Superficial Dose Research Articles

Superficial Dosimetry Study of the Frequency of Bolus Using in Volumetric Modulated Arc Therapy after Modified Radical Mastectomy.

To investigate the effect of various frequencies of bolus use on the superficial dose of volumetric modulated arc therapy after modified radical mastectomy for breast cancer. Based on the computed tomography images of a female anthropomorphic breast phantom, a 0.5 cm silicone-based 3D-printed bolus was created. Nine points evenly distributed on the breast skin were selected for assessing the skin dose, and a volume of subcutaneous lymphatic drainage of the breast (noted as ROI2-3) was delineated for assessing the chest wall dose. The treatment plans with and without bolus (plan_wb and plan_nb) were separately designed using the prescription of 50 Gy in 25 fractions following the standard dose constraints of the adjacent organ at risk. To characterize the accuracy of treatment planning system (TPS) dose calculations, the doses of the nine points were measured five times by thermoluminescence dosimeters (TLDs) and then were compared with the TPS calculated dose. Compared with Plan_nb (144.46 ± 10.32 cGy), the breast skin dose for plan_wb (208.75 ± 4.55 cGy) was significantly increased (t = -18.56, P < 0.001). The deviation of skin dose was smaller for Plan_wb, and the uniformity was significantly improved. The calculated value of TPS was in good agreement with the measured value of TLD, and the maximum deviation was within 5%. Skin and ROI2-3 doses were significantly increased with increasing frequencies of bolus applications. The mean dose of the breast skin and ROI2-3 for 15 and 23 times bolus applications were 45.33 Gy, 50.88 Gy and 50.36 Gy, 52.39 Gy, respectively. 3D printing bolus can improve the radiation dose and the accuracy of the planned dose. Setting Plan_wb to 15 times for T1-3N+ breast cancer patients and 23 times for T4N+ breast cancer patients can meet the clinical need. Quantitative analysis of the bolus application frequency for different tumor stages can provide a reference for clinical practice.

Exploring the Benefits of 3D-Printed Bolus in Cone Beam CT for Modified Radical Mastectomy Breast Cancer

To improve the accuracy of superficial dose coverage for patients undergoing modified radical mastectomy (MRM), boluses are frequently applied to the skin surface of targeted region during radiotherapy. The irregular curvature of MRM breast cases results in discrepancies of dosimetry and poor contact. Additionally, the presence of an air gap causes unnecessary high-dose escalation and uncertainty in dose calculation, making the use of commercial flat bolus problematic. This study evaluated the effectiveness of 3D-printed bolus by comparing it to commercial bolus in setup variations and dosimetric compliance through daily cone-beam computed tomography (CBCT) scans. Ten patients underwent MRM were divided into 2 groups. 5 patients treated with 0.5 cm commercial bolus as group A while the other 5 patients covered by 3D Bolus as group B. 3D bolus was made of polylactic acid filament (PLA) and contoured in advance with thickness of 0.5 cm on the surface of target. Positional errors were recorded through daily image guidance and compared with the images. Both groups had right-sided breast with neck lymph nodes involved and received 50 Gy in 25 fractions using 4 partial arcs of the volumetric modulated arc therapy (VMAT) technique. The CBCTs were recalculated through treatment planning system (TPS) to assess superficial dose coverage. A two-tailed student's t-test was applied. Groups A and B pitch angles were -0.203 ± 0.837° and 0.334 ± 0.909° (p = 0.0003). Roll angle were 0.313 ± 0.728° and -0.633 ± 1.286° (p = 0.0000013). Yaw angle were -0.034 ± 0.872° and 0.018 ± 0.883° (p = 0.721). There was a trend of differences in the Z-axis and significant statistical differences in the pitch and roll angles due to aligning gel layers directly with group A's body surface, while 3D bolus rigidly adherence to group B's delineated curve. In the planned target volume (PTV), the CI and HI of group A were 0.9448±0.0208 and 1.2061±0.0448, respectively, while 0.9776±0.0144 and 1.1472±0.0206 (p<0.005) were for group B. As for the superficial region, which is defined from body surface to 0.5cm inside PTV, CI and HI of A were 0.8290±0.0599 and 1.4675±0.2434, while for B, they were 0.9753±0.0214 and 1.1330±0.0126, respectively (p<0.005). The study analyzed the results through image comparison and investigated the CI and HI between two types of boluses. The 3D bolus reduces setup errors and improves dose coverage especially when superficial region is concerned. Better consistency of patient repositioning and dosimetry can be achieved and proved by daily assessment of CBCT scan. The customization of 3D bolus with integration of TPS and CT scans provides a solution to the inadequacies of commercial bolus. The results suggest that the use of 3D bolus is a promising development in radiation therapy for MRM breast cancer patients.

