevant inhomogeneities.

The Leksell Gamma Plan Convolution algorithm (LGP-Convolution) has not been widely adopted. This mainly stems from the higher calculated beam-on times relative to the standard ray tracing-based LGP-TMR10 dose calculation algorithm. This study aims to evaluate the accuracy of the LGP-Convolution in scenarios where the treated lesions are in the vicinity of or encompassed by bone and/or air inhomogeneities. The solid water dosimetry phantom provided by the vendor was modified to include bone and air inhomogeneities. Two treatment planning scenarios were investigated involving a single shot and multiple shots, respectively. Treatment planning and dose prescription were performed using the LGP-Convolution algorithm. Triple channel film dosimetry was performed using GafChromic EBT3 films calibrated in terms of absorbed dose to water in a 60 Co beam. Monte Carlo (MC) simulation dosimetry was also performed in the inhomogeneous experimental geometry using the EGSnrc MC platform and a previously validated sector-based phase-space source model. MC simulations were also employed to determine correction factors required for converting EBT3 measurements at points within the bone and air inhomogeneities from dose-to-water values to the corresponding dose to medium values. EBT3 dose to medium correction factors ranged with field size (4, 8, or 16mm) within 0.941-0.946 for bone and 0.745-0.749 for air inhomogeneities. An excellent agreement was found between the LGP-Convolution calculations with corresponding EBT3 and MC dose to medium results at all measurement points, except those located inside the air inhomogeneity. The latter is of no clinical importance and excluding them yielded gamma index passing rates of nearly 100% for 3% local dose difference and 1mm distance-to-agreement criteria. The excellent agreement observed between LGP-Convolution calculations and film as well as MC results of dose to medium indicates that the latter is the quantity reported by the LGP-Convolution.

Total marrow irradiation versus total body irradiation using intensity-modulated helical tomotherapy

BackgroundTotal body irradiation (TBI) is often a component of the conditioning regimen prior to hematopoietic stem cell transplantation in patients with hematological malignancies. However, total marrow irradiation (TMI) could be an alternative method for reducing radiation therapy-associated toxicity, as it specifically targets the skeleton and thus could better protect organs at risk. Here, we compared dosimetric changes in irradiation received by the target volume and organs at risk between TBI and TMI plans.Materials and methodsTheoretical TMI plans were calculated for 35 patients with various hematological malignancies who had already received TBI in our clinic. We then statistically compared irradiation doses between the new TMI plans and existing TBI plans. We examined whether TMI provides greater protection of organs at risk while maintaining the prescribed dose in the targeted skeletal area. We also compared beam-on times between TBI and TMI.ResultsTMI planning achieved significant reductions in the mean, minimum, and maximum irradiation doses in the lungs, kidneys, liver, spleen, and body (i.e., remaining tissue except organs and skeleton). In particular, the mean dose was reduced by 49% in the liver and spleen and by 55–59% in the kidneys. Moreover, TMI planning reduced the corpus beam-on time by an average of 217 s.ConclusionTMI planning achieved significant dose reduction in organs at risk while still achieving the prescribed dose in the target volume. Additionally, TMI planning reduced the beam-on time for corpus plans despite a high modulation factor.Graphical abstract

Dosimetric Comparison of Unmatched Flattening Filter-free and Flattened Beams in Volumetric Arc Therapy Plans for Head-and-neck Cancer.

Conventionally, the fattened beam is being used in radiotherapy for routine clinical cases even after introduction of intensity-modulated radiotherapy with incorporation of multi-leaf collimator system. With the removal of the flattening filter (FF) from the beam's path average energy of the photon gets reduced resulting in reduced scatter, reduction in treatment time, and reduced neutron contamination for high-energy beam and ultimately resulting in treatment plan quality deviations. This study aims to investigate the usefulness of the FF-free (FFF) beam for routine head-and-neck cancer (HNC) cases treated with volumetric arc therapy (VMAT) and dosimetrically compares the result with the FF beam. In this study, 20 patients treated on HalcyonTM (unmatched 6 megavoltage [6MV] FFF beam) medical linear accelerator with VMAT of different HNC selected and for comparison with 6MV FF beam, 20 equivalent treatment plans are created for TruebeamTM configuration and the plans have been evaluated for target coverage, doses to the organ at risk (OAR), and other dose quality indices. Comparable target coverage, doses to OARs except for rest right parotid (P = 0.02) between 6MV FFF beam and 6MV FF beam is observed. Insignificant differences in conformity index, homogeneity index, and gradient index have been observed. Higher monitor unit (MU) (P ≤ 0.001) and lesser beam on time (BOT) (P = 0.003) have been observed in 6MV FFF. 6MV FFF beam provides comparable target coverage and improved dose-sparing effect to most of the OARs. 6MV FFF beam has lesser BOT, but on the other hand number of MUs is higher as compared to 6MV-FF plans.

