The Impact of Dose Rate on the Tumor Microenvironment Using Flattening-filter-free Beams

AIMS

Recently, dose delivery technology has rapidly evolved with flattening filter-free beams (FFF), and the biological effects of high dose rates are a matter of interest. We hypothesized that FFF beams at different dose rates obtained with modern linear accelerators have different effects on the TME.

MATERIALS AND METHODS

The B16-F10 melanoma syngeneic tumor model was established, and mice were randomized to 2 different doses (2 Gy and 10 Gy) and 3 different dose rates (1 Gy/min, 6 Gy/min, and 14 Gy/min) along with the control group. Euthanasia was performed on the seventh day after RT, and intracardiac blood was collected for a comet assay. Tumors were harvested and examined histomorphologically and immunohistochemically. Statistical analyses were performed using SPSS software version 23 (SPSS Inc., Chicago, IL, USA).

RESULTS

The daily growth rate was uniform, and no difference was observed between tumor volumes across all three dose rates for each dose. Deoxyribonucleic acid (DNA) damage in blood mononuclear cells was not affected by dose or dose rate. In the TME histomorphological examination, the number of mitosis is less in the 10 Gy arm, whereas the pleomorphism score was greater. Nevertheless, varying dose rates had no effect on the number of mitosis or the pleomorphism score. The severity of the inflammation, cell densities in the TME, and expression of immunohistochemical markers were comparable across all doses and dose rates.

CONCLUSION

In our study involving the B16-F10 syngeneic tumor model, varying dose rates obtained with FFF beams had no effect on tumor volume, blood mononuclear cell DNA damage, or TME parameters. However, in order to fully understand the biological impacts of novel techniques, our study should be validated with alternative preclinical setups.

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

Introduction

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.

Materials and Methods

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.

Results

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.

Conclusion

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.

Linac primary barrier transmission: Flattening filter free and field size dependence

There is widespread consensus in the literature that flattening filter free (FFF) beams have a lower primary barrier transmission than flattened beams. Measurements presented here, however, show that for energy compensated FFF beams, the barrier transmission can be as much as 70% higher than for flattened beams. The ratio of the FFF barrier transmission to the flattened beam barrier transmission increases with increasing barrier thickness. The use of published FFF TVL data for energy compensated FFF beams could lead to an order of magnitude underestimate of the air kerma rate. There are little data in the literature on the field size dependence of the barrier transmission for flattened beams. Barrier transmission depends on the field size at the barrier, not at isocenter Measurements are presented showing the relative dependence of barrier transmission on the field size, measured at the barrier, for 6 MV and 10 MV beams. An analytical fitting formula is provided for the field size dependence. For field sizes greater than about 150 cm in side length, the field size dependence is minimal. For field sizes less than about 100 cm, the transmission declines rapidly as the field size decreases.

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

Background

The 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 methods

Dynamic 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.

Results

BOT 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.

Conclusion

Major 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.

Technical Note: An alternative approach to verify 6FFF beam dosimetry for Ethos and MR Linac without using a 3D water tank

PURPOSE

The current approach to Linac beam dosimetry verification is typically performed utilizing a three-dimensional (3D) water tank system. The 3D beam scanning process is cumbersome, labor intensive, error-prone, and costly. This is especially challenging for the new Ethos system and MR Linacs with a ring gantry. This work proposes an alternative approach to verify 6FFF beam dosimetry for Ethos, ViewRay MRIdian® Linac, and other Linacs with 6FFF beam quality using two-dimensional (2D) ion chamber arrays.

METHODS

Percentage depth dose (PDD) and profiles of an Ethos, an MRIdian® Linac, and several Linacs with 6FFF beams were measured at the nominal beam current. The beam energy was detuned by changing the bending magnet current on one TrueBeam. PDDs and profiles were measured for detuned beam energies. The peak shape of the 6FFF profile was defined by a "slope" parameter and unflatness. Correlations between peak slope and unflatness metrics vs PDDs were used to evaluate the sensitivity of beam energy to beam profile changes at different field sizes and depths.

RESULTS

Strong correlations were found between peak slope and PDDs for all Linacs with 6FFF beam. The R-squared values in the linear regression fitting between PDD and peak slope and unflatness were 0.99 and 0.84, respectively. Both profile slope and unflatness were proportional to PDD at the 10 cm depth and the peak slope was 4.3 times more sensitive than PDD. We have identified that measurements with a shallow depth are preferred to quantify the beam energy consistency.

CONCLUSIONS

Our work shows the feasibility of verifying 6FFF beam quality of Ethos, MR Linac, and other Linacs by defining a profile slope measured from 2D ionization chambers array devices. This new approach provides a simplified method for performing a routine beam quality check without using a 3D water tank system while maximizing cost effectiveness and efficiency.

Is Synchronous Bilateral Breast Irradiation Using Flattening Filter-Free Beam-Based Volumetric-Modulated Arc Therapy Beneficial? A Dosimetric Study

Objective:

The aim of this study is to validate the clinical use of flattening filter-free (FFF) beam-based volumetric-modulated arc therapy (VMAT) in synchronous bilateral breast carcinoma (SBBC) patient treatments and to compare with flattening filtered (FF) beam-based VMAT.