Comparison of four commercial dose calculation algorithms in different evaluation tests.

Accurate and fast dose calculation is crucial in modern radiation therapy. Four dose calculation algorithms (AAA, AXB, CCC, and MC) are available in Varian Eclipse and RaySearch Laboratories RayStation Treatment Planning Systems (TPSs). This study aims to evaluate and compare dosimetric accuracy of the four dose calculation algorithms applying to homogeneous and heterogeneous media, VMAT plans (based on AAPM TG-119 test cases), and the surface and buildup regions. The four algorithms are assessed in homogeneous (IAEA-TECDOCE 1540) and heterogeneous (IAEA-TECDOC 1583) media. Dosimetric evaluation accuracy for VMAT plans is then analyzed, along with the evaluation of the accuracy of algorithms applying to the surface and buildup regions. Tests conducted in homogeneous media revealed that all algorithms exhibit dose deviations within 5% for various conditions, with pass rates exceeding 95% based on recommended tolerances. Additionally, the tests conducted in heterogeneous media demonstrate high pass rates for all algorithms, with a 100% pass rate observed for 6 MV and mostly 100% pass rate for 15 MV, except for CCC, which achieves a pass rate of 94%. The results of gamma index pass rate (GIPR) for dose calculation algorithms in IMRT fields show that GIPR (3% /3 mm) for all four algorithms in all evaluated tests based on TG119, are greater than 97%. The results of the algorithm testing for the accuracy of superficial dose reveal variations in dose differences, ranging from -11.9% to 7.03% for 15 MV and -9.5% to 3.3% for 6 MV, respectively. It is noteworthy that the AXB and MC algorithms demonstrate relatively lower discrepancies compared to the other algorithms. This study shows that generally, two dose calculation algorithms (AXB and MC) that calculate dose in medium have better accuracy than other two dose calculation algorithms (CCC and AAA) that calculate dose to water.

Comparing brass mesh to tissue equivalent bolus materials for volumetric modulated arc therapy chest wall irradiation.

To compare the superficial dose when using brass mesh bolus (BMB), no bolus, or 3mm tissue-equivalent bolus with a pseudo-flash volumetric modulated arc therapy (VMAT) breast treatment planning technique. Two different beam arrangements for right-sided irradiation and one beam arrangement for bilateral irradiation were planned on an inhomogeneous thorax phantom in accordance with our clinical practice for VMAT postmastectomy radiotherapy (PMRT). Plans were optimized using pseudo-flash and representative critical organ optimization structures were used to shape the dose. Plans were delivered without bolus, with 3mm tissue-equivalent bolus (TEB), or with one-layer BMB. Optically stimulated luminescence dosimeter (OSLD) and radiochromic film measurements were taken and analyzed to determine the superficial dose in each case and the relative enhancement from the no bolus delivery. Superficial dose measured with OSLDs was found to be 76.4±4.5%, 103.0±6.1%, and 98.1±5.8% of prescription for no physical bolus (NB), TEB, and BMB, respectively. Superficial dose was observed to increase from lateral to medial points when measured with film. However, the relative increase in superficial dose from NB was consistent across the profile with an increase of 43±2.1% and 34±3.3% of prescription for TEB and BMB, respectively. The results are in good agreement with expectations from the literature and the experience with tangential radiotherapy. Three millimeter TEB and one-layer BMB were shown to provide similar enhancement to the superficial dose compared to delivery without bolus. BMB, which does not significantly affect dose at depth and is more conformal to the patient surface, is an acceptable alternative to 3mm TEB for chest wall PMRT patients treated with pseudo-flash PMRT.