A planning-based feasibility study of MR-Linac treatment for anal cancer radiation therapy.

The hybrid magnetic resonance image (MRI) scanner and radiation therapy linear accelerator (MR-Linac) has the potential to enhance clinical outcomes for anal cancer (AC) patients with improved soft tissue visualization and daily plan adaption but has planning and delivery limitations due to the incorporation of MRI. We aimed to identify if Elekta Unity MR-Linac-based radiation therapy is feasible for anal cancer. Ten prospectively enrolled AC patients treated with radical chemoradiotherapy were replanned for MR-Linac treatment using departmental planning criteria. For comparison, and to reduce interobserver variability, volumetric modulated arc radiation therapy (VMAT) plans were also created for each patient by the same single senior radiation therapist. Plans were compared using departmental dosimetric plan criteria, as well as conformity and homogeneity indices, monitor units (MUs) and measured plan delivery (beam-on) time. Results were deemed clinically acceptable. Target and organ at risk (OAR) doses were comparable between MR-Linac plans and VMAT plans, although PTV45Gy D98% coverage was compromised in 3 of 10 MR-Linac plans due to caudocranial length exceeding the limits of the MR-Linac. MR-Linac plans had lower MUs, median of 689.1 vs 849.65 (p = 0.002), but took over twice as long to deliver, 529.5s vs 224s (p = <0.0001) as VMAT plans. MR-Linac planning and treatment of AC is feasible for a subset of patients. The current physical limitations of the Elekta Unity system mean patients with large caudocranial elective PTV45Gy target volumes may not be covered dosimetrically to the required clinical standard. Longer image verification and treatment delivery times of the MR-Linac also mean patient selection and intrafractional IGRT are likely to be integral to ensuring high quality clinical outcomes in this rare cancer.

Patient wait times for daily outpatient radiotherapy appointments (a single-centre study)

AbstractPurpose:Patient wait time for every single fraction of every patient treated at our centre for the past year has been presented in this study. The waiting time data were analysed across different treatment sites and modalities.Materials and Methods:Between March 2021 and March 2022, all patients and their corresponding recorded measurements of waiting time were analysed. Times recorded included check-in time (CK), scheduled time to start treatment (SC) and beam-on time for the first beam of therapy (ST). SPSS version 18 was used for statistical calculations, correlations and assessing significance.Results:A total of 181 patients were treated during this duration. The total number of radiotherapy (RT) sessions recorded was 3011. Out of these 3011 sessions, number of times treated by rapid arc (RA), intensity-modulated radiotherapy (IMRT), three-dimensional conformal radiotherapy (3DCRT), stereotactic body radiotherapy (SBRT), stereotactic radiosurgery and stereotactic radiotherapy (SRS/SRT) were 68.18%, 30.19%, 0.167%, 0.565% and 0.19%, respectively. The mean (± standard deviation) times for scheduled time to start treatment (SC) to check-in time (CK), SC to ST (beam-on time for the first beam of treatment), CK to ST and (CK or SC) to ST were −14 ± 48 min, 6 ± 50 min, 19 ± 24 min and −4 ± 31 min, respectively.Conclusion:Patient wait times during RT were presented in this study. This study covered the daily waiting times before RT during modern-day RT treatment sessions. This vast series of consecutive patient data will be a valuable resource for the future planning and management of any modern RT department.

Estimating Carbon Dioxide Emissions and Direct Power Consumption of Linear Accelerator–Based External Beam Radiation Therapy

Climate change is one of the direst health threats that humanity faces. We aim to estimate the carbon dioxide (CO2) emissions associated with the energy usage from linear accelerator (LINAC)-based external beam radiation therapy (EBRT) for the most common cancer diagnoses. We identified patients with the 4 most common cancer types treated with curative-intent EBRT. Beam-on time for each fraction was extracted from the treatment planning system and averaged over each site and treatment modality. The power was multiplied by the beam-on time in hours to yield kilowatt hours (kWh). Using the US Environmental Protection Agency Greenhouse Gas Equivalencies calculator, we converted the kWh into estimates of CO2-equivalent emissions for the average US power grid. Idle time of the LINAC was estimated via Varian Medical Systems. A total of 10 patients were included for each of the following modalities: conventionally fractionated for prostate cancer (28 fractions [fx]), prostate stereotactic body radiation therapy (SBRT) (5 fx), 15- and 5-fx regimens for early-stage breast cancer, 3- and 5-fx SBRT regimens for early-stage lung cancer, conventional EBRT (30 fx) for locally advanced lung cancer, and short- (5 fx) and long-course (25-28 fx) for rectal cancer. The modality with the lowest and highest carbon emissions per course, on average, was prostate SBRT (2.18 kg CO2; interquartile range, 1.92-2.30) and conventional treatment for prostate cancer (17.34 kg CO2; interquartile range, 10.26-23.79), respectively. This corresponds to CO2-equivalent emissions of driving an average of 5.4 miles and 41.2 miles in a standard vehicle, respectively. "Standby" mode for a LINAC TrueBeam and Clinac IX uses 112 kWh and 64.8 kWh per day, respectively. We have estimated CO2 emissions arising from direct energy usage of a LINAC for 4 common cancers treated with EBRT. "Standby" mode of a LINAC uses the most energy per day. Comprehensive studies are warranted to minimize the environmental effects of health and cancer care.

