Technical note: A comparison of in-house 3D-printed and commercially available patient-specific skin collimators for use with electron beam therapy

PURPOSE

Skin collimation is a useful tool in electron beam therapy (EBT) to decrease the penumbra at the field edge and minimize dose to nearby superficial organs at risk (OARs), but manually fabricating these collimation devices in the clinic to conform to the patient's anatomy can be a difficult and time intensive process. This work compares two types of patient-specific skin collimation (in-house 3D printed and vendor-provided machined brass) using clinically relevant metrics.

METHODS

Attenuation measurements were performed to determine the thickness of each material needed to adequately shield both 6 and 9 MeV electron beams. Relative and absolute dose planes at various depths were measured using radiochromic film to compare the surface dose, flatness, and penumbra of the different skin collimation materials.

RESULTS

Clinically acceptable thicknesses of each material were determined for both 6 and 9 MeV electron beams. Field width, flatness, and penumbra results between the two systems were very similar and significantly improved compared to measurements performed with no surface collimation.

CONCLUSION

Both skin collimation methods investigated in this work generate sharp penumbras at the field edge and can minimize dose to superficial OARs compared to treatment fields with no surface collimation. The benefits of skin collimation are greatest for lower energy electron beams, and the benefits decrease as the measurement depth increases. Using bolus with skin collimation is recommended to avoid surface dose enhancement seen with collimators placed on the skin surface. Ultimately, the appropriate choice of material will depend on the desire to create these devices in-house or outsource the fabrication to a vendor.

[Investigation of the Usefulness of Monte Carlo Simulation in Electron Beam Therapy Using Body Surface Lead Cutout]

PURPOSE

The purpose of this study was to understand the PDD and OAR during electron beam therapy using lead cutout on the body surface.

METHODS

The Monte Carlo code PHITS version 3.24 was used to simulate PDD and OAR. The simulation results were compared with actual measurements using a silicon diode detector to evaluate the validity of the simulation results.

RESULTS

The simulated PDD and OAR parameters of the linac agreed with the measured values within 2 mm. When the lead cutout on the body surface was used, all parameters except for R100 agreed with the measured values within 2 mm. The cutout sizes of the broad-beam square irradiation fields were 3 cm for 6 MeV, 5 cm for 12 MeV, and 8 cm for 18 MeV when the lead cutout on the body surface was used.

CONCLUSION

The Monte Carlo simulation was useful for understanding the PDD and OAR of the lead cutout irradiation fields, which are difficult to measure.

Development of a real-time in vivo dosimetry tool for electron beam therapy using a flexible thin film solar cell coated with scintillator powder

PURPOSE

A real-time solar cell based in vivo dosimetry system (SC-IVD) was developed using a flexible thin film solar cell and scintillating powder. The present study evaluated the clinical feasibility of the SC-IVD in electron beam therapy.

METHODS

A thin film solar cell was coated with 100 mg of scintillating powder using an optical adhesive to enhance the sensitivity of the SC-IVD. Calibration factors were obtained by dividing the dose, measured at a reference depth for 6-20 MeV electron beam energy, by the signal obtained using the SC-IVD. Dosimetric characteristics of SC-IVDs containing variable quantities of scintillating powder (0-500 mg) were evaluated, including energy, dose rate, and beam angle dependencies, as well as dose linearity. To determine the extent to which the SC-IVD affected the dose to the medium, doses at R90 were compared depending on whether the SC-IVD was on the surface. Finally, the accuracy of surface doses measured using the SC-IVD was evaluated by comparison with surface doses measured using a Markus chamber.

RESULTS

Charge measured using the SC-IVD increased linearly with dose and was within 1% of the average signal according to the dose rate. The signal generated by the SC-IVD increased as the beam angle increased. The presence of the SC-IVD on the surface of a phantom resulted in a 0.5%-2.2% reduction in dose at R90 for 6-20 MeV electron beams compared with the bare phantom. Surface doses measured using the SC-IVD system and Markus chamber differed by less than 5%.

CONCLUSIONS

The dosimetric characteristics of the SC-IVD were evaluated in this study. The results showed that it accurately measured the surface dose without a significant difference of dose in the medium when compared with the Markus chamber. The flexibility of the SC-IVD allows it to be attached to a patient's skin, enabling real-time and cost-effective measurement.