Effect of 0.35 T and 1.5 T magnetic fields on superficial dose in MR-guided radiotherapy of laryngeal cancer

BackgroundTreatment of head and neck cancer on linear accelerators with on-board magnetic resonance imaging (MR-linac) might be beneficial to reduce side effects and increase accuracy. For many head and neck cancer patients, dose coverage of the often superficially located planning target volumes (PTVs) is required. This study examines the impact of the electron return effect (ERE) on the surface dose in MR-guided radiotherapy (MRgRT) compared to conventional radiotherapy. Materials and methodsFor this bicentric dosimetric study, 14 cases of laryngeal carcinomas with PTVs reaching up to the skin surface were included. For each patient, five different plans were compared, two VMAT plans (with and without a 5 mm bolus) and three IMRT MRgRT plans (0.35 T, 1.5 T and 0 T, each without bolus). Dose distributions were also validated with film measurements. ResultsA similar coverage on the most superficial 3–5 mm of the PTV was achieved in the VMAT plans with bolus and the MRgRT plans for both 0.35 T and 1.5 T. However, coverage on this region was usually not achieved for VMAT without bolus and the 0 T plans. The film measurements on phantoms confirmed the results with the relative error never exceeding the calculated differences between the plans. ConclusionThe present study could demonstrate that the ERE for both commercially available MR-linac variants provides sufficient coverage of the superficial tissue layers in MRgRT-plans for laryngeal carcinoma.

Comparison of surface dose during whole breast radiation therapy on Halcyon and TrueBeam using Cherenkov imaging.

The emergence of the Halcyon linear accelerator has allowed for increased patient throughput and improved treatment times for common treatment sites in radiation oncology. However, it has been shown that this can lead to increased surface dose in sites like breast cancer compared with treatments on conventional machines with flattened radiation beams. Cherenkov imaging can be used to estimate surface dose by detection of Cherenkov photons emitted in proportion to energy deposition from high energy electrons in tissue. Phantom studies were performed with both square beams in reference conditions and with clinical treatments, and dosimeter readings and Cherenkov images report higher surface dose (25% for flat phantom entrance dose, 5.9% for breast phantom treatment) from Halcyon beam deliveries than for equivalent deliveries from a TrueBeam linac. Additionally, the first Cherenkov images of a patient treated with Halcyon were acquired, and superficial dose was estimated.

Modeling scatter through sides of island blocks used for intensity-modulated bolus electron conformal therapy.

Passive Radiotherapy Intensity Modulators for Electrons (PRIME) devices are comprised of cylindrical tungsten island blocks imbedded in a machinable foam slab within the patient's cutout. Intensity-modulated bolus electron conformal therapy (IM-BECT) uses PRIME devices to reduce dose heterogeneity caused by the irregular bolus surface. Heretofore, IM-BECT dose calculations used the pencil beam redefinition algorithm (PBRA) assuming perfect collimation. This study investigates modeling electron scatter into and out the sides of island blocks. Dose distributions were measured in a water phantom at 7, 13, and 20 MeV for devices having nominal intensity reduction factors of 1.000 (foam only), 0.937, 0.812, and 0.688, corresponding to nominal island block diameters (dnom ) of 0.158, 0.273, and 0.352cm, respectively. Pencil beam theory derived an effective diameter (dIS ) to account for in-scattered electrons as a function of dnom and beam energy (Ep,0 ). However, for out-scattered electrons, an effective diameter (dmod ) was estimated by best fitting measured data. In the modulated region (under island blocks, depth<R90 ), modified PBRA-calculated dose distributions showed 2%/2mm passing rates for dnom =0.158, 0.273, and 0.352cm of (100%, 100%, 100%) at 7 MeV, (100%, 100%, 93.5%) at 13 MeV, and (99.8%, 85.4%, and 71.5%) at 20 MeV. The largest dose differences (≤6%) occurred at the highest energy (20 MeV), largest dnom , shallowest depths (≤2cm), and on central axis. An equation for modeling island block scatter, dmod (dnom , Ep,0 ), has been developed for use in the PBRA, insignificantly impacting calculation time. Although inaccuracy sometimes exceeded our 2%/2mm criteria, it could be clinically acceptable, as superficial dose differences often fall inside the bolus. Also, patient PRIME devices are expected to have fewer large diameter island blocks than did test devices. Inaccuracies are attributed to out-scattered electrons having energy spectra different than the primary beams.