Evaluation of Lung and Liver Tumor Dose Coverage Treated With the CyberKnife Synchrony System With Consideration of Measured Tracking Errors.

Lung and liver tumor dose coverage was evaluated for the CyberKnife synchrony respiratory tracking system (SRTS) with consideration of the motion tracking accuracy measured for motion patterns of individual patients. Seven treatment plans of six cases treated with the SRTS were evaluated. The motion phantom was moved with the motion data derived from the treatment log files. A laser emitted from the linac head to the moving phantom block was recorded with a webcam, and the tracking accuracy was evaluated. The dose volume histogram (DVH) of planning target volume (PTV) and gross tumor volume (GTV) were calculated by a pencil beam algorithm with shifting the beams with Gaussian random numbers mimicking the measured tracking errors. The tracking errors measured with the motion phantom in the lateral direction were within ±2 mm for 90% of beam-on time. The tracking errors in the longitudinal direction were within ±3.0 mm and ±1.1 mm for 90% and 50% of beam-on time, respectively. Although one case showed a decrease in the dose covering 95% of PTV (D95%) by 1.8%, the change in the dose covering 99% of GTV (D99%) was within 1%. This study evaluated the motion tracking errors of the SRTS by a motion phantom moved with the patients' respiration signal, and the impact of the tracking errors on the target coverage was calculated. Even for respiratory patterns with large maximum tracking errors, sufficient GTV coverage is achievable if the beam is accurately delivered for high percentage of beam-on time.

Stereotactic radiosurgery for limited brain metastasis using three different techniques: helical tomotherapy, volumetric modulated arc therapy, and cone-based LINAC radiosurgery.

Specific radiation delivered to tumors by stereotactic radiosurgery (SRS) has become widely used in the treatment of brain metastasis. This study aimed to compare radiation therapy planning and its parameters from SRS using three different modalities: helical tomotherapy (HT), volumetric modulated arc therapy (VMAT), and cone-based linac radiosurgery (Cone-based). Each contouring dataset of patents who experienced one to four brain metastasis received SRS in our center was re-planned to create radiation therapy planning in all three treatment systems (HT, VMAT, and Cone-based). The parameters of conformity index (CI), homogeneity index (HI), CI50, and gradient index (CGI) were analyzed to compare the effects of the three techniques. Decision score analysis was used to evaluate the performance on dosimetric and organs-at-risk parameters among the different techniques by applying the Cone-based technique as a benchmark. A total of 21 patients with 39 lesions were included in this study. The results from the decision score analysis demonstrated statistically identical CI, CI50, and CGI values between Cone-based and VMAT for single lesions. For multiple lesions, VMAT also provided better CI when compared to Cone-based technique while HT exhibited the poorest dosimetric parameters. Moreover, VMAT exhibited the lowest BrainV5Gy value and displayed the shortest beam-on time calculation. We have conducted a comprehensive comparison of SRS planning approaches. The Cone-based technique revealed the highest HI value, while VMAT provided the best estimated beam-on time value. HT displayed a feasible SRS modality for single lesions, but not for multiple lesions.

Craniospinal Irradiation: A Dosimetric Comparison Between O-Ring Linac and Conventional C-arm Linac

PurposeThe aim of this study was to perform a dosimetric evaluation between craniospinal irradiation volumetric modulated arc therapy plans designed for an O-Ring and a conventional C-arm Linac. Methods and MaterialsTwo adult patients were selected for this study. Two plans were designed one for a TrueBeam Edge and one for Halcyon O-ring Linac for each patient. The evaluation of the plans was conducted in terms of dose volume histogram analysis of the target volume and organs at risk (OARs) along with total plan monitor units and beam-on time. Paired sample t test was performed to compare Dmax and Dmean of OARs for each plan's comparison. The delivery of volumetric modulated arc therapy plans was evaluated using Octavius 4D phantom. ResultsAll plans demonstrated dose distributions with sufficient planned target volume conformity and homogeneity. The Homogeneity Index and Conformity Index for all plans were found comparable. The paired sample t test did not demonstrate significant difference in terms of Dmax and Dmean of OARs between plans for both patients. All plans met the threshold of 90%, with Halcyon plans having higher gamma passing rates. ConclusionsCraniospinal irradiation plans generated for Halcyon and Edge are equivalent in terms of plan quality and dose sparing at OARs. The small variations may have originated from the differences in beam profile or mean energy, the degree of the modulation for each plan and characteristics of multi leaf collimators for each treatment unit. Halcyon is able to deliver a distinctly faster treatment, but Edge provides an automatic rotational correction for patient positioning.

