Absorption ratio of treatment couch and effect on surface and build-up region doses

Self-Crosslinking AuNPs Composite Hydrogel Bolus for Radiophotothermal Therapy

Radiophotothermal therapy is a promising treatment for superficial tumors. Traditional radiotherapy requires tissue boluses on the patient's skin to increase therapeutic effectiveness due to the dose-buildup effect of high-energy radiation. However, combining radiotherapy with photothermal therapy leads to uncertainties as the low-penetration near-infrared light dose is reduced after penetrating the bolus. To enhance precision and effectiveness, this study introduces a novel bolus made of AuNPs@poly(AM-THMA-DMAEMA) composite hydrogel. This hydrogel is prepared through a one-pot method involving the reduction of trihydrate chloroauric acid (HAuCl4·3H2O) and copolymerization of acrylamide (AM) and N-[Tris(hydroxymethyl)methyl]acrylamide (THMA) in a redox system with dimethylaminoethyl methacrylate (DMAEMA) and potassium persulfate (KPS). The gold nanoparticles (AuNPs) improve the mechanical strength (tensile strength of 320.84 kPa, elongation at break of 830%) and antibacterial properties (>99% against Staphylococcus aureus). The local surface plasmon resonance (LSPR) effect of AuNPs enables the hydrogel to absorb near-infrared light for precise monitoring of the infrared radiation dose. The hydrogel's biocompatibility is enhanced by the absence of additional crosslinking agents, and its excellent surface adhesion strength is due to numerous hydrogen bonds and electrostatic interactions. This study offers new possibilities for nanoparticle composite hydrogels as tissue boluses, achieving high precision and efficiency in radiophotothermal therapy.

A Study to Analyze Impact of Treatment Couch and Immobilization Devices on Surface Dose for Megavoltage Photon Beams

PURPOSE/OBJECTIVE

The purpose of this study is to investigate the effect of treatment couch and immobilization devices on surface dose for megavoltage photon beams.

MATERIAL/METHODS

Percentage surface dose (PSD) measurement was carried out in Elekta Synergy™ Linear accelerator using PTW Markus® Parallel plate ionization chamber of volume 0.05cm3 with water equivalent RW3 Slab phantom (PTW, Germany). The measurement depth was considered at 0.07mm. The reference PSD was measured at 0° gantry angle with 10×10cm2, 20×20cm2 and 30×30cm2 field sizes and 100cm SSD for 4MV, 6MV and 15MV photon beams. For comparison, PSD measurement was carried out at 180° gantry angle inclusion of treatment couch (TC), All in One positioning system (AIO - PS) and Vac lok Cushions (VLC).

RESULTS

Beam angle at 0°, for field sizes 10×10cm2, 20×20cm2 and 30×30cm2, the PSD was observed as 30.9%, 40.5%, 48.7% for 4MV; 23.7%, 33.8%, 42.2% for 6MV; and 17.0%, 29.6%, 38.6% for 15MV respectively. Beam angle at 180° with TC, an increase in PSD by maximum of 65.0% for 4MV, 64.9% for 6MV and 55.9% for 15MV as compared to 0° angle. The PSD increased when beam angle was 180° with TC and AIO - PS were 65.0% for 4MV, 67.4% for 6MV, and 60.9% for 15MV than 0° angle. Similarly, increased PSD for beam angle at 180° with TC and VLC were 66.8% for 4MV, 66.8% for 6MV and 61.3% for 15MV as compared to 0° angle.

CONCLUSION

For all three-photon energies, at 180° gantry angle, the PSD increased significantly in case of TC, VLC, and AIO - PS for all the field sizes as compared to gantry angle at 0°. It is necessary to consider TC, AIO - PS and VLC during dose calculation to ensure accuracy of patient treatment delivery.

Minimizing the Risk of Acute Skin Toxicity Due to Hip Prosthesis Avoidance in Prostate Radiation Therapy

Limiting the entrance dose through hip prostheses to improve dosimetric accuracy can result in unfavorable skin toxicity. We propose a volumetric modulated arc therapy solution that strikes a better balance between dose accuracy and skin dosimetry. Our current planning strategy limits the entrance dose through hip prostheses using stringent optimization objectives on an avoidance structure. Avoidance efficiency is evaluated by recalculating the plan with prosthesis density set at 20 g/cc, and evaluating the loss of target coverage from increased attenuation. We require this loss to be ≤5% of the original values. This approach has resulted in an uncommon skin toxicity for a prostate-bed patient with bilateral hip prostheses. Thus, the dosimetric tradeoffs between skin dose and prosthesis avoidance were investigated by incrementally reducing prosthesis avoidance to achieve maximum skin doses (D_max) between 30 and 50 Gy. When prosthesis avoidance is prioritized, the skin dose increases and the target dose coverage and conformity decrease. A large degradation in target coverage for plans with the lowest skin D_max of 30 to 35 Gy indicates that a significant proportion of the target dose arises from beams entering the prostheses. The plan with a skin D_max of 40 Gy provides a better compromise between skin and prosthesis entrance doses, with a <20% reduction in target coverage at an increased prosthesis density of 20 g/cm³. Skin dose needs to be considered when using prosthesis avoidance planning strategies. Allowing for a minimal dose through the prosthesis may be required to restrict skin dose and reduce the risk of toxicity.

Effect of linear accelerator carbon fiber couch on radiotherapy dose

This study aimed to explore the effect of carbon fiber couch on radiotherapy dose attenuation and gamma pass rate in intensity-modulated radiotherapy (IMRT) plans. A phantom inserted with an ionization chamber was placed at different positions of the couch, and the dose was measured by the chamber. Under the same positioning, the phantom dose was calculated using the real and virtual couch images, and the difference in the planned dose of radiotherapy was compared. Ten clinical IMRT plans were selected as dose verification data, and the gamma pass rates were compared between couch addition and non-addition conditions. When the radiation field was near 110° and 250°, the measured value attenuation coefficient of the ionization chamber at the joint of the couch was up to 34%; the attenuation coefficient of the treatment couch from the actual couch image calculated using the treatment planning system (TPS) was up to 33%; the attenuation coefficient of the virtual couch calculated using the TPS was up to 4.0%. The gamma pass rate of the dose verification near gantry angles 110° and 250° was low, and that of the joint could be lower than 85% under the condition of 3%/3 mm. The gamma pass rates of the radiation field passing through the couch were all affected. The dose was affected by the radiation field passing through the couch, with the largest effect when passing through the joint part of the treatment couch, followed by that of the main couch plate and extension plate. When the irradiation field passed through the joint and near 110° and 250° of the main couch, the dose difference was large, making it unsuitable for treatment.