Materials and Methods:

Computed tomography images of 15 SBBC patients were taken for this study. A dose of 50 Gy in 25 fractions was prescribed to planning target volume (PTV). VMAT plans were generated using both FFF and FF 6 MV X-ray beams in Eclipse treatment planning system. PTV and organs at risk (OARs) doses were analyzed quantitatively using dose–volume histograms (DVHs) to meet plan objectives. Pretreatment point and planar dosimetry were performed.

Results:

The findings were reported as mean ± 1 standard deviation. PTV volume receiving 95% of the prescribed dose was 95.71% ± 0.65% for FF-VMAT and 95.45% ± 1.33% for FFF-VMAT (P = 0.743). Conformity index was 1.12 ± 0.31 (FF-VMAT) and 1.12 ± 0.02 (FFF-VMAT). Right lung mean dose was 10.95 ± 1.33 Gy (FF-VMAT) and 10.60 ± 98.5 (FFF-VMAT). Left lung mean dose was 9.73 ± 1.56 (FF-VMAT) and 9.61 ± 1.53 Gy (FFF-VMAT). Tumor control probability (TCP) was 99.68% ± 0.02% (FF-VMAT) and 99.67% ± 0.01% (FFF-VMAT) (P = 0.390). Uncomplicated TCP was 98.72% ± 0.02% (FF-VMAT) and 98.72% ± 0.01% (FFF-VMAT) (P = 0.508).

Conclusion:

The planning objective parameters achieved using FFF-based VMAT showed that FFF can also be used clinically to treat bilateral breast carcinomas and the low-dose lung volumes were still lesser with FFF-VMAT plans than FF-VMAT.

Radioprotective effect of melatonin against radiotherapy-induced cerebral cortex and cerebellum damage in rat

PURPOSE

The present study aims to investigate the radioprotective effect of melatonin (MEL) against early period brain damage caused by different dose rate beams in the experimental rat model.

MATERIALS AND METHODS

Forty-eight Sprague Dawley rats were randomly divided into six groups; the control, only melatonin, low dose rate-radiotherapy (LDR-RT), high dose rate-radiotherapy (HDR-RT) groups and (LDR-RT) + MEL and (HDR-RT) + MEL radiotherapy plus melatonin groups. Each rat administered melatonin was given a dose of 10 mg/kg through intraperitoneal injection, 15 minutes before radiation exposure. The head and neck region of each rat in only radiotherapy and radiotherapy plus melatonin groups was irradiated with a single dose of 16 Gy in LDR-RT and HDR-RT beams. Rats in all groups were examined for histopathology and biochemistry analysis 10 days after radiotherapy.

RESULTS

Comparing the findings for LDR-RT and HDR-RT only radiotherapy groups and the control group, there was a statistically significant difference in histopathological and biochemical parameters, however, melatonin administered in radiotherapy plus melatonin groups contributed improving these parameters (p < .05). There was no statistically significant difference between LDR-RT and HDR-RT beams (p > .05).

CONCLUSIONS

It was concluded that melatonin applied before LDR-RT and HDR-RT radiotherapy protected early period radiotherapy-induced brain damage. The effects of clinically low and high dose beams on the cerebral cortex and cerebellum were investigated histopathologically for the first time. HDR beams can be safely applied in brain radiotherapy. However, more experimental rat and clinical studies are needed to explain the radiobiological uncertainties about the clinic dose rate on different cancerous and healthy tissues.

Radiobiological comparison of flattening filter (FF) and flattening filter-free (FFF) beam in rat laryngeal tissue

PURPOSE

The purpose of this study is to investigate the radioprotective effect of melatonin by analyzing histopathological changes and serum biochemical levels on experimental rat models exposed to flattening filter (FF) and flattening filter-free (FFF) beam.

MATERIALS AND METHODS

Forty-eight healthy adult Sprague Dawley rats were randomly divided into six groups. The control (Group 1) was given no treatment, the melatonin (Group 2) was given 10 mg/kg melatonin only, the FF (Group 3) and FFF (Group 5) were given fractionated dose (Total 32 Gy, 5 consecutive days) radiotherapy only, and the FF plus melatonin (Group 4) and FFF plus melatonin (Group 6) were given 10 mg/kg melatonin 15 minutes prior to irradiation. Rats were examined for histopathology and biochemical analysis 10 days after irradiation.

RESULTS

When results of FF and FFF radiotherapy only groups are compared to control group, statistically significant difference in histopathological and biochemical parameters are observed; however, melatonin administration in radiotherapy plus melatonin groups improved these parameters (p <.05). In addition, there was no statistically significant difference between FF and FFF beams (p > .05).

CONCLUSIONS

The effect of low- and high-dose beams on the rat larynx and serum samples were investigated histopathologically and biochemically for the first time. We observed that melatonin supplemented before FF and FFF radiotherapy protected early period radiotherapy-induced laryngeal mucosal damage. Since the radiobiological results of FF and FFF beams are similar, FFF beams can be safely applied in laryngeal irradiation. However, more experimental rat and clinical studies are needed to clarify the radiobiological uncertainy concerning dose rate on cancerous and healthy tissue.