Calibration and time fading characterization of a new optically stimulated luminescence film dosimeter.

Optically stimulated luminescence (OSL) dosimeters produce a signal linear to the dose, which fades with time due to the spontaneous recombination of energetically unstable electron/hole traps. When used for radiotherapy (RT) applications, fading affects the signal-to-dose conversion and causes an error in the final dose measurement. Moreover, the signal fading depends to some extent on treatment-specific irradiation conditions such as irradiation times. In this work, a dose calibration function for a novel OSL film dosimeter was derived accounting for signal fading. The proposed calibration allows to perform dosimetry evaluation for different RT treatment regimes. A novel BaFBr:Eu2+ -based OSL film (Zeff , 6MV =4.7) was irradiated on a TrueBeam STx using a 6MV beam with setup: 0° gantry angle, 90cm SSD, 10cm depth, 10×10cm2 field. A total of 86 measurements were acquired for dose-rates ( ) of 600, 300, and 200MU/min for irradiation times (tir ) of 0.2, 1, 2, 4.5, 12, and 23min and various readout times (tscan ) between 4 and 1440min from the start of the exposure (beam-on time). The OSL signal, , was modeled via robust nonlinear regression, and two different power-law fading models were tested, respectively, independent (linear model) and dependent on the specific (delivery-dependent model). After 1 day from the exposure, the error on the dose measurement can be as high as 48% if a fading correction is not considered. The fading contribution was characterized by two accurate models with adjusted-R2 of 0.99. The difference between the two models is <4.75% for all and . For different beam-on times, 3, 10.5, and 20min, the optimum was calculated in order to achieve a signal-to-dose conversion with a model-related error <1%. In the case of a 3min irradiation, this condition is already met when the OSL-film is scanned immediately after the end of the irradiation. For an irradiation of 10.5 and 20min, the minimum scanning time to achieve this model-related error increases, respectively, to 30 and 90min. Under these conditions, the linear model can be used for the signal-to-dose conversion as an approximation of the delivery-dependent model. The signal-to-dose function, D(Mi , j , ), has a residual mean error of 0.016, which gives a residual dose uncertainty of 0.5mGy in the region of steep signal fading (i.e., =4min). The function of two variables is representable as a dose surface depending on the signal (Mi , j ) measured for each i,j-pixel and the time of scan ( ). The calibration of a novel OSL-film usable for dosimetry in different RT treatments was corrected for its signal fading with two different models. A linear calibration model independent from the treatment-specific irradiation condition results in a model-related error <1% if a proper scanning time is used for each irradiation length. This model is more practical than the delivery-dependent model because it does not need a pixel-to-pixel fading correction for different .

Dosimetric comparison of volumetric modulated arc therapy with TrueBeam LINAC and hypofractionated radiosurgery with gamma knife ICON for large (>10 cm3) skull base meningiomas.

Multi Fractionated stereotactic radiosurgery (MF-SRS) of Linac has an essential role in the treatment of skull base meningiomas (sbMNG). However, Gamma Knife Icon (GK) allows MF-SRS using mask immobilization with onboard image guidance. This dosimetric study aims to investigate whether equivalent plan quality can be achieved with Volumetric Modulated Arc Therapy (VMAT) in patients with large sbMNG (>10 cm3) previously treated with GK. Twenty patients with the median target volume of 19.7cm3 are re-planned by using VMAT with 20 Gy in 5 fractions. Plan qualities are compared to tumor coverage, paddick conformity index (PCI), gradient index (GI), V4 Gy, V10 Gy, V12 Gy, optic chiasm V20 Gy, brainstem V23 Gy, optic nerve V25 Gy volumes, and maximum doses for all. Additionally, beam-on time and approximate planning time are also analyzed and compared. All plans provide adequate clinical requirements. First, the CI is comparable for the GK and VMAT (0.99±0.01 vs. 1.13±0.20; p = 0.18). Second, VMAT has a significantly higher GI than GK (3.81±0.35 vs. 2.63±0.09; p < 0.001). Third, the PCI is significantly higher in GK than VMAT (0.76±0.05 vs. 0.70±0.07; p < 0.001). The lower GI of the GK also results in significantly lower V4 Gy (156.1±43.8 vs. 207.5±40.1 cm3, p < 0.001) and V10 Gy (26.1±9.0 vs. 28.9±7.7 cm3, p < 0.001) compared to VMAT. Last, the VMAT reduces beam-on time (4.8±0.5 vs. 19±1.1 min.; p < 0.001). Although both systems have succeeded in creating effective plans in clinical practice, the GK reveals more effective lower normal brain tissue doses. However, the shorter treatment time with LINAC, excluding the total procedure time, can be considered advantageous over GK.

