Characterization of an X-ray irradiation system (Hitachi MBR-1618R-BE) based on Monte Carlo simulations for biomedical study.

Radiation therapy depends on predictably and reliably delivering dose to tumors and sparing normal tissues. Protons with kinetic energy of a few hundred MeV can selectively deposit dose to deep seated tumors without an exit dose, unlike x-rays. The better dose distribution is attributed to a phenomenon known as the Bragg peak. The Bragg peak is due to relatively high energy deposition within a given distance or high Linear Energy Transfer (LET). In addition, biological response to radiation depends on the dose, dose rate, and localized energy deposition patterns or LET. At present, the LET can only be measured at a given fixed point and the LET spatial distribution can only be inferred from calculations. The goal of this study is to develop and test a method to measure LET over extended areas. Traditionally, radiochromic films are used to measure dose distribution but not for LET distribution. We report the first use of these films for measuring the spatial distribution of the LET deposited by protons. The radiochromic film sensitivity diminishes for large LET. A mathematical model correlating the film sensitivity and LET is presented to justify relating LET and radiochromic film relative sensitivity. Protons were directed parallel to radiochromic film sandwiched between solid water slabs. This study proposes the scaled-normalized difference (SND) between the Treatment Planning system (TPS) and measured dose as the metric describing the LET. The SND is correlated with a Monte Carlo (MC) calculation of the LET spatial distribution for a large range of SNDs. A polynomial fit between the SND and MC LET is generated for protons having a single range of 20 cm with narrow Bragg peak. Coefficients from these fitted polynomial fits were applied to measured proton dose distributions with a variety of ranges. An identical procedure was applied to the protons deposited from Spread Out Bragg Peak and modulated by 5 cm. Gamma analysis is a method for comparing the calculated LET with the LET measured using radiochromic film at the pixel level over extended areas. Failure rates using gamma analysis are calculated for areas in the dose distribution using parameters of 25% of MC LET and 3 mm. The processed dose distributions find 5%-10% failure rates for the narrow 12.5 and 15 cm proton ranges and 10%-15% for proton ranges of 15, 17.5, and 20 cm and modulated by 5 cm. It is found through gamma analysis that the measured proton energy deposition in radiochromic film and TPS can be used to determine LET. This modified film dosimetry provides an experimental areal LET measurement that can verify MC calculations, support LET point measurements, possibly enhance biologically based proton treatment planning, and determine the polymerization process within the radiochromic film.

Radiotherapy (RT) using ultra-high dose rate (UHDR) radiation, known as FLASH RT, has shown promising results in reducing normal tissue toxicity while maintaining tumor control. However, implementing FLASH RT in clinical settings presents technical challenges, including limited depth penetration and complex treatment planning. Monte Carlo (MC) simulation is a valuable tool for dose calculation in RT and has been investigated for optimizing FLASH RT. Various MC codes, such as EGSnrc, DOSXYZnrc, and Geant4, have been used to simulate dose distributions and optimize treatment plans. Accurate dosimetry is essential for FLASH RT, and radiation detectors play a crucial role in measuring dose delivery. Solid-state detectors, including diamond detectors such as microDiamond, have demonstrated linear responses and good agreement with reference detectors in UHDR and ultra-high dose per pulse (UHDPP) ranges. Ionization chambers are commonly used for dose measurement, and advancements have been made to address their response nonlinearities at UHDPP. Studies have proposed new calculation methods and empirical models for ion recombination in ionization chambers to improve their accuracy in FLASH RT. Additionally, strip-segmented ionization chamber arrays have shown potential for the experimental measurement of dose rate distribution in proton pencil beam scanning. Radiochromic films, such as GafchromicTM EBT3, have been used for absolute dose measurement and to validate MC simulation results in high-energy X-rays, triggering the FLASH effect. These films have been utilized to characterize ionization chambers and measure off-axis and depth dose distributions in FLASH RT. In conclusion, MC simulation provides accurate dose calculation and optimization for FLASH RT, while radiation detectors, including diamond detectors, ionization chambers, and radiochromic films, offer valuable tools for dosimetry in UHDR environments. Further research is needed to refine treatment planning techniques and improve detector performance to facilitate the widespread implementation of FLASH RT, potentially revolutionizing cancer treatment.

