Dosimetric evaluation of ABS, PLA and NR boluses for electron radiotherapy

Background: The use of boluses for radiation therapy is very necessary to overcome the problem of sending inhomogeneous doses in the target volume due to irregularities on the surface of the skin. The bolus materials for radiation therapy need to be evaluated. Objective: The present study aims to evaluate some handmade boluses for megavoltage electron and photon radiation therapy. Several dosimetric properties of the synthesized boluses, including relative electron density (RED), transmission factor, mass attenuation coefficient, percentage depth dose (PDD), and percentage surface dose (PSD) were investigated.Material and Methods: In this experimental study, we evaluated natural rubber, silicone rubber mixed either with aluminum or bismuth, paraffin wax, red plasticine, and play-doh as soft tissue equivalent. CT-simulator, in combination with ECLIPSE software, was used to determine bolus density. Meanwhile, Linear Accelerator (Linac) Clinac iX (Varian Medical Systems, Palo Alto), solid water phantom, and Farmer ionization chamber were used to measure and analyze of dosimetric properties. Results: The RED result analysis has proven that all synthesized boluses are equivalent to the density of soft tissue such as fat, breast, lung, and liver. The dosimetric evaluation also shows that all synthesized boluses have a density similar to the density of water and can increase the surface dose with a value ranging from 6-20% for electron energy and 30-50% for photon energy.Conclusion: In general, all synthesized boluses have an excellent opportunity to be used as an alternative tissue substitute in the surface area of the body when using megavoltage electron and photon energy.

EP-1459: Dosimetric evaluation of arc-based modulated electron radiation therapy

Purpose: In conventional photon breast therapy, the effect of setup and breathing motion is accounted for by adding a margin to the clinical target volume (CTV). In modulated electron radiation therapy (MERT), since the electron beams can be arranged nearly along the direction of the organ motion, the effect of breathing motion is greatly reduced. This work is aimed to estimate the dosimetric changes caused by setup and breathing motion for MERT of breast cancer. Method and Materials: Monte Carlo (MC) based inverse treatment planning was used based on the CT data of a breast phantom. The dosimetric accuracy of the MERT delivery using the Siemens photon MLC (pMLC) was verified previously. In this work, five Monte Carlo calculated plans were compared with different static displacements (along the beam direction) of the phantom from its normal position, i.e. ±1cm, ±0.5cm and 0cm. Comparisons were performed in terms of 2D isodose distributions, dose‐volume histograms (DVHs), minimum dose (Dmin), mean dose (Dmean) and maximum dose (Dmax). Results: For ±1cm target displacements, dose differences from the nominal plan (without target motion) are relatively large in both 2D dose distributions and DVHs. For example, the difference in Dmean for this case is about 3.4%, indicating a visible target dose reduction. For ±0.5cm target displacements, a minor difference is seen for the 90% isodose lines, and the difference in Dmean is less than 2%. The integrated dose distributions remain unchanged, indicating the effect of breathing motion on the dose coverage of the target is ignorable. Conclusions: Due to the relatively small SSDs used for pMLC‐based MERT the effect of patient setup may become significant (displacement >0.5cm). Breathing motion has little effect for en‐face electron irradiation. The magnitude of the setup effect can be estimated with the inverse‐square relationship with an accurately determined “effective” SSD.

For inversely planned multi-field modulated electron radiation therapy (MERT), the impact of different levels of energy and intensity modulation on the dosimetric characteristics of MERT plans was investigated. Four cases (breast, skin, larynx, parotid) were selected for this study. One to four different energies and one to four different intensity levels per energy were considered resulting in sixty optimized plans. The optimized plans with the best combinations of one, two, three and four energies considering all intensity levels were selected for final dose calculation. The influence of energy and intensity levels on the homogeneity index (HI) in the planning target volume (PTV) and on organs at risk (OAR) sparing was investigated. Additionally, the difference in the HI between final and optimized plans ΔHI was studied. Energy and intensity modulation both improved the HI in the PTV for the final plans. While intensity modulation had negligible influence on OAR sparing, energy modulation could also improve OAR sparing depending on the selected energies. To achieve a HI > 90% in the PTV, the minimal number of energies required were four for the breast case, three for the parotid and skin cases and one for the larynx case. ΔHI decreased with increasing number of apertures. Overall, energy modulation had a larger impact on the dosimetric characteristics of MERT plans than intensity modulation.

The CLEAR (CERN Linear Electron Accelerator for Research) facility has significantly advanced high-energy electron radiotherapy, particularly for treating deep-seated tumors. However, achieving precise and accessible treatment delivery while minimizing damage to surrounding healthy tissues remains challenging. Very High Energy Electrons Beam (VHEE) offer notable potential due to their deep penetration capabilities. However, their nearly uniform dose distribution raises concerns about unintended exposure to healthy tissues. A key innovation in this field is the use of focused VHEE beams, which deliver a concentrated dose to a small defined area at a high dose rate, potentially enhancing treatment precision. This study evaluates the dosimetric characteristics of focused VHEE beams compared to collimated beams using GEANT4/TOPAS Monte Carlo simulations. A beamline with two quadrupole magnet triplets was designed to focus VHEE beams on a water phantom, simulating clinical conditions. The findings show that focused VHEE beams increased the dose to the prostate by 5.24 % while significantly reducing the dose to adjacent organs at risk: 16.93 % to the bladder, 50.81 % to the rectum, and 68.75 % to the femoral heads. These reductions highlight the dosimetric advantage of focused VHEE beams in sparing non-targeted tissues. While these results underscore the potential benefits of focused VHEE beams for deep-seated tumor treatment, additional research, including clinical validation and patient-specific modeling, is essential to fully evaluate their clinical utility. This study lays the groundwork for optimizing VHEE beam applications in cancer therapy by demonstrating improved dose delivery accuracy and reduced risk to adjacent organs.