A novel and clinically useful weight-optimized dynamic conformal arc in stereotactic radiation therapy of non-small cell lung cancer: Dosimetric comparison of treatment plans with volumetric‐modulated arc therapy

PurposeThis study constitutes a feasibility assessment of dynamic conformal arc (DCA) therapy as an alternative to volumetric‐modulated arc therapy (VMAT) for stereotactic body radiation therapy (SBRT) of lung cancer with the free-breathing technique using four-dimensional computed tomography. MethodsA total of 25 patients who received 50 Gy in four fractions treated using VMAT technique having two partial coplanar arcs with 6 MV beams for non-small cell lung cancer (NSCLC) were included. Plans were re‐planned using a novel and clinically technique of weight optimized based on dynamic conformal Arcs with two coplanar partial dynamic conformal arcs (WO-DCA). For the two partial arcs, MLC aperture around the PTV was automatically generated at different margins for both arcs and maintained dynamically around the target during arc rotation. Weight of the two arcs using optimization method was adjusted between the arcs to maximize tumor coverage and protect organs at risk (OAR). The clinical VMAT and WO-DCA plans were compared via the RTOG-0915 protocol for conformity and dose to the organs at risk (OAR). Additionally, delivery efficiency, quality assurance pass rate, monitor unit and beam treatment time were recorded. ResultsThe mean value of quality assurance (QA) pass rate 98.16 ± 1.27 in WO-DCA and 92.87 ± 1.56 in VMAT. The rate was higher in WO-DCA (p < 0.001 and t = 8.75). The values of beam‐on time (BOT) and monitor units (MU) in the VMAT technique were 4.20 (3.45–4.95) and 3155 (2279–4867) and they were 3.10 (2.85–3.35) and 2167 (1702–2948) in WO-DCA. These values were significantly improved with WO-DCA (p < 0.001 and p < 0.001) ConclusionsAs there is, no beam modulation through the target, WO‐DCA plans could potentially minimize small‐field dosimetry error without MLC interplay effects via respiratory motion and provide similar doses to OAR and the tumor while providing faster treatment delivery by significantly reducing MU and BOT in lung cancer for tumors of appropriate localization. Additionally, providing WO‐DCA eliminate patient‐specific VMAT quality assurance; potentially offering cost‐effective, same day SBRT treatments.

Investigation of optimal planning strategy in gamma knife perfexion for vestibular schwannoma tumor using hybrid plan technique

Purpose. Stereotactic radiosurgery (SRS) for vestibular schwannoma (VS) is clinically challenging because of surrounding critical structures. We generated and compared the forward plan (FP), inverse plan (IP), and hybrid plan (HP) for the optimal planning strategy in Gamma Knife stereotactic radiosurgery (GKSRS) for vestibular schwannoma tumors (VS). Methods and materials. In this study, 51 planning scenarios of 17 patients with VS were planned for GKSRS using FP, IP, and HP in Leksell Gamma plan (LGP10.1) using the TMR10 algorithm. The planning images were obtained using the following MRI (GE, USA) scan parameters: T1W images-MPRAGE sequence, FOV-256 mm × 256 mm, matrix size-512 mm × 512 mm, and the slice thickness 1 mm. The total dose was prescribed12Gy and normalized at 50% isodose level. Results and discussion. The plan parameters were compared dosimetrically by maintaining FP as a base plan. The statistical analysis, including one-factor, repeated measures ANOVA and Bonferroni correction tests, were performed. The p-value for planning parameters such as brainstem dose, beam ON time, and gradient index significantly favored HP. Conclusion. Overall results show that HP is an efficient method for GKSRS of VS The p-value was less than 0.001 and statistically significant for various plan indices.

Stereotactic body radiotherapy using helical tomotherapy for hepatocellular carcinoma: clinical outcome and dosimetric comparison of Hi-ART vs. Radixact.

Helical tomotherapy (HT), a unique rotational dose delivery machine, has been updated from Hi-ART to Radixact. We retrospectively evaluated the treatment outcomes of stereotactic body radiotherapy (SBRT) using HT for hepatocellular carcinoma (HCC) and compared the dosimetric details of Hi-ART and Radixact. Between April 2014 and November 2020, 28 patients with HCC were treated with SBRT using HT for a cure at Soonchunhyang University College of Medicine, Bucheon. According to the Barcelona Clinic Liver Cancer classification, 9 patients had stage 0 disease, 12 had stage A, 4 had stage B, and 3 had stage C. The tumor size ranged from 1 cm to 8 cm (median, 2 cm). The SBRT dose ranged from 40 to 60 Gy (median, 48 Gy) with 4 fractions. Twenty-three patients were treated with Hi-ART and 5 patients were treated with Radixact. To compare the dosimetric parameters between Hi-ART and Radixact, we created treatment plans with the same constraints, pitch, modulation factor, and field width for the same patient in pairs. The median follow-up time from the date of SBRT administration was 24 months (range, 3-67 months). The local failure-free survival and intrahepatic failure-free survival rates were 96% and 58% at 1 year, 84% and 36% at 2 years, and 76% and 18% at 3 years, respectively. The overall survival rate was 93% at 1 year, 93% at 2 years, and 53% at 3 years, respectively. When the paired treatment plans were reviewed, the beam-on time and intermediate dose-spillage were found to be significantly reduced in Radixact than Hi-ART (P<0.001). With regard to normal organ sparing, the irradiated dose to the total liver, normal liver, heart, and kidney was significantly lower with Radixact (P<0.001). SBRT using HT for HCC showed favorable treatment outcomes. Radixact, the latest version, physically improved treatment efficiency by reducing treatment time and provided better organ sparing than Hi-ART.