Oral Extramedullary Plasmacytoma Treatment Using a Combination of Marginal Excision and Strontium-90 Therapy in Five Dogs.

Extramedullary plasmacytoma (EMP) is a benign round cell tumor that is most commonly found in cutaneous locations in dogs and occurs less frequently in the oral cavity. They are highly radiosensitive, are distinct from systemic multiple myeloma syndrome and wide surgical excision is typically curative. This report describes five cases of non-invasive oral EMP in dogs treated with a combination of marginal excision and strontium-90 plesiotherapy. All five cases had narrow or incomplete margins on histopathologic evaluation but experienced no recurrence after combination therapy. Plesiotherapy radiation may offer a potential adjunct treatment for non-invasive oral EMP by providing a superficial dose of radiation that complements a less invasive surgical removal. The combination of plesiotherapy and marginal excision may offer an alternative to wide surgical excision for non-invasive oral EMPs.

Performance evaluation of buildup bolus during external radiotherapy of mastectomy patients: treatment planning and film dosimetry.

A buildup bolus is used during the post-mastectomy radiotherapy (PMRT) to overcome under-dosage issues in the chest wall. The current study is aimed at evaluating the performance of a bolus in dose enhancement through both film dosimetry and treatment planning approaches. Twenty patients were enrolled in current research. The received dose by the skin at the lateral and medial regions of the chest wall in the presence and absence bolus was evaluated. Film dosimetry results showed that the presence of the bolus can averagely increase the skin dose by about 80% (P value < 0.001) and 92% (P value < 0.001) in lateral and medial regions, respectively. No significant difference was observed between the measured and treatment planning system (TPS)-calculated dose values in the presence of bolus. The presence of the bolus can considerably increase the absorbed dose by superficial chest wall regions. The TPS shows a favorable performance in superficial dose calculations in the presence of the buildup bolus. Hosseini et al.: demonstration of implemented research in the current study.

Outcomes Associated with the Use of Tissue Equivalent Bolus Tailored by Skin Involvement Status in Postmastectomy Radiotherapy: A Single-Blinded Randomized Clinical Trial

<h3>Purpose/Objective(s)</h3> To assess the impact of tissue equivalent bolus in postmastectomy radiotherapy (PMRT), the present study compared the use of different bolus regimens tailored by skin involvement status. <h3>Materials/Methods</h3> Patients with breast cancer who required PMRT were recruited (NCT01925651) and divided in two subgroups: standard-risk (SR, without skin involvement) and high-risk (HR, with skin involvement). The SR was randomized between no bolus or 5mm-bolus on alternate days and the HR between 5mm-bolus on alternate days or 5mm-bolus daily. Skin changes were graded weekly with the Radiation Therapy Oncology Group (RTOG) toxicity scale (single-blinded). Subsequently, patients were followed to assess local control (LC) in the chest wall (CW). <h3>Results</h3> Fifty-eight patients were enrolled (34 SR and 24 HR). The median age was 48 years and 35.3% had BMI > 30 kg/m². All the baseline characteristics were similar between each arm within the same risk subgroup. The radiotherapy technique used was tridimensional (3D) in 30 cases (58.8%) and conventional (2D) in 21 cases (41.2%). Overall, the maximal radiodermatitis rates were 29.4% (G2) and 15.7% (G3). In the SR arms, there was no difference in the incidence of G2 radiodermatitis between no bolus and alternate days bolus (p=0.70), and no G3 event occurred. In the HR arms, the incidences of G2 (100% vs. 44.5%, p=0.01) and G3 radiodermatitis (70% vs. 11.1%, p=0.02) were higher with daily bolus. G2 events occurred earlier with daily bolus, but four of the six G3 events occurred 1-3 weeks after the last RT fraction. After a median follow-up of 6.2 years, the 5-year LC was 95.8% (95%CI: 88.2%-100%) in the SR and 91.7% (95%CI: 77.3%-100%) in the HR subgroups. Per randomization arm, there was no LC difference between the SR arms (p=0.90) or between the HR arms (p=0.70). All CW failures occurred in the tangent field's margins and when using 2D technique. <h3>Conclusion</h3> Within the same risk subgroup, no difference in LC was detected with a more intense bolus regimen. Due to increased G3 radiodermatitis and location of the CW failures (field borders), further studies testing the benefit of increasing superficial dose within the field (by adding bolus or increasing its intensity) are warranted.