Purpose: This study aimed to investigate the high dose rate (HDR) 192Ir brachytherapy, including near source dosimetry, of a catheter‐based applicator from 0.5 mm to 1 cm along the transverse axis. Methods: Radiochromic film and Monte Carlo (MC) simulation were used to generate absolute dose for the catheter‐based applicator. Results from radiochromic film and MC simulation were compared directly to the treatment planning system (TPS) based on the AAPM Updated Task Group 43 (TG‐43U1) dose calculation formalism. Results: Difference between dose measured using radiochromic film along the transverse plane at 0.5 mm from the surface and the predicted dose by the TPS was 24%±13%. Dose difference between the MC simulation along the transverse plane at 0.5 mm from the surface and the predicted dose by the TPS was 22.1%±3%. For distances from 1.5 mm to 1 cm from the surface, radiochromic film and MC simulation agreed with TPS within an uncertainty of 3%. Conclusion: The TPS under‐predicts the dose at the surface of the applicator, i.e., 0.5 mm from the catheter surface, as compared to the measured and MC simulation predicted dose. MC simulation results demonstrated that 15% of this error is due to neglecting the beta particles and discrete electrons emanating from the sources and not considered by the TPS and 7% of the difference was due to the photon alone, potentially due to the differences in MC dose modeling, photon spectrum, scoring techniques, and effect of the presence of the catheter and the air gap. Beyond 1mm from the surface, the TPS dose algorithm agrees with the experimental and MC data within 3%.

A comparison of HDR near source dosimetry using a treatment planning system, Monte Carlo simulation, and radiochromic film

BackgroundUltra‐high dose rate radiotherapy elicits a biological effect (FLASH), which has been shown to reduce toxicity while maintaining tumor control in preclinical radiobiology experiments. FLASH depends on the dose rate, with evidence that higher dose rates drive increased normal tissue sparing. The pattern of dose delivery also has significance for conformal proton FLASH delivered via pencil beam scanning (PBS) given its unique spatio‐temporal distribution of dose deposition.PurposeIn PBS, the machine‐generated log file contains information on the spatio‐temporal pattern of PBS delivery measured by the segmented ionization chambers in the treatment nozzle. The spot position and monitor unit (MU) obtained from log files have previously been used to reconstruct the treatment dose by Monte Carlo (MC) simulations. The incorporation of spot timing allows reconstruction of the 3D temporal dose distribution. The log‐based dose and dose rate can have a role in quality assurance (QA) and FLASH treatment verification if the reconstruction can be shown to be accurate in spatial and temporal domains of dose deposition. Thus, the objective of this study is to validate the accuracy of dose rate reconstruction using input data from machine log files of PBS delivery. By analyzing the delivered spot timing, position, and MU extracted from the logs, we aim to evaluate the reliability and precision of the log data for dose and dose rate reconstruction.MethodsFLASH PBS spread‐out Bragg peak (SOBP) treatment fields were delivered using a cyclotron accelerated proton beam. This method involves a patient and field‐specific conformal energy modulator (CEM) to achieve a SOBP at the tumor site. Log files record spot positions and the delivered MU with timing information at 250 µs resolution. To validate timing information, a 9.9 mm diameter parallel plate ionization chamber was positioned at various locations within the SOBP. An electrometer sampling at 20 kHz recorded the time‐resolved ionization current collected by the ionization chamber. These measurements were used to determine spot dose, dose rate, duration, and transition times. Disparities between the measured and logged spot map MU and timing were determined. Dose average and PBS dose rates were compared between the measurement and log‐based MC simulations.ResultsThere was a good agreement between the measured dwell time and transition time and the logged information across various detector positions. The median disparities for inter‐spot dwell time range from ‐0.041 to 0.024 ms. Differences between logged and planned spot positions are minimal, measuring less than 1.08 mm in the x direction and 1.15 mm in the y direction, consistent with prior studies and the spatial resolution of the PBS nozzle ionization chamber. Delivered MU were within 1.9% of the planned MU. Measured dose and dose rates are consistent with simulated outcomes derived from MC simulation.ConclusionWe validated the precision and accuracy of PBS log file data through measurements and MC simulations. These findings support the use of log files in MC calculations as one part of patient‐specific quality assurance (PSQA) and dose rate delivery verification for conformal proton FLASH radiotherapy with SOBP.