The Carbon Footprint of Radiation Oncology on Climate Change: A Model in Early-Stage Breast Cancer

<h3>Purpose/Objective(s)</h3> Tipping points in climate change are imminent and with them heightened urgency to identify drivers of carbon (CO₂) emissions. In healthcare, little is known of the influence oncology systems have on the environment. We sought to develop a model that evaluates CO₂ emissions produced due to radiation therapy (RT), incorporating contributions from treatment commute and radiation delivery/fractionation (fx) in early-stage breast cancer (ESBC) patients. <h3>Materials/Methods</h3> To model CO₂ emissions generated per patient at completion of RT for ESBC, we identified key emission-altering variables impacting commute and RT delivery (Table 1). CO₂ emissions from commute were modeled by: 1) average distance to a hospital from rural, suburban, and urban community settings using government data and 2) CO2 release per mile by vehicle type (gas car, electric car, and public transport) per EPA data. CO₂ emissions from RT delivery were modeled by: 1) linear accelerator (LINAC) beam-on power per manufacturing data; 2) beam-on time, estimated by average time for 5 patients receiving moderate hypofx (42.4Gy/16fx) and 5 ultra-hypofx (26Gy/5fx) RT; and 3) LINAC energy source (Es) comparing high CO2 emitting (coal: 820 gCO2/kWh) to low CO2 emitting (nuclear: 12 gCO2/kWh) energy plants per IPCC data. Contributions from commute and RT delivery variables were multiplied by the total fx course, comparing moderate vs ultra-hypofx 3D RT schedules. C02 emission analyses between variables were performed using t-test comparisons. <h3>Results</h3> CO₂ emissions produced from a patient's commute to RT were associated with a 6.5-fold increase (8484 vs 1302 gCO₂) when traveling from a rural area by gas car (least efficient) compared to an urban area via public (metro) transportation (most efficient). During treatment, CO₂ emissions varied by RT fx course and LINAC Es. There was a 4.12-fold increase in CO2 emissions with moderate vs ultra-hypofx RT regimens (150 vs 36 kgCO2, p<0.01) with coal energy. In comparison, a 3.22-fold increase in CO2 emissions was detected with moderate vs ultra-hypofx RT regimens (67 vs 21 kgCO2, p<0.01) with nuclear energy. Finally, for a standard moderate hypofx regimen, coal Es use led to a 68-fold increase (5235 vs 77 gC02, p<0.01) in CO2 emissions vs nuclear Es use. <h3>Conclusion</h3> To our knowledge, we present the first model of key emission-altering variables and CO₂ production in RT of ESBC patients. We identify the largest CO₂ drivers to be related to patient commute and LINAC Es—both of which are exacerbated by extended fx courses. This model provides a foundation to further investigate the impact of cancer care on the environment and inform clinical practice changes related to sustainable energy policies.

Carbon Footprint of Proton Therapy: How Many Trees do We Need to Offset It?