Feasibility of a superficial dose reduction technique with a lead foil in Halcyon™

Feasibility of a superficial dose reduction technique with a lead foil in Halcyon™

Treatment planning of total marrow irradiation with intensity-modulated spot-scanning proton therapy.

PurposeThe goal of this study is to investigate treatment planning of total marrow irradiation (TMI) using intensity-modulated spot-scanning proton therapy (IMPT). The dosimetric parameters of the intensity-modulated proton plans were evaluated and compared with the corresponding TMI plans generated with volumetric modulated arc therapy (VMAT) using photon beams.MethodsIntensity-modulated proton plans for TMI were created using the Monte Carlo dose-calculation algorithm in the Raystation 11A treatment planning system with spot-scanning proton beams from the MEVION S250i Hyperscan system. Treatment plans were generated with four isocenters placed along the longitudinal direction, each with a set of five beams for a total of 20 beams. VMAT-TMI plans were generated with the Eclipse-V15 analytical anisotropic algorithm (AAA) using a Varian Trilogy machine. Three planning target volumes (PTVs) for the bones, ribs, and spleen were covered by 12 Gy. The dose conformity index, D80, D50, and D10, for PTVs and organs at risk (OARs) for the IMPT plans were quantified and compared with the corresponding VMAT plans.ResultsThe mean dose for most of the OARs was reduced substantially (5% and more) in the IMPT plans for TMI in comparison with VMAT plans except for the esophagus and thyroid, which experienced an increase in dose. This dose reduction is due to the fast dose falloff of the distal Bragg peak in the proton plans. The conformity index was found to be similar (0.78 vs 0.75) for the photon and proton plans. IMPT plans provided superior superficial dose coverage for the skull and ribs in comparison with VMAT because of increased entrance dose deposition by the proton beams.ConclusionTreatment plans for TMI generated with IMPT were superior to VMAT plans mainly due to a large reduction in the OAR dose. Although the current IMPT-TMI technique is not clinically practical due to the long overall treatment time, this study presents an enticing alternative to conventional TMI with photons by providing superior dose coverage of the targets, increased sparing of the OARs, and enhanced radiobiological effects associated with proton therapy.

Influencing Factors of Total Skin Irradiation With Helical Tomotherapy.