Secondary malignancy risk for patients with localized prostate cancer after intensity-modulated radiotherapy with and without flattening filter

Men treated for localized prostate cancer by radiotherapy have often a remaining life span of 10 yr or more. Therefore, the risk for secondary malignancies should be taken into account. Plans for ten patients were evaluated which had been performed on an Oncentra® treatment planning system for a treatment with an Elekta Synergy™ linac with Agility™ head. The investigated techniques involved IMRT and VMTA with and without flattening filter. Different dose response models were applied for secondary carcinoma and sarcoma risk in the treated region and also in the periphery. As organs at risk we regarded for carcinoma risk urinary bladder, rectum, colon, esophagus, thyroid, and for sarcoma risk bone and soft tissue. The excess absolute risk (EAR) was found very similar in the treated region for both techniques (IMRT and VMAT) and also for both with and without flattening filter. The secondary sarcoma risk resulted about one magnitude smaller than the secondary carcinoma risk. The EAR to the peripheral organs was statistically significant reduced by application of the flattening filter free mode concerning the flattening filter as main source of scattered dose. Application of flattening filter free mode can thus support to reduce second malignancy risk for patients with localized prostate cancer.

Initial Clinical Experience Treating Patients With Lung Cancer on a 6MV-Flattening-Filter-Free O-Ring Linear Accelerator

Introduction Modern technologies, like intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT), have improved the therapeutic ratio of thoracic radiotherapy (TRT) for lung cancer (LC). Halcyon™ (Varian Medical Systems, Palo Alto, CA, USA), a novel 6MV-flattening-filter-free O-ring linear accelerator (6X-FFF ORL), was designed to deliver IMRT and VMAT with greater speed than a C-arm linac. Herein, we report our initial clinical experience treating patients with LC on this linac. Methods All patients who received TRT for LC on the 6X-FFF ORL at our institution were retrospectively identified. Patients' clinicopathologic data, radiotherapy details, early disease-control and toxicity outcomes, dosimetric data, couch corrections, and treatment times are reported. Results Between 10/2018-12/2019, 30 consecutive patients (median age 66 years, range 54-94 years) received definitive or post-operative TRT for LC (median 66 Gy/33 fractions; range 5-70 Gy/2-37 fractions) following four-dimensional computed tomography (CT) simulation (97%) using daily kilovoltage KV cone-beam CT (CBCT) (100%) on a 6X-FFF ORL for non-small cell LC (84%) or small cell LC (16%), with 53% receiving VMAT, 43% receiving static-field IMRT, and 77% receiving concurrent systemic therapy. All plans were approved through institutional peer review. The average three-dimensional vector couch correction based on CBCT guidance was 0.90 ± 0.50 cm. The average beam-on and beam on plus CBCT times were 1.7 ± 1.1 min, and 5.0 ± 3.2 min, respectively. Grade 3 dyspnea and fatigue occurred in 3% and 3% of patients, respectively. There were no grade ≥4 toxicities. Conclusion In this first clinical report of TRT for LC on a 6X-FFF ORL, daily CBCT-guided treatment was fast and safe with respect to dosimetry and clinical outcomes. Thus, use of this linac for TRT may increase LC patient throughput without a detriment in radiotherapy quality.

Dosimetric Comparison of Hypofractionated Multi-Beam Intensity-Modulated Radiation Therapy and Volumetric Modulated Arc Therapy With Flattened Beam and Flattening-Filter-Free Beam for Skull Base Meningioma Adjacent to Optic Pathways

Background Since the optic pathways are the most vulnerable to radiation, the treatment of skull base tumors involving them is challenging. In this study simulation plans by multi-beam (MB) intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), both with the flattened beam (FB) and flattening-filter-free beam (FFF), were compared in terms of covering of the target and sparing of the optic pathways. Materials and methods Treatment planning was simulated by MB-IMRT with FB and FFF and by 2-rotational VMAT with FB and FFF in three cases of skull base meningioma [volume of the planned target volume (PTV; PTV margin=2 mm except for overlapping area with optic pathways or brainstem): 8.6 ml, 34.6 ml, and 55.3 ml respectively], which were treated previously by multi-fractionated MB-IMRT [45 Gy/18 fx. (fraction) with 7-, 6-, and 5-beam] using a conventional Novalis (BrainLAB, Tokyo, Japan) planned by iPlan (BrainLAB, Tokyo, Japan). In all three cases, the optic pathways were adjacent to the lesion. The reference CT with contouring data set of target volumes [gross tumor volume (GTV) and PTV] and OARs (organs at risk) was transferred from iPlan to Eclipse (Varian Medical Systems, Tokyo, Japan). In this study, hypofractionated radiation therapy by 30 Gy/5 fx. was designed; 95% dose (28.5 Gy/5 fx.) was prescribed to D95 (dose to 95% volume of PTV). Conformity index (CI), homogeneity index (HI, D5/D95), D[0.1 ml] (dose to 0.1 ml) for optic pathways, and D[1 ml] for brainstem and eyes, and V[20 Gy] (volume delivered with 20 Gy or more/5 fx.) of the whole brain were evaluated. Results The indices did not differ between FB and FFF, in either MB-IMRT or VMAT. Between MB-IMRT and VMAT, the indices were similar. The mean dose of PTV and HI was a little larger with MB-IMRT than with VMAT. D[0.1 ml] of the optic pathways and D[1 ml] of the ipsilateral eye were smaller with VMAT in all three cases. D[1 ml] of the brainstem was smaller with VMAT in two cases, though it was similar in one case. Conclusion Based on our findings, VMAT with FFF might be the optimal method to treat cases of skull base meningioma involving optic pathways. However, further studies involving more cases are required to arrive at a conclusive verdict.