<h3>Purpose/Objective(s)</h3> There is an increasing concern about rising carbon dioxide (CO2) levels and its hazardous impact on human health and climate change. Radiation oncology uses high energy machines; however, their carbon footprint is not well understood. This study quantifies the energy utilization of proton therapy, and estimates corresponding carbon footprint. Additionally, the study evaluates possible ways to offset CO2 with planting new trees. <h3>Materials/Methods</h3> Patients treated between 07/2020 and 06/2021 using Mevion S250 proton passive scattering system were evaluated. Current measurements were recorded at 2 second intervals and converted to kilowatt (kW) power consumption. Patients treated were reviewed for disease, site treated, dose, number of fractions, and duration of beam during each fraction. EPA web calculator was used to convert power consumption to tons of CO2, and the number of new trees required to offset it. <h3>Results</h3> There were 185 patients treated (average 19.9 per day); 63% had full course with protons, 31% had photon boost, and 6% had protons as boost. Total of 5,176 fractions were delivered (average 28 fractions per treatment course). Total BeamOn time was 538,546 seconds (149.6 hours), average 104 seconds per fraction. Power consumption was 55.8 kW in standby/night mode and 64.4 kW during BeamOn. Average daily power use was 1,346 kWh, and annual consumption was ∼490,000 kWh. BeamOn consumption accounted for 1.7% of total machine power draw. Breast cancer with RNI required highest power consumption (140 kWh per patient), while prostate cancer lowest (28 kWh), average across all patients was 52 kWh. Highest cumulative power use was for CNS tumors (2,753 kWh), lowest for lymphomas (208 kWh). Power draw of the clinical and administrative areas was 11 kW, and annual consumption was ∼96,000 kWh, for a program total of 586 MWh. The corresponding carbon footprint for BeamOn time was 6.83 metric tons of CO2, or 37 kg per patient course (highest for breast cancer at 99 kg, lowest for prostate cancer 20 kg). Carbon footprint for the annual machine power consumption was 347.5 tons CO2, and for the proton program was 415.5 tons of CO2. Attributed footprint was thus 415.5 tons / 185 = 2,200 kg CO2 per patient treated. The carbon offset required for BeamOn time would be 113 new trees planted (average 0.6 trees per patient courses). The corresponding offset required to operate the Mevion system for a year would be 5,743 new trees planted, and the proton program composite offset would be 6,867 new trees planted. The attributed carbon offset per patient treated would thus be 37 new trees planted. <h3>Conclusion</h3> Carbon footprint varied by disease treated and was on average 37 kg of CO2 per patient, and 415.5 tons of CO2 for the proton program. The number of new trees to be planted to offset the power use would be 6,867 new trees annually, or 37 per patient treated. We hope this study inspires radiation oncology practices to evaluate their own carbon footprint.

An Optimal Solution Study on a Non-Invasive Stereotactic Radiosurgery Instrument Optimizer for Stereotactic Radiosurgery of Single Brain Metastasis

<h3>Purpose/Objective(s)</h3> This study looks at a fast inverse planning (FIP) optimizer that enables more efficient and less planner dependent stereotactic radiosurgery (SRS) treatment planning. FIP generates a plan using multiple user-inputted variables that affect plan quality metrics, such as target coverage (TC), maximum dose to the target and organs-at-risk, low dose (LD), and beam-on time (BOT). The objective of this study is to systematically characterize each of these variables and determine which combination leads to the best SRS plan quality metrics. <h3>Materials/Methods</h3> Thirty patients with a single brain metastasis who were previously treated with SRS were evaluated. Each lesion was planned using FIP by varying the LD and BOT variables in 0.1 increments from 0.0 to 1.0 (i.e., plan #1 [0.0, 0.0], plan #2 [0.0, 0.1], ..., plan #120 [1.0, 0.9] and plan #121 [1.0, 1.0]). All plans were generated with the same prescription dose (24 Gy/1 fraction), TC (100%), maximum allowed dose (43.2 Gy), and Gamma angle (90°). The following metrics were recorded for each plan to evaluate plan quality: Paddick conformity index (PCI), gradient index (GI), and BOT. Heat maps for each of the above metrics as a function of LD and BOT penalties were analyzed. An objective score matrix was calculated for each plan using a linear weighted combination of scaled and normalized GI, PCI, and BOT. An optimal solution space containing the 5 lowest values of the score matrix per patient was extracted. A frequency histogram was subsequently created in the LD-BOT coordinate space. <h3>Results</h3> The target volumes in the study ranged from 1.0 cc to 2.5 cc. For every patient, 121 plans were generated across the range of LD and BOT penalties, resulting in 3630 plans included in this analysis. The range of PCI, GI, and BOT across all plans were 0.71 to 0.95, 2.5 to 2.9, and 17.2 to 221 minutes, respectively. The variation of the LD/BOT penalties had a much larger impact resulting in a higher deviation about the mean for BOT (169%), followed by PCI (28%). Heat map analysis revealed that the plans that maximized PCI and minimized GI values occupied the upper triangular matrix of the LD/BOT penalty space, while plans that minimized BOT occupied the lower triangular space. The optimal solutions frequently occupied the diagonal space in between these zones. Specifically, the histogram analysis showed that 4 highest frequencies occurred at the user-inputted settings (LD, BOT) of (0.6, 0.5), (0.8, 0.6), (0.7, 0.5), and (0.7, 0.6), in order of plan quality. <h3>Conclusion</h3> In this study, the user-inputted variables (LD and BOT) were systematically varied to determine the optimal solutions to simultaneously maximize PCI while minimizing GI and BOT. The results of this study may permit SRS planners to generate single brain metastasis plans more efficiently using the study-defined optimal settings of LD and BOT.