PurposeTo investigate the influencing factors of total skin irradiation (TSI) with helical tomotherapy for guiding the clinical selection of the suitable parameters and optimizing the plan quality and efficiency.Materials and MethodsSix patients with mycosis fungoides (MF) who received TSI were retrospectively selected. They were all dressed with 5 mm thick diving suits during the CT scan and treatment as a bolus to increase the superficial dose through buildup. The dose prescription was 24 Gy in 20 fractions and 5 times per week. During the planned pretreatment, Ring0, Ring1, Ring2, Ring3, and Ring4 of 1 cm thick away from the planning target volume (PTV) at the distances of 0, 1, 2, 3, and 4 cm and other normal tissues (NTs) were generated, respectively. The auxiliary structures were completely blocked during planning; while the field widths were 5 and 2.5 cm, the pitches were 0.287 and 0.215, the modulation factors were 4 and 3, and the other parameters remained consistent. Finally, the dose parameters of PTV and auxiliary structures, as well as the beam on time (BOT) and gantry period, were compared and analyzed.Resultswhen the auxiliary structures were completely blocked with distance to PTV (dPTV) above 3 cm were used, the mean dose (Dmean), conformity index (CI), and heterogeneity index (HI) of the PTV met the clinical requirements. As the dPTV gradually increased, the BOT decreased while the volume of normal tissue that received excessive radiation increased correspondingly. If the dPTV was less than 3 cm, the clinical requirements were not met. The field widths (FWs), pitches, and modulation factors (MFs) had no effect on PTVmean and the HI. The FW of 2.5 cm was slightly better than 5 cm for the CI. The FW and MF had a significant impact on the BOT, which gradually increased with decreasing FW and increasing MF. Pitch had no effect on the BOT.ConclusionDuring planning with TSI patients, dPTV is the key factor that has a significant influence on the plan quality. We found that the plan with the dPTV above 3 cm can meet clinical objectives. The BOT increases as the dPTV increases. The FWs also have an effect on the CI and BOT. Therefore, it is necessary to comprehensively balance these factors to optimize the quality and efficiency of the plan. We also found that different MFs and pitches have no obvious effect on the results.

464 A Novel High Dose Rate Brachytherapy Device for Preventing Local Recurrence of Pancreatic Cancer Dosimetry Verification

OBJECTIVES/GOALS: To demonstrate safety and limit damage to off-target organs, we will be determining dosimetry parameters through experimentation and Monte Carlo simulations with our brachy therapy applicator designed to improve upon current designs to treat a 3-dimensional volume. METHODS/STUDY POPULATION: Low-cost materials were used to manufacture our High Dose Rate (HDR) applicator and a readily available after loading system was used to load our configuration with a radioactive source. The dosimetry of our device was analyzed using commercially available software and external beam therapy films to generate depth dose profiles and superficial dose distribution. Additionally, we attempt to confirm Task Group No. 43 (TG-43) dosimetry parameters using Monte Carlo simulations for our device. These data were compared with currently available applicators used for intraoperative radiotherapy. RESULTS/ANTICIPATED RESULTS: We anticipate that we will be able to validate dosimetry parameters for our device in preparation for clinical use. We aim to show our dose distributions align well with proposed target volumes while considering the composition and shape of our applicator. We hope to demonstrate that, unlike current applicators, our design is more effective at treating a 3-dimensional target volume. DISCUSSION/SIGNIFICANCE: By 2040, pancreatic cancer will be the second-largest cause of cancer-related deaths. Even with current brachytherapy applicators, 30-40% of pancreatic cancer seems to recur near the surgical site after surgery. By preventing local recurrence, we hope to improve patient outcomes.

Efficacy of tangential irradiation with volumetric modulated arc therapy on scalp angiosarcoma using medical linac.

To increase the superficial dose and reduce the brain dose for radiotherapy of scalp angiosarcoma, we propose a novel irradiation technique of tangential irradiation volumetric modulated arc therapy (TI-VMAT). TI-VMAT and the conventional VMAT treatment plans for thirteen scalp angiosarcoma patients were created with a prescribed dose of 70Gy. Each treatment was normalized to cover 95% of the planning target volume (PTV) with its prescribed dose. To realize TI-VMAT, an avoidance structure (AS) function was applied. AS was defined as a contour subtracted PTV by a certain space from the brain contour. TI-VMAT treatment plans for six different spaces between PTV and AS were developed and compared with the conventional VMAT treatment plan with respect to the following dosimetric parameters: homogeneity index (HI) and conformity index (CI) of the PTV, mean brain dose, and brain volume irradiated with 20% (V20% [cc]), 40% (V40% [cc]), 60% (V60% [cc]), 80% (V80% [cc]), and 100% (V100% [cc]) of the prescribed dose. HI and CI were comparable between TI-VMAT and the conventional VMAT, the mean brain dose for TI-VMAT with AS defined by a space of 2.0cm and jaw tracking was 14.27Gy, which was significantly lower than that for the conventional VMAT (21.20Gy). In addition, dosimetric parameters such as V20% [cc] were significantly suppressed compared to those for high doses. Our proposed irradiation technique TI-VMAT shows the potential to reduce radiation doses in the brain with maintaining higher dose coverage on the PTV.