Beam flatness modulation for a flattening filter free photon beam utilizing a novel direct leaf trajectory optimization model

Flattening filter free (FFF) linear accelerators produce a fluence distribution that is forward peaked. Various dosimetric benefits, such as increased dose rate, reduced leakage and out of field dose has led to the growth of FFF technology in the clinic. The literature has suggested the idea of vendors offering dedicated FFF units where the flattening filter (FF) is removed completely and manipulating the beam to deliver conventional flat radiotherapy treatments. This work aims to develop an effective way to deliver modulated flat beam treatments, rather than utilizing a physical FF. This novel optimization model is an extension of the direct leaf trajectory optimization (DLTO) previously developed for volumetric modulated radiation therapy (VMAT) and is capable of accounting for all machine and multileaf collimator (MLC) dynamic delivery constraints, using a combination of linear constraints and a convex objective function. Furthermore, the tongue and groove (T&G) effect was also incorporated directly into our model without introducing nonlinearity to the constraints, nor nonconvexity to the objective function. The overall beam flatness, machine deliverability, and treatment time efficiency were assessed. Regular square fields, including field sizes of 10 × 10 cm² to 40 × 40 cm² were analyzed, as well as three clinical fields, and three arbitrary contours with "concave" features. Quantitative flatness was measured for all modulated FFF fields, and the results were comparable or better than their open FF counterparts, with the majority having a quantitative flatness of less than 3.0%. The modulated FFF beams, due to the included efficiency constraint, were able to achieve acceptable delivery time compared to their open FF counterpart. The results indicated that the dose uniformity and flatness for the modulated FFF beams optimized with the DLTO model can successfully match the uniformity and flatness of their conventional FF counterparts, and may even provide further benefit by taking advantage of the unique FFF beam characteristics.

Standardized flattening filter free volumetric modulated arc therapy plans based on anteroposterior width for total body irradiation

In this work, the feasibility of using flattening filter free (FFF) beams in volumetric modulated arc therapy (VMAT) total body irradiation (TBI) treatment planning to decrease protracted beam‐on times for these treatments was investigated. In addition, a methodology was developed to generate standardized VMAT TBI treatment plans based on patient physical dimensions to eliminate plan optimization time. A planning study cohort of 47 TBI patients previously treated with optimized VMAT ARC 6 MV beams was retrospectively examined. These patients were sorted into six categories depending on height and anteroposterior (AP) width at the umbilicus. Using Varian Eclipse, clinical 40 cm × 10 cm open field arcs were substituted with 6 MV FFF. Mid‐plane lateral dose profiles in conjunction with relative arc output factors (RAOF) yielded how far a given multileaf collimator (MLC) leaf must move in order to achieve a mid‐plane 100% isodose for a specific control point. Linear interpolation gave the dynamic MLC aperture for the entire arc for each patient AP width category, which was subsequently applied through Python scripting. All FFF VMAT TBI plans were then evaluated by two radiation oncologists and deemed clinically acceptable. The FFF and clinical VMAT TBI plans had similar Body–5 mm D98% distributions, but overall the FFF plans had statistically significantly increased or broader Body–5 mm D2% and mean lung dose distributions. These differences are not considered clinically significant. Median beam‐on times for the FFF and clinical VMAT TBI plans were 11.07 and 18.06 min, respectively, and planning time for the FFF VMAT TBI plans was reduced by 34.1 min. In conclusion, use of FFF beams in VMAT TBI treatment planning resulted in dose homogeneity similar to our current VMAT TBI technique. Clinical dosimetric criteria were achieved for a majority of patients while planning and calculated beam‐on times were reduced, offering the possibility of improved patient experience.

Basic Properties of a New Polymer Gel for 3D-Dosimetry at High Dose-Rates Typical for FFF Irradiation Based on Dithiothreitol and Methacrylic Acid (MAGADIT): Sensitivity, Range, Reproducibility, Accuracy, Dose Rate Effect and Impact of Oxygen Scavenger