IMRT and SBRT Treatment Planning Study for the First Clinical Biology-Guided Radiotherapy System

<h3>Purpose/Objective(s)</h3> The first clinical biology-guided radiation therapy (BgRT) system – RefleXion^TM X1 - was installed and commissioned for clinical use at our institution. This study aimed at evaluating the treatment plan quality and delivery efficiency for IMRT/SBRT cases without PET-guidance. <h3>Materials/Methods</h3> Forty-two patient plans across six cancer sites (conventionally-fractionated lung, head and neck, anus, prostate, brain, and lung SBRT) planned with the Eclipse^TM treatment planning system (TPS) and treated with either a TrueBeam® or Trilogy® were selected for this retrospective study. For each Eclipse VMAT plan, two corresponding plans were generated on the X1 TPS with 10 mm jaws (X1 10mm) and 20 mm jaws (X1 20mm) using our institutional planning constraints. All clinically relevant metrics in this study, including PTV D95%, PTV D2%, Conformity Index (CI), R50, organs- at-risk (OAR) constraints, and beam-on time were analyzed and compared between 126 VMAT and RefleXion plans using paired t-tests. <h3>Results</h3> All but three planning metrics were either equivalent or superior for the X1 10mm plans as compared to the Eclipse VMAT plans across all planning sites investigated. The Eclipse VMAT and X1 10mm plans generally achieved superior plan quality and sharper dose fall-off superior/inferior to targets as compared to the X1 20mm plans, however, the X1 20mm plans were still considered acceptable for treatment. Strong correlation was observed between the TPS-calculated beam-on time and PTV length in the superior-inferior direction for the X1 10mm (<i>R</i>² = 0<i>.</i>87) and X1 20mm (<i>R</i>² = 0<i>.</i>85) plans. On average, the required beam-on time increased by a factor of 1.6 across all sites for X1 10mm compared to X1 20mm plans. <h3>Conclusion</h3> Clinically acceptable IMRT/SBRT treatment plans were generated with the X1 TPS for both the 10 mm and 20 mm jaw settings. The X1 10mm plans were equivalent or superior to the Eclipse VMAT plans, however the beam-on times were longer.

Intensity Modulated Radiotherapy on a Novel Ring Gantry-Based Linac with Integrated Dual PET-CT Imaging Systems: Dosimetric Plan Comparison and Initial Clinical Delivery Experience

<h3>Purpose/Objective(s)</h3> This study aims to report our initial clinical experience on using a new external-beam radiotherapy delivery system on a ring gantry with dual PET-CT imaging capabilities for IMRT treatments at different anatomical sites. The study also evaluates the system's plan quality compared to an existing ring gantry-based Linac. This new external-beam radiotherapy delivery system has the potential to be used for biology-guided radiotherapy. <h3>Materials/Methods</h3> We performed a single-institution retrospective review of patients treated on the machine since commissioning. The initial five patients treated to a thoracic site and five patients treated to a pelvic site were included in this study. The dose per fraction ranged from 1.8 to 3.0 Gy. The treatment plans for the machine used a 2-cm jaw size along the longitudinal direction and used the IMRT delivery mode. A 64-leaf binary MLC was used to modulate a flattening filter free 6-MV photon beam. A helical tomotherapy plan (HT plan) using a 2.5-cm jaw size was generated for each RFX plan to serve as the backup plan in clinical treatments. Dosimetric parameters used in institutional treatment protocols were compared between RFX plans and HT plans. Daily delivery records were reviewed, and actual beam-on time was compared between the two machine types. <h3>Results</h3> On average, 12 dosimetric parameters (range: 7 – 14) for critical organs and the primary PTV were used in each treatment plan as clinically relevant planning optimization parameters and were analyzed in this study. Among the critical organ dose parameters, 19 (22.4%) dosimetric parameters were lower by more than 10% in the study's plans compared to the HT plans, while 11 (12.9%) dosimetric parameters were higher by more than 10% in the study's plans compared to the HT plans. The other dosimetric parameters had less than 10% or no clinically relevant difference between the two plans. There was no statistically significant difference in integrated dose between the new machine and HT plans. All the plans were normalized so that the prescription dose covered 95% of the primary planning target volume (PTV). The average D_mean of the primary PTV were 103.5±1.1% and 102.6±0.8% of the prescription dose in the new external-beam radiotherapy delivery system and HT plans, respectively, and the average D(1%) of the primary PTV (dose covering the hottest 1% of the PTV) were 107.1±2.0% and 105.2%±1.6% in the new external-beam radiotherapy delivery system and HT plans, respectively (two-tailed p-value = 0.06 in the paired t-test). The beam-on time was not significantly different between the two machines. The average daily beam-on time was 438.8±151.2 seconds and 427.6±183.1 seconds with the new external-beam radiotherapy delivery system and HT systems, respectively (two-tailed p-value = 0.76 in the paired t-test). <h3>Conclusion</h3> Our initial clinical experience with the external-beam radiotherapy delivery system showed that image guided IMRT could be delivered to different anatomical sites with adequate clinical workflow efficiency. The new machine achieved clinically comparable treatment plan quality compared to helical tomotherapy. The beam-on time was comparable to that on helical tomotherapy.