PURPOSE: Breast skin thickness is 3 mm (1). The subcutaneous layer (SL) lies immediately beneath the skin and is at risk for local recurrence after breast cancer surgery and adjuvant radiotherapy. As the SL lies within the buildup region for megavoltage radiation treatment, placement of bolus for patients receiving chest wall (CW) radiotherapy (RT) is routine at many centers. The dermal toxicity of bolus is well known: in 12 studies of CW RT, the pooled risk of Grade 3 acute toxicity is 9.6% with bolus and 1.2% without bolus (2). Meanwhile, bolus is rarely used after breast-conserving surgery (BCS) RT for patients with similar cancers. This study examines variation in tangential RT dose coverage of the SL in the intact breast and bolus-free CW with clinically relevant photon beam energies using Monte Carlo (MC) calculations and a commercial treatment planning system (TPS). METHODS: Thirty CT datasets from patients without skin involvement were identified. There were two groups of patients: 15 treated with BCS RT and 15 treated with CW RT. In each group, 5-patient subgroups had tangent-beam RT planned without bolus with 6, 10, and 15 MV photons, respectively, using the Analytical Anisotropic Algorithm (AAA) algorithm (v.13.6.23) in Eclipse v.15.6 (Varian Medical Systems Inc., Palo Alto, CA). On each CT, the SL was segmented as a high-resolution shell from 3 to 5 mm below the body contour in the Eclipse TPS v.15.6 (Varian Medical Systems Inc., Palo Alto, CA). A 1x1x1 mm MC dose simulation was performed using EGSnrc code (BEAMnrc/DOSXYZnrc). The MC dose distributions were imported back into the TPS for comparison with AAA calculations. The V95% and V90% for the SL were calculated for each case and the mean V95% and V90% were reported for each subgroup. A t-test was used with a two-sided alpha = 0.05 for statistical analysis. RESULTS: The mean separation increased with use of higher energies for both BCS and CW RT. The MC-calculated mean SL V90% and V95% were higher for CW RT than for BCS RT at each energy. The V90% coverage was 91.5% for CW and 74.4 % for BCS at 6 MV (p<0.001), 89.3% for CW and 61.3% for BCS (p<0.001) at 10 MV and 87.1% for CW and 60.9% for BCS (p<0.001) at 15 MV (Table 1). For SL V95% the CW coverage was higher than the BCS coverage for every energy. For SL V90% at 6 MV, the AAA and MC calculations agreed within 2.5%, with the MC being slightly higher. The agreement between AAA and MC decreased for higher energies with MC reporting higher SL V90% coverage by up to 16.3%. The higher MC-calculated dose to the SL is consistent with the literature (3). CONCLUSION: MC and AAA SL dose calculations agreed well for 6 MV, but AAA underestimated the dose for 10 and 15 MV. For 6-15 MV photons, the MC-calculated dosimetric coverage of the SL is higher for CW RT than BCS RT. Since radiation oncologists are satisfied with the SL coverage by BCS RT, bolus is not needed for CW RT, because, without bolus, CW RT delivers a higher SL dose than BCS RT. REFERENCES:. 1.Pope TL Jr, et al.J Can Assoc Radiol. 1984 Dec;35(4):365-8. PMID: 6526847. 2.Dahn HM, et al. Crit Rev Oncol Hematol. 2021 Jun 5;163:10339. PMID: 34102286. 3.Panettieri V, et al., Radiother Oncol 2009; 93: 94-101 Table 1.Monte Carlo calculated mean V90% and V95% for Breast and Chest Wall for each energyV95%V95%V95%V90%V90%V90%Energy(MV)Chest Wall(Mean)Breast. (Mean)p-valueChest Wall(Mean)Breast. (Mean)p-value661.9%35.1%<0.00191.5%74.4%<0.0011065.2%39.7%<0.00189.3%61.3%<0.0011561.7%38.9%0.01287.1%60.9%<0.001 Citation Format: Dylan Narinesingh, Alan Nichol, Alanah Bergman, Tony Popescu. Subcutaneous layer dosimetry of the breast and chest wall at clinical beam energies without bolus: A Monte Carlo and analytical anisotropic algorithm (AAA) calculation study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-19-10.