The photon induced radical-initiated polymerization in polymer gels can be used for high-resolution tissue equivalent dosimeters in quality control of radiation therapy. The dose (D) distribution in radiation therapy can be measured as a change of the physical measurement parameter T2 using T2-weighted magnetic resonance imaging. The detection by T2 is relying on the local change of the molecular mobility due to local polymerization initiated by radicals generated by the ionizing radiation. The dosimetric signals R2 = 1/T2 of many of the current polymer gels are dose-rate dependent, which reduces the reliability of the gel for clinical use. A novel gel dosimeter, based on methacrylic acid, gelatin and the newly added dithiothreitol (MAGADIT) as an oxygen-scavenger was analyzed for basic properties, such as sensitivity, reproducibility, accuracy and dose-rate dependence. Dithiothreitol features no toxic classification with a difference to THPC and offers a stronger negative redox-potential than ascorbic acid. Polymer gels with three different concentration levels of dithiothreitol were irradiated with a preclinical research X-ray unit and MR-scanned (T2) for quantitative dosimetry after calibration. The polymer gel with the lowest concentration of the oxygen scavenger was about factor 3 more sensitive to dose as compared to the gel with the highest concentration. The dose sensitivity (α = ∆R2/∆D) of MAGADIT gels was significantly dependent on the applied dose rate D˙ (≈48% reduction between D˙ = 0.6 Gy/min and D˙ = 4 Gy/min). However, this undesirable dose-rate effect reduced between 4–8 Gy/min (≈23%) and almost disappeared in the high dose-rate range (8 ≤ D˙≤ 12 Gy/min) used in flattening-filter-free (FFF) irradiations. The dose response varied for different samples within one manufacturing batch within 3%–6% (reproducibility). The accuracy ranged between 3.5% and 7.9%. The impact of the dose rate on the spatial integrity is demonstrated in the example of a linear accelerator (LINAC) small sized 5 × 10 mm² 10 MV photon field. For MAGADIT the maximum shift in the flanks in this field is limited to about 0.8 mm at a FFF dose rate of 15 Gy/min. Dose rate sensitive polymer gels likely perform better at high dose rates; MAGADIT exhibits a slightly improved performance compared to the reference normoxic polymer gel methacrylic and ascorbic acid in gelatin initiated by copper (MAGIC) using ascorbic acid.

Whole Breast and Lymph Node Irradiation using Halcyon™ 2.0 Utilizing Automatic Multi-isocenter Treatment Delivery and Daily Kilovoltage Cone-beam Computed Tomography

The Halcyon^TM is a newly introduced platform to provide a simplified high-throughput workflow. A substantial fraction of patients treated with radiation are receiving breast irradiation. The Halcyon has a smaller maximum field size (28 x 28 cm²) compared to traditional C-arm linacs, limiting treatment of larger breast fields. With the use of autofeathering and linked multiple isocenters, non-divergent beams can be used to treat whole breast with large mid-tangent separation. In this case report, a multiple isocenter whole breast treatment with nodal involvement is described with the Halcyon 2.0 platform. The patient was simulated with the same immobilization techniques as used for C-arm linac treatments. The treatment planning time was around 20-30 minutes, which is similar to traditional planning for C-arm linacs, and dosimetric analysis resulted in satisfactory dose-volume histogram (DVH) parameters that met all planning objectives. Treatment times were shorter with an average of 9:32 minutes from the beginning of imaging to the end of treatment and a total in-room time of around 15 minutes per fraction. The use of multiple isocenters for the extended treatment field created a half-beam block type field arrangement, which has been previously reported to produce superior dosimetry results at the supraclavicular-tangents junction area

Whole Breast Irradiation with Halcyon™ 2.0: Workflow and Efficiency of Field-in-Field Treatment with Dynamic Beam Flattening Technique and kV Cone Beam Computed Tomography

Whole breast irradiation accounts for a substantial fraction of patients treated in Radiation Oncology clinics. The recently introduced Halcyon™ platform provided a high-throughput, simplified workflow. The 2.0 version introduced new features such as the dynamic beam flattening (DBF) technique that uses the upper layer of the multi-leaf collimator (MLC) to create a flat beam profile at depth and an improved kV cone beam computed tomography (kV CBCT). In this case report, we described our experience in whole breast irradiation with Halcyon 2.0 new features. The patient was simulated in the supine position with the same immobilization technique used on C-arm linacs and an additional contralateral elbow position measurement to ensure clearance. The treatment planning process using DBF and field-in-field technique was similar to the traditional flattened beam planning and did not require additional training. Dosimetric analysis showed satisfactory dose-volume histogram (DVH) parameters that met all the planning objectives, with maximal dose at 107% and V105% at 3.6% of the breast volume. Daily image-guided radiation therapy (IGRT) using improved CBCT showed excellent soft tissue contrasts and sufficient field of view. The average imaging and treatment time was nine minutes, and the average in-room time was 16.2 minutes. These treatment times were substantially higher than those for breast treatments on our Halcyon platform using an irregular surface compensator technique. The use of DBF contributed to the majority of treatment time increase due to the motion of the upper layer of the MLC to create a flat beam profile. The total treatment time using DBF might be too long for patients with deep inhalation breast hold (DIBH) and can be drastically reduced using an irregular surface compensator technique, also known as the electronic tissue compensation (ECOMP) technique, instead of the DBF-enabled field-in-field technique.

Comparison of Measured and Monte Carlo Calculated Dose Distributions from Indigenously Developed 6 MV Flattening Filter Free Medical Linear Accelerator

Purpose:

Monte Carlo simulation was carried out for a 6 MV flattening filter-free (FFF) indigenously developed linear accelerator (linac) using the BEAMnrc user-code of the EGSnrc code system. The model was benchmarked against the measurements. A Gaussian distributed electron beam of kinetic energy 6.2 MeV with full-width half maximum of 1 mm was used in this study.