Effect of auto flash margin on superficial dose in breast conserving radiotherapy for breast cancer.

PurposeTo investigate the dose‐effect of Auto Flash Margin (AFM) on breast cancer's superficial tissues based on the Treatment Planning System (TPS) in the breast‐conserving radiotherapy plan.MethodsA total of 16 breast‐conserving patients with early stage breast cancer were selected, using the X‐ray Voxel Monte Carlo (XVMC) algorithm. Then, every included case plan was designed using a 2 cm‐AFM (the value of AFM is 2 cm) and N‐AFM (without AFM). Under the condition of ensuring the same configuration of #MU and collimator, the absorbed dose after a simulated inspiratory motion was calculated again using the new plan center, which moved backward to the linac source. The dose difference between the measurement points between AFM and N‐AFM groups was compared.ResultsIn the dose results, PTVV50Gy of the AFM group was superior to that of the N‐AFM group, PTVD2%, PTVDmean, Lung_IpsiV20Gy, Lung_IpsiDmean, and BodyDmax. Also, the dose results of the N‐AFM group were significantly higher than those of the AFM group. However, there was no significant difference between Lung_ContraV5Gy, HeartDmean, and Breast_ContraV10Gy in the two groups. In the collimator alignments at the same angle between groups, the AFM group formed an apparent air region outside the collimator compared with the N‐AFM group. In the XVMC algorithm feature parameter, the AFM group had less #MU, higher QE, and slightly longer optimization time. The #segments of both groups were close to the 240 control points preset by the plan. The validation results of EBT3 film in both groups were more significant than 95%, meeting the clinical plan's application requirements. The difference in film results between groups was mainly reflected in the dose distribution at the near‐source. 4DCT was used to summarize the maximum and minimum inspiratory motion distances of 7.31 ± 0.45 and 3.42 ± 0.91 mm respectively.ConclusionsThese results suggest that the AFM function application could significantly reduce the possibility of insufficient tumor target caused by inspiratory motion and ensure sufficient tumor target exposure.

Effects on skin dose from unwanted air gaps under bolus in an MR-guided linear accelerator (MR-linac) system

Bolus is commonly used in MV photon radiotherapy to increase superficial dose and improve dose uniformity for treating shallow lesions. However, irregular patient body contours can cause unwanted air gaps between a bolus and patient skin. The resulting dosimetric errors could be exacerbated in MR-Linac treatments, as secondary electrons generated by photons are affected by the magnetic field. This study aimed to quantify the dosimetric effect of unwanted gaps between bolus and skin surface in an MR-Linac. A parallel-plate ionization chamber and EBT3 films were utilized to evaluate the surface dose under bolus with various gantry angles, field sizes, and different air gaps. The results of surface dose measurements were then compared to Monaco 5.40 Treatment Planning System (TPS) calculations. The suitability of using a parallel-plate chamber in MR-Linac measurement was validated by benchmarking the percentage depth dose and output factors with the microDiamond detector and air-filled ionization chamber measurements in water. A non-symmetric response of the parallel-plate chamber to oblique beams in the magnetic field was characterized. Unwanted air gaps significantly reduced the skin dose. For a frontal beam, skin dose was halved when there was a 5 mm gap, a much larger difference than in a conventional linac. Skin dose manifested a non-symmetric pattern in terms of gantry angle and gap size. The TPS overestimated skin dose in general, but shared the same trend with measurement when there was no air gap, or the gap size was larger than 5 mm. However, the calculated and measured results had a large discrepancy when the bolus-skin gap was below 5 mm. When treating superficial lesions, unwanted air gaps under the bolus will compromise the dosimetric goals. Our results highlight the importance of avoiding air gaps between bolus and skin when treating superficial lesions using an MR-Linac system.