In radiation therapy, irradiating healthy normal tissues in the beam trajectories is inevitable. This unnecessary dose means that patients undergoing treatment risk developing side effects. Recently, FLASH radiotherapy delivering ultra-high-dose-rate beams has been re-examined because of its normal-tissue-sparing effect. To confirm the mean and instantaneous dose rates of the FLASH beam, stable and accurate dosimetry is required. Detailed verification of the FLASH effect requires dosimeters and a method to measure the average and instantaneous dose rate stably for 2- or 3-dimensional dose distributions. To verify the delivered FLASH beam, we utilized machine log files from the built-in monitor chamber to develop a dosimetry method to calculate the dose and average/instantaneous dose rate distributions in two or three dimensions in a phantom. To create a spread-out Bragg peak (SOBP) and deliver a uniform dose in a target, a mini-ridge filter was created with a 3D printer. Proton pencil beam line scanning plans of 2×2 cm2 , 3×3 cm2 , 4×4 cm2 , and round shapes with 2.3cm diameter patterns delivering 230 MeV energy protons were created. The absorbed dose in the solid water phantom of each plan was measured using a PPC05 ionization chamber (IBA Dosimetry, Virginia, USA) in the SOBP region, and the log files for each plan were exported from the treatment control system console. Using these log files, the delivered dose and average dose rate were calculated using two methods: a direct method and a Monte Carlo (MC) simulation method that uses log file information. The computed and average dose rates were compared with the ionization chamber measurements. Additionally, instantaneous dose rates in user-defined volumes were calculated using the MC simulation method with a temporal resolution of 5ms. Compared to ionization chamber dosimetry, 10 of 12 cases using the direct calculation method and 9 of 11 cases using the MC method had a dose difference below ±3%.Nine of 12 cases using the direct calculation method and 8 of 11 cases using the MC method had dose rate differences below ±3%.The average and maximum dose differences for the direct calculation and MC method were-0.17, +0.72%, and -3.15, +3.32%, respectively. For the dose rate difference, the average and maximum for the direct calculation and MC method were +1.26, +1.12%, and +3.75, +3.15%, respectively. In the instantaneous dose rate calculation with the MC simulation, a large fluctuation with a maximum of 163Gy/s and a minimum of 4.29Gy/s instantaneous dose rate was observed in a specific position, whereas the mean dose rate was 62Gy/s. We successfully developed methods in which machine log files are used to calculate the dose and the average and instantaneous dose rates for FLASH radiotherapy and demonstrated the feasibility of verifying the delivered FLASH beams.

Introduction: Proton radiotherapy offers an advantage in sparing healthy tissue compared to photon therapy due to the specific interaction of protons with the patient’s body. In radiobiological experiments, alpha sources are commonly used instead of proton accelerators for convenience, but ensuring a uniform dose distribution is challenging. Properly designing the cell irradiation setup is crucial to reliably measure the average cellular response in such experiments. The objective of this research is to underscore the importance of dosimetric validation in radiobiological investigations. While Monte Carlo (MC) simulations offer valuable insights, their accuracy needs experimental confirmation. Once consistent results are obtained, the reliance on simulations becomes viable, as they are more efficient and less cumbersome compared to experimental procedures. Material and methods: The simulations are performed with three MC code-based tools: Geant4-DNA, GATE, and SRIM to model the alpha radiation source and calculate dose distributions for various cell irradiation scenarios. Dosimetric verification of the experimental setup containing a 241Am source is performed using radiochromic films. Additionally, a clonogenic cell survival assay is carried out for the DU145 cell line. Results: The study introduces a novel source simulation model derived from dosimetric measurements. The comparison between dosimetric results obtained with simulations and measured experimentally yields a gamma (3%/3mm) parameter value exceeding 99%. Furthermore, the LQ model parameters fitted to survival data of DU145 cells irradiated with particles emitted from 241Am source demonstrate consistency with previously published findings. Conclusions: Radiobiological experiments investigate cellular responses to various irradiation scenarios. Challenges arise with densely ionizing radiation used in clinical practice, particularly in ensuring uniform dose delivery for reliable experiments. MC codes aid in simulating dose distribution and designing irradiation systems for consistent cell treatment. However, experimental validation is essential before relying on simulation results. Once confirmed, these results offer a cost-effective and time-efficient approach to planning radiobiological experiments compared to traditional laboratory work.