Methods:

The simulation of indigenously developed linac unit has been carried out by using the Monte Carlo-based BEAMnrc user-code of the EGSnrc code system. Using the simulated model, depth and lateral dose profiles were studied using the DOSXYZnrc user-code. The calculated dose data were compared against the measurements using an RFA dosimertic system made by PTW, Germany (water tank MP3-M and 0.125 cm³ ion chamber).

Results:

The BEAMDP code was used to analyze photon fluence spectra, mean energy distribution, and electron contamination fluence spectra. Percentage depth dose (PDD) and beam profiles (along both X and Y directions) were calculated for the field sizes 5 cm × 5 cm - 25 cm × 25 cm. The dose difference between the calculated and measured PDD and profile values were under 1%, except for the penumbra region where the maximum deviation was found to be around 3%.

Conclusions:

A Monte Carlo model of indigenous FFF linac (6 MV) has been developed and benchmarked against the measured data.

Second cancer risk after radiation therapy of ependymoma using the flattening filter free irradiation mode of a linear accelerator

Pediatric patients suffering from ependymoma are usually treated with cranial or craniospinal three‐dimensional (3D) conformal radiotherapy (3DCRT). Intensity‐modulated techniques spare dose to the surrounding tissue, but the risk for second malignancies may be increased due to the increase in low‐dose volume. The aim of this study is to investigate if the flattening filter free (FFF) mode allows reducing the risk for second malignancies compared to the mode with flattening filter (FF) for intensity‐modulated techniques and to 3DCRT. A reduction of the risk would be advantageous for treating pediatric ependymoma. 3DCRT was compared to intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT) with and without flattening filter. Dose–volume histograms (DVHs) were compared to evaluate the plan quality and used to calculate the excess absolute risk (EAR) to develop second cancer in the brain. Dose verification was performed with a two‐dimensional (2D) ionization chamber array and the out‐of‐field dose was measured with an ionization chamber to determine the EAR in peripheral organs. Delivery times were measured. Both VMAT and IMRT achieved similar plan quality in terms of dose sparing in the OAR and higher PTV coverage as compared to 3DCRT. Peripheral dose in low‐dose region, which is proportional to the EAR in organs located in this region, for example, gonads, bladder, or bowel, could be significantly reduced using FFF. The lowest peripheral EAR and lowest delivery times were hereby achieved with VMAT_FFF. The EAR calculated based on DVH in the brain could not be reduced using FFF mode. VMAT_FFF improved the target coverage and homogeneity and kept the dose in the OAR similar compared to 3DCRT. In addition, delivery times were significantly reduced using VMAT_FFF. Therefore, for radiotherapy of ependymoma patients, VMAT_FFF may be considered advantageous for the combination of Elekta Synergy linac and Oncentra External Beam planning system used in this study.

Emerging technologies in stereotactic body radiotherapy

Stereotactic body radiation therapy (SBRT) stems from the initial developments of intra-cranial stereotactic radiosurgery (SRS). Despite similarity in their names and clinical goals of delivering a sufficiently high tumoricidal dose, maximal sparing of the surrounding normal tissues and a short treatment course, SBRT technologies have transformed from the early days of body frame-based treatments with X-ray verification to primarily image-guided procedures with cone-beam CT or stereoscopic X-ray systems and non-rigid body immo-bilization. As a result of the incorporation of image-guidance systems and multi-leaf col-limators into mainstream linac systems, and treatment planning systems that have also evolved to allow for routine dose calculations to permit intensity modulated radiotherapy and volumetric modulated arc therapy (VMAT), SBRT has disseminated rapidly in the community to manage many disease sites that include oligometastases, spine lesions, lung, prostate, liver, renal cell, pelvic tumors, and head and neck tumors etc. In this article, we review the physical principles and paradigms that led to the widespread adoption of SBRT practice as well as technical caveats specific to individual SBRT technologies. From the perspective of treatment delivery, we categorically described (I) C-arm linac-based SBRT technologies; (II) robotically manipulated X-band CyberKnife® technology; and (III) emerging specialized systems for SBRT that include integrated MRI-linear accelerators and the imaged-guided Gamma Knife Perfexion Icon system with expanded multi-isocenter treatments of skull-based tumors, head-and-neck and cervical-spine lesions.

[Evaluation of a 2D Diode Array Corresponding to Flattening Filter Free X-ray Beams]

Recently, a medical linear accelerator with a flattening filter free (FFF) mode has led to the use of FFF X-ray beams at clinical sites. The usefulness of FFF X-ray beams in high-precision radiation therapy has been reported. Therefore, the quality assurance and quality control for FFF X-ray beams have become necessary. In this study, the characteristics of the detectors of a newly developed 2-D diode array (MapCHECK2, Sun Nuclear Corporation) for FFF X-ray beams, i.e., dose reproducibility, dose rate dependence, dose linearity, and output factor, were evaluated. For the measurements, 6 and 10 MV FFF beams were used. The results showed that the coefficient of variation for dose reproducibility was within 0.08%, the dose rate dependence was less than 1.0%, the coefficient of determination of dose linearity was found to be R²=1.0, which was high, and the output factor agreed within 2.5% as compared with the farmer ion chamber, diode E, and pinpoint ion chamber for field sizes greater than 2×2 cm². The results suggested that MapCHECK2 could be a useful tool for quality assurance and quality control for FFF X-ray beams.