Evaluation of the surface dose for total body irradiation (TBI) technique with parallel-opposed anterior posterior geometry

AbstractAim:Total body irradiation (TBI) is an external radiotherapy technique in which the whole body including the superficial regions is required to receive the therapeutic dose. The purpose of this study is to evaluate the received surface dose during TBI technique.Methods and materials:The anterior/posterior (AP/PA) TBI was implemented with 18-MV photon beam at 312-cm treatment distance for human-like phantom. The GAFCHROMIC-EBT3 films were used for superficial dose measurements.Results and discussion:The percentage of surface-absorbed dose relative to the prescription point for 8 points of measurements was between 102·78–121·48% and 104·51–127·43% at 5 and 10 mm depth, respectively. In the chest wall region due to the presence of lung blocks, the absorbed dose was below the acceptable level, so an electron boost was required to increase the chest wall absorbed dose.Conclusions:According to the results, the implemented technique was able to deliver sufficient dose to the shallow surface of phantom’s body.

Reduction of superficial radiation dose with bolus in passive scattering proton beam therapy.

PurposeIn passive scattering proton beam therapy, scattered protons from the snout and aperture increase the superficial dose, however, treatment planning systems (TPSs) based on analytic algorithms (such as proton convolution superposition) are often inaccurate in this aspect. This additional dose can cause permanent alopecia or severe radiation dermatitis. This study aimed to evaluate the effect of bolus on the superficial radiation dose in passive scattering proton beam therapy.MethodsWe drew a clinical target volume (CTV) and a scalp‐p (phantom), and created plans using a TPS for a solid water phantom with and without bolus. We calculated the dose distribution in the established plans independently with Monte Carlo (MC) simulation and measured the actual dose distribution with an array of ion chambers and radiochromic films. To assess the clinical impact of bolus on scalp dose, we conducted independent dose verification using MC simulation in a clinical case.ResultsIn the solid water phantom without bolus, the calculated scalp‐p volume receiving 190 cGy was 20% with TPS but 80% with MC simulation when the CTV received 200 cGy. With 2 cm bolus, this decreased from 80% to 10% in MC simulation. With the measurements, average superficial dose to the scalp‐p was reduced by 5.2% when 2 cm bolus was applied. In the clinical case, the scalp‐c (clinical) volume receiving 3000 cGy decreased from 74% to 63% when 2 cm bolus was applied.ConclusionThis study revealed that bolus can reduce radiation dose at the superficial body area and alleviate toxicity in passive scattering proton beam therapy.

Evaluation of the effects of implementing a diode transmission device into the clinical workflow.

This work investigated effects of implementing the Delta4 Discover diode transmission detector into the clinical workflow. PDD and profile scans were completed with and without the Discover for a number of photon beam energies. Transmission factors were determined for all beam energies and included in Eclipse TPS to account for the attenuation of the Discover. A variety of IMRT plans were delivered to a Delta4 Phantom+ with and without the Discover to evaluate the Discover's effects on IMRT QA. An imaging QA phantom was used to assess the detector's effects on MV image quality. OSLDs placed on the Phantom+ were used to determine the detector's effects on superficial dose. The largest effect on PDDs after dmax was 0.5%. The largest change in beam profile symmetry and flatness was 0.2% and 0.1%, respectively. An average difference in gamma passing rates (2%/2mm) of 0.2% was observed between plans that did not include the Discover in the measurement and calculation to plans that did include the Discover in the measurement and calculation. The Discover did not significantly change the MV image quality, and the largest observed increase in the relative superficial dose when the Discover was present was 1%. The effects the Discover has on the linac beam were found to be minimal. The device can be implemented into the clinic without the need to alter the TPS beam modeling, other than accounting for the device's attenuation. However, a careful workflow review to implement the Discover should be completed.