Evaluation of Uncertainty-Based Stopping Criteria for Monte Carlo Calculations of Intensity-Modulated Radiotherapy and Arc Therapy Patient Dose Distributions

We present a rigorously defined dosimetric Monte Carlo (MC) model of the MultiRad225 kilovoltage X-ray irradiator, which is validated using experimental techniques. Previous MC studies performed with the MultiRad225 lacked rigorous dosimetric validation. Experimental measurements are conducted with an ionization chamber and radiochromic film with beam energies of 119, 160, and 200 kV. MC simulations are conducted mimicking each experiment. Both beam quality (half-value layer, HVL) and quantity (dose rate) are assessed with these methods. MC simulated dose rates and HVL values show close agreement with experimental results across varying source-to-surface distances (SSD) for each beam energy with most dose rate simulations falling within 5% of the experimentally measured values. This work provides a validated MC model as a foundation for future studies with the MultiRad225. The MC model utilizes a comprehensive geometry which accurately captures relevant physical phenomena.

The aims of this study were (i) to design a new high-dose-rate (HDR) brachytherapy applicator for treating surface lesions with planning target volumes larger than 3 cm in diameter and up to 5 cm in size, using the microSelectron-HDR or Flexitron afterloader (Elekta Brachytherapy) with a (192)Ir source; (ii) to calculate by means of the Monte Carlo (MC) method the dose distribution for the new applicator when it is placed against a water phantom; and (iii) to validate experimentally the dose distributions in water. The penelope2008 MC code was used to optimize dwell positions and dwell times. Next, the dose distribution in a water phantom and the leakage dose distribution around the applicator were calculated. Finally, MC data were validated experimentally for a (192)Ir mHDR-v2 source by measuring (i) dose distributions with radiochromic EBT3 films (ISP); (ii) percentage depth-dose (PDD) curve with the parallel-plate ionization chamber Advanced Markus (PTW); and (iii) absolute dose rate with EBT3 films and the PinPoint T31016 (PTW) ionization chamber. The new applicator is made of tungsten alloy (Densimet) and consists of a set of interchangeable collimators. Three catheters are used to allocate the source at prefixed dwell positions with preset weights to produce a homogenous dose distribution at the typical prescription depth of 3 mm in water. The same plan is used for all available collimators. PDD, absolute dose rate per unit of air kerma strength, and off-axis profiles in a cylindrical water phantom are reported. These data can be used for treatment planning. Leakage around the applicator was also scored. The dose distributions, PDD, and absolute dose rate calculated agree within experimental uncertainties with the doses measured: differences of MC data with chamber measurements are up to 0.8% and with radiochromic films are up to 3.5%. The new applicator and the dosimetric data provided here will be a valuable tool in clinical practice, making treatment of large skin lesions simpler, faster, and safer. Also the dose to surrounding healthy tissues is minimal.