Gamma Knife and volumetric modulated arc therapy stereotactic radiosurgery plan quality and OAR sparing comparison for pituitary adenomas and vestibular schwannomas

PURPOSE

To compare the plan quality and organs at risk (OAR) sparing of auto-planned volumetric modulated art therapy (VMAT) and Gamma Knife (GK) for stereotactic radiosurgery of pituitary adenomas (PA) and vestibular schwannomas (VS).

METHODS

VMAT radiosurgery plans were made using auto planning tool for eight vestibular schwannoma and eight pituitary adenoma patients previously treated with GK. VMAT plans were made with three non-coplanar arcs using 315, 0 and 45 degrees angles, 6MV FFF energy at 1400 MU/min dose rate and 2.5 mm thick MLC leaves. Both GK and VMAT plans were prescribed to similar isodose lines (50% - 60%).

RESULTS

Respectively for GK and VMAT, the mean Paddick conformity index (PCI) was 0.62 ± 0.08 and 0.67 ± 0.10 (p > 0.05) for PA and 0.72 ± 0.09 and 0.660 ± 0.13 (p > 0.05) for VS; the mean gradient index (GI) was 2.76 ± 0.14 and 3.14 ± 0.40 Gy (p < 0.05) for PA and 3.71 ± 1.83 and 3.60 ± 0.84 Gy (p > 0.05) for VS; mean brainstem maximum dose was 9.13 ± 3.50 Gy and 7.31 ± 2.01 Gy (p > 0.05) for PA and 11.67 ± 4.56 Gy and 12.22 ± 4.55 Gy (p > 0.05) for VS; mean optic nerve maximum dose was 9.66 ± 1.0 Gy and 7.67 ± 2.58 Gy (p < 0.05); mean cochlea mean dose was 7.31 ± 2.7 Gy and 7.23 ± 3.13 Gy (p > 0.05); and mean treatment time was 68 min and 5 min for PA and 40 min and 3 min for VS.

CONCLUSIONS

Auto planning with standard template simplified the planning stage for VMAT and provided clinically acceptable plans. Comparison of GK and VMAT for plan quality and OAR sparing varied across patients but both were overall comparable.

Flattened Beams for Lung Stereotactic Body Radiation Therapy

BACKGROUND/AIM

To assess the clinical impact of high dose rate stereotactic body radiation therapy (SBRT) in patients with lung neoplastic lesions.

PATIENTS AND METHODS

From January 2014 to June 2016, a single-center retrospective analysis was performed including all patients treated by either flattening filter free (FFF) beams or flattening filter beams (FF) three-dimensional (3D) SBRT for lung neoplastic lesions.

RESULTS

A total of 99 SBRT were performed on 75 patients. Among these, 29 SBRT were performed using a FFF technique while 70 other SBRT were done using a FF technique. Median follow-up time was 12.9 months. Overall, no difference between the two groups was found except for the mean beam on time which was reduced by 3.3 to 0.9 minutes in the FFF group (p<0.001).

CONCLUSION

We report a low toxicity rate and a shortened beam on time in patients treated with 3D FFF SBRT for lung neoplastic lesions.

Volumetric-modulated arc therapy and intensity-modulated radiation therapy treatment planning for prostate cancer with flattened beam and flattening filter free linear accelerators

This study on patients with localized prostate cancer was set up to investigate valuable differences using flattened beam (FB) and flattening filter free (FFF) mode in the application of intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT). For ten patients, four different plans were calculated with Oncentra planning system of Elekta, using Synergy machines: IMRT and VMAT, with and without flattening filter. Homogeneity and conformity indexes, dose to the organs at risk, and measurements of peripheral dose and dosimetric plan verification including record of the delivery times were analyzed and statistically evaluated. The indexes for homogeneity and conformity (CTV and PTV) are either advantageous or not significantly different for FFF compared to FB with one minor exception. Regarding the doses to the organs at risk and the measured peripheral dose, equivalent or lower doses were delivered for FFF than with FB. Furthermore, the delivery times were significantly shorter for FFF. VMAT compared to IMRT reveals benefits or at least equivalent values. VMAT-FFF combines the most advantageous plan quality parameters with the shortest delivery times and reduced peripheral dose and is therefore recommended for the given equipment and cancer localization.

Development of a flattening filter free multiple source model for use as an independent, Monte Carlo, dose calculation, quality assurance tool for clinical trials

PURPOSE

The Imaging and Radiation Oncology Core-Houston (IROC-H) Quality Assurance Center (formerly the Radiological Physics Center) has reported varying levels of compliance from their anthropomorphic phantom auditing program. IROC-H studies have suggested that one source of disagreement between institution submitted calculated doses and measurement is the accuracy of the institution's treatment planning system dose calculations and heterogeneity corrections used. In order to audit this step of the radiation therapy treatment process, an independent dose calculation tool is needed.

METHODS

Monte Carlo multiple source models for Varian flattening filter free (FFF) 6 MV and FFF 10 MV therapeutic x-ray beams were commissioned based on central axis depth dose data from a 10 × 10 cm² field size and dose profiles for a 40 × 40 cm² field size. The models were validated against open-field measurements in a water tank for field sizes ranging from 3 × 3 cm² to 40 × 40 cm² . The models were then benchmarked against IROC-H's anthropomorphic head and neck phantom and lung phantom measurements.