Empirical formulation for small circular electron fields

An empirical model for small circular electron fields was developed. This can be of great help in the treatment planning process for small circular electron fields. A complete dosimetric analysis of the circular fields defined by electron cutouts diameters (2 cm-9 cm) was done for nominal electron energies ranging between 6 MeV and 20 MeV using a 3D water phantom and a pin-point ion chamber. Properties studied included depth dose, in-air inverse-square fall-off, and beam profiles. The Varian Clianc 2100 C accelerator was modelled, benchmarked and Monte Carlo simulations were performed using the EGSnrc/BEAMnrc code for the small circular cutouts. A simple exponential model was found to accurately predict the very important therapeutic depth (90% of Dmax) for the small circular field size within an accuracy of better than 2 mm in most cases. The model has only two parameters (d₉₀and 'b'). Also, the penumbra widths (90% of the off-axis profiles) of these small circular electron fields were studied and least square fitted to a simple quadratic model. Full dosimetric profiles of these small circular electron fields were further studied using the benchmarked Monte Carlo simulations. This study presents a simple model to predict the very important therapeutic depth (90% of Dmax) and a recipe to develop such an electron treatment model for any linear accelerator system. Such predictions can be extremely valuable and time saving prior to treatment planning involving not only small circular shaped electron fields but also irregularly shaped electron fields.

Conformal electron beam radiotherapy using custom-made step bolus for postmastectomy chest wall irradiation: An institutional experience

Background

Postmastectomy radiation (PMRT) to the chest wall using electron beam treatment with uniform bolus was practiced at our institution. The planning target volume (PTV) included the chest wall and the internal mammary nodes (IMN) along with supraclavicular nodal regions. The varying thickness of the postmastectomy chest wall and the varying position of the IMN resulted in dose inhomogeneity in the PTV. In addition, there was the risk of increased lung and cardiac doses. In this prospective study, we report the making of a custom-made bolus using dental wax called "step bolus."

Materials and Methods

From March 2010 to January 2011, 167 patients received PMRT. As conformal photon plans were not acceptable in 48 patients, they were treated with single energy electrons and custom-made bolus.

Results

Addition of the step bolus improved dose distribution to the PTV reduced the mean lung dose %, the mean heart dose % and lung dose (D10, D20, D30, D50, and D70). Forty-seven patients had Grade 2, and one patient had Grade 3 skin toxicity. Acute symptomatic radiation pneumonitis was observed in one patient. At 5 years, 29 patients were alive with a median follow-up of 32 months and no local recurrences were observed. One patient died of myocardial infarction unrelated to treatment, one patient did not come for follow-up, 22 patients had systemic metastases, and 24 patients were disease free.

Conclusion

A custom-made step bolus using dental wax can be used for tissue compensation in electron beam therapy with resulting good local disease control and acceptable toxicity.

Heart-sparing radiotherapy with three-dimensional printing technology after mastectomy for patients with left breast cancer

The purpose of this study was to analyze the effectiveness of electron beam therapy (EBT) with patient-tailored bolus (PTB) using three-dimensional printing technology to reduce heart and lung doses during post-mastectomy radiotherapy (PMRT). For 28 patients with left breast cancer, we designed customized virtual bolus for PMRT to compensate for surface irregularities on computed tomography images and developed optimized plans for EBT. As comparison between the PTB and tangential plans, the PTB plan reduced unnecessary exposure to heart and ipsilateral lung with better target coverage compared with the tangential technique.

Radiation therapy techniques in the treatment of skin cancer: an overview of the current status and outlook

Nonmelanoma skin cancer (NMSC) is a major health concern due to its high incidence rate, its negative impact on the quality of life of patients as well as the associated economic burden to the healthcare system. Surgery is currently the primary treatment offered for skin cancer patients but not applicable or available in all cases. Radiation therapy (RT), with its long successful history in the management of cancer, has shown to be an effective alternative or complementary method in cutaneous oncology. Specifically, for dermatology applications, RT is very often the preferred option due to its favorable cosmetic results, besides the excellent control rate of the tumor. During the last 120 years since the introduction of treatments based on ionizing radiation, several techniques in this area have been developed. Radionuclide brachytherapy, electronic brachytherapy, X-ray therapies with kilovolt (kV) to megavolt (MV) photons and electron beam therapy are the established methods that are currently used on skin cancer patients. The purpose of this article is to overview these techniques and discuss the pros and cons of these methods in dermatology practices. Additionally, a new approach of beta RT of superficial skin tumors is discussed, which may offer exciting features in the management of NMSC.

Novel treatment options for nonmelanoma skin cancer: focus on electronic brachytherapy

Nonmelanoma skin cancer (NMSC) is an increasing health care issue in the United States, significantly affecting quality of life and impacting health care costs. Radiotherapy has a long history in the treatment of NMSC. Shortly after the discovery of X-rays and ²²⁶Radium, physicians cured patients with NMSC using these new treatments. Both X-ray therapy and brachytherapy have evolved over the years, ultimately delivering higher cure rates and lower toxicity. Electronic brachytherapy for NMSC is based on the technical and clinical data obtained from radionuclide skin surface brachytherapy and the small skin surface applicators developed over the past 25 years. The purpose of this review is to introduce electronic brachytherapy in the context of the history, data, and utilization of traditional radiotherapy and brachytherapy.