Purpose: To investigate the tongue‐and‐groove (TG) effect in terms of the amount of underdosage and full width at half maximum (FWHM) at the interleaf position of dose profiles as varying off‐axis distances and depths in water. Methods: The Varian millennium 120 MLC was investigated by using Monte Carlo(MC) simulations, film measurements and treatment planning system (TPS) calculations for 6 MV photon beam. To estimate the TG effect, the MLC fields were overlapped at four off‐axis distances of 0, 5, 10, 13 cm. The TG effect was estimated at 1.5, 5, and 10 cm depths in water, compared to dose profiles of the open field. The BEAMnrc and DOSXYZnrc code were used to simulate the in‐water dose distributions. The measurements were performed using the radiochromic films. The Varian TPS calculated the 3D dose distributions for the same MLC fields. Results: The results from the MC and film were in good agreement. Compared to the open field, the amount of TG underdosage for both methods generally decreased with increasing off‐axis distances and depths; approximately 9%–16% at 1.5 cm depth and 5%–11% at 10 cm depth depending on off‐axis distances. However, the TPS results showed the almost same amount of underdosage regardless of off‐axis distances; 8%–9% at 1.5 cm depth and 6%–7% at 10 cm depth. The FWHM at the interleaf position slightly increased with increasing depths. It was determined to be 0.31 cm on average for the MC and film but 0.63 cm for the TPS. Conclusions: There was the fair agreement between MC simulations and film measurements in estimating the TG effect for the 120 MLC. Since the TPS underestimated the TG effect, our results from the MC and film can be useful data to correct some modeling parameters for the 120 MLC in the TPS.

Purpose: To generate and validate a linac Monte Carlo (MC) model for surface dose prediction. Methods: BEAMnrc V4-2.4.0 was used to model 6 and 18 MV photon beams for a commercially available linac. DOSXYZnrc V4-2.4.0 calculated 3D dose distributions in water. Percent depth dose (PDD) and beam profiles were extracted for comparison to measured data. Surface dose and at depths in the buildup region was measured with radiochromic film at 100 cm SSD for 4 × 4 cm2 and 10 × 10 cm2 collimator settings for open and MLC collimated fields. For the 6 MV beam, films were placed at depths ranging from 0.015 cm to 2 cm and for 18 MV, 0.015 cm to 3.5 cm in Solid Water™. Films were calibrated for both photon energies at their respective dmax. PDDs and profiles were extracted from the film and compared to the MC data. The MC model was adjusted to match measured PDD and profiles. Results: For the 6 MV beam, the mean error(ME) in PDD between film and MC for open fields was 1.9%, whereas it was 2.4% for MLC. For the 18 MV beam, the ME in PDD for open fields was 2% and was 3.5% for MLC. For the 6 MV beam, the average root mean square(RMS) deviation for the central 80% of the beam profile for open fields was 1.5%, whereas it was 1.6% for MLC. For the 18 MV beam, the maximum RMS for open fields was 3%, and was 3.1% for MLC. Conclusion: The MC model of a linac agreed to within 4% of film measurements for depths ranging from the surface to dmax. Therefore, the MC linac model can predict surface dose for clinical applications. Future work will focus on adjusting the linac MC model to reduce RMS error and improve accuracy.

Purpose: To calculate the dose distribution with Monte Carlo (MC) around an intracavitary brachytherapy (ICBT) ovoid type applicator and compare these calculations with radiochromic film (RCF) measurements acquired in a liquid water phantom. Methods: Detailed physical measurements of a CT-MR compatible Fletcher ovoid applicator (Nucletron Co.) were performed in-house to facilitate construction of a MC model of the device. MC calculations were simulated utilizing MCNPX version 2.7.b of a microSelectron version 2 (mHDR v2) Ir-192 source (Nucletron Co.) at multiple dwell-positions within a single ovoid. Oncentra brachytherapy treatment planning system (Nucletron Co.) was used to determine the dwell-time required for three active dwell-positions to deliver 10 Gy to a prescription point taken as the center of each film. RCF measurements were performed in two perpendicular planes, (a) 1 cm beyond the distal end and (b) 3.3 cm from, and parallel to, the long axis of the ovoid. RCF measurements were performed in a liquid water phantom. The ovoid, liquid water phantom, and RCF were physically aligned using in-room patient setup lasers. The MC model application was confirmed superimposing simulated dose distributions resulting from three active dwell-positions and comparing with RCF. Results: Brachytherapy measurements could be acquired in a liquid water phantom consistent with AAPM TG-43 protocol. For both planes, 96% of the absolute simulated and measured dose points agreed within 2% or 2 mm distance-to-agreement. Conclusions: The MC model is sufficient to predict measured RCF dose distributions acquired in a liquid water phantom accurately.

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.