RESULTS

Validation results, assessed with a ±2%/2 mm gamma criterion, showed average agreement of 99.9% and 99.0% for central axis depth dose data for FFF 6 MV and FFF 10 MV models, respectively. Dose profile agreement using the same evaluation technique averaged 97.8% and 97.9% for the respective models. Phantom benchmarking comparisons were evaluated with a ±3%/2 mm gamma criterion, and agreement averaged 90.1% and 90.8% for the respective models.

CONCLUSIONS

Multiple source models for Varian FFF 6 MV and FFF 10 MV beams have been developed, validated, and benchmarked for inclusion in an independent dose calculation quality assurance tool for use in clinical trial audits.

Structural Shielding Design of a 6 MV Flattening Filter Free Linear Accelerator: Indian Scenario

Detailed structural shielding of primary and secondary barriers for a 6 MV medical linear accelerator (LINAC) operated with flattening filter (FF) and flattening filter free (FFF) modes are calculated. The calculations have been carried out by two methods, one using the approach given in National Council on Radiation Protection (NCRP) Report No. 151 and the other based on the monitor units (MUs) delivered in clinical practice. Radiation survey of the installations was also carried out. NCRP approach suggests that the primary and secondary barrier thicknesses are higher by 24% and 26%. respectively, for a LINAC operated in FF mode to that of a LINAC operated in both FF and FFF modes with an assumption that only 20% of the workload is shared in FFF mode. Primary and secondary barrier thicknesses calculated from MUs delivered on clinical practice method also show the same trend and are higher by 20% and 19%, respectively, for a LINAC operated in FF mode to that of a LINAC operated in both FF and FFF modes. Overall, the barrier thickness for a LINAC operated in FF mode is higher about 20% to that of a LINAC operated in both FF and FFF modes.

Fidelity of dose delivery at high dose rate of volumetric modulated arc therapy in a truebeam linac with flattening filter free beams

The purpose of this study is to assess fidelity of radiation delivery between high and low dose rates of the flattening filter free (FFF) modes of a new all-digital design medical linear accelerator (Varian TrueBeam™), particularly for plans optimized for volumetric modulated arc therapy (VMAT). Measurements were made for the two energies of flattening filter free photon beams with a Varian TrueBeam™ linac: 6 MV (6 XFFF) at 400 and 1400 MU/min, and 10 MV (10 XFFF) at 400 and 2400 MU/min. Data acquisition and analysis was performed with both ionization chambers and diode detector system Delta(4), for square radiation fields and for 8 VMAT treatment plans optimized for SBRT treatment of lung tumors. For the square fields, a percent dose difference between high and low dose rate of the order of 0.3-0.4% for both photon energies was seen with the ionization chambers, while the contribution to the difference from ion recombination was found to be negligible. For both the VMAT and square-field deliveries, the Delta(4) showed the same average percent dose difference between the two dose rates of ~0.8% and ~0.6% for 10 MV and 6 MV, respectively, with the lower dose rate values giving the greater measured dose compared to the high dose rate. Thus, the VMAT deliveries introduced negligible dose differences between high and low dose rate. Finally, reproducibility of dose measurements was good for both energies.

Ion recombination corrections of ionization chambers in flattening filter-free photon radiation

The flattening filter free (FFF) X-rays can provide much higher dose rate at the treatment target compared to the conventional flattened X-rays. However, the substantial increase of dose rate for FFF beams may affect the ion recombination correction factor, which is required for accurate measurements using ionization chambers in clinical dosimetry. The purpose of this work is to investigate the ion recombination of three types of commonly used ion chambers (Farmer, PinPoint and plane-parallel) in the FFF photon radiation. Both 6 MV and 10 MV flattened and FFF beams were fully commissioned on a Varian TrueBeam linear accelerator. The ion recombination correction factor, P(ion), was determined using the two-voltage technique for a 0.6 cc Farmer chamber, a 0.015 cc PinPoint chamber, and a 0.02 cc parallel-plate chamber at different source-to-axis distances (SAD) in a solid water phantom or water tank phantom at a depth of 10 cm in a 10 × 10 cm(2) field. Good repeatability of measurements was demonstrated. Less than 1% difference in P(ion) between the flattened and FFF photons for all three ion chambers was observed. At a SAD of 100 cm and a depth of 10 cm for a 10 × 10 cm(2) field, P(ion) for the Farmer chamber was 1.004 and 1.008 for the 6 MV flattened and FFF beams, respectively. At the same setup using the Farmer chamber, P(ion) was 1.002 and 1.015 for the 10MV flattened and FFF beams, respectively. All P(ion) results for the Farmer, PinPoint, or parallel plate chamber in the 6 MV and 10 MV flattened and FFF beams were within 2% from the unity (1 ≤ P(ion) < 1.02). The P(ion) ratio of the FFF to flattened beams was 0.99~1.01 for both 6 MV and 10 MV photons. The ion recombination effect of the Farmer, PinPoint, and plane-parallel chamber in the FFF beams is not substantially different from that in the conventional flattened beams.