Radiation therapy for deep periocular cancer treatments when protons are unavailable: is combining electrons and orthovoltage therapy beneficial?

A Monte Carlo study on dose distribution of an orthovoltage radiation therapy system

It is important to plan radiotherapy treatment and establish optimal dose distribution to reduce the chances of side effects and injury. Because there are no commercially available tools for calculating dose distribution in orthovoltage radiotherapy in companion animals, we developed an algorithm to accomplish this and verified its characteristics using tumor disease cases. First, we used the Monte Carlo method to develop an algorithm to calculate the dose distribution of orthovoltage radiotherapy (280 kVp; MBR-320, Hitachi Medical Corporation, Tokyo, Japan) using BEAMnrc at our clinic. Using development of Monte Carlo method, dose distribution for tumor and normal organs were evaluated in brain tumors, squamous cell carcinomas of the head, and feline nasal lymphomas. In all cases of brain tumors, the mean dose delivered to the GTV ranged from 36.2 to 76.1% of the prescribed dose due to the decrease through the skull. In the nasal lymphoma in cats, the eyes with covered a 2 mm-thick lead plate, the respective average dose to the eyes was 71.8% and 89.9% less than that to the uncovered eyes. The findings may be useful for informed decision making in orthovoltage radiotherapy with more effective and targeted irradiation and data collection allowing detailed informed consent.

Current advances in the selection of adjuvant radiotherapy regimens for keloid

Keloid is a common benign skin tumor in the outpatient department, and patients are often accompanied by itching and pain. Since the pathogenesis is unknown, the effect of single method treatment is unsatisfactory, and therefore the recurrence rate is high. Therefore, comprehensive treatment is mostly used in clinical treatment. Adjuvant radiotherapy is currently one of the most effective treatments for keloid. After long-term clinical practice, brachytherapy and electron beam radiotherapy has increasingly become the gold standard of treatment, because brachytherapy provides more focused radiation treatment to focal tissue to significantly reduce recurrence rate, and better preserve normal tissue. With the development of new radiotherapy techniques, more options for the treatment of keloid. Currently, adjuvant radiotherapy has been widely recognized, but there is no consensus on the optimal protocol for adjuvant radiotherapy for keloids. This review provides a review of published treatment options and new radiotherapy techniques for adjuvant radiotherapy of keloids and gives a comprehensive evaluation for clinical treatment.

Using patient-specific bolus for pencil beam scanning proton treatment of periorbital disease

Purpose

A unique mantle cell lymphoma case with bilateral periorbital disease unresponsive to chemotherapy and with dosimetry not conducive to electron therapy was treated with pencil beam scanning (PBS) proton therapy. This patient presented treatment planning challenges due to the thin target, immediately adjacent organs at risk (OAR), and nonconformal orbital surface anatomy. Therefore, we developed a patient‐specific bolus and hypothesized that it would provide superior setup robustness, dose uniformity and dose conformity.

Materials/Methods

A blue‐wax patient‐specific bolus was generated from the patient's face contour to conform to his face and eliminate air gaps. A relative stopping power ratio (RSP) of 0.972 was measured for the blue‐wax, and the HUs were overridden accordingly in the treatment planning system (TPS). Orthogonal kV images were used for bony alignment and then to ensure positioning of the bolus through fiducial markers attached to the bolus and their contours in TPS. Daily CBCT was used to confirm the position of the bolus in relation to the patient's surface. Dosimetric characteristics were compared between (a) nonbolus, (b) conventional gel bolus and (c) patient‐specific bolus plans. An in‐house developed workflow for assessment of daily treatment dose based on CBCT images was used to evaluate inter‐fraction dose accumulation.

Results

The patient was treated to 24 cobalt gray equivalent (CGE) in 2 CGE daily fractions to the bilateral periorbital skin, constraining at least 50% of each lacrimal gland to under 20 Gy. The bolus increased proton beam range by adding 2–3 energy layers of different fields to help achieve better dose uniformity and adequate dose coverage. In contrast to the plan with conventional gel bolus, dose uniformity was significantly improved with patient‐specific bolus. The global maximum dose was reduced by 7% (from 116% to 109%). The max and mean doses were reduced by 6.0% and 7.7%, respectively, for bilateral retinas, and 3.0% and 13.9% for bilateral lacrimal glands. The max dose of the lens was reduced by 2.1%. The rigid shape, along with lightweight, and smooth fit to the patient face was well tolerated and reported as “very comfortable” by the patient. The daily position accuracy of the bolus was within 1 mm based on IGRT marker alignment. The daily dose accumulation indicates that the target coverage and OAR doses were highly consistent with the planning intention.

Conclusion

Our patient‐specific blue‐wax bolus significantly increased dose uniformity, reduced OAR doses, and maintained consistent setup accuracy compared to conventional bolus. Quality PBS proton treatment for periorbital tumors and similar challenging thin and shallow targets can be achieved using such patient‐specific bolus with robustness on both setup and dosimetry.

Determination of the absorbed dose to water for medium-energy x-ray beams using Fricke dosimetry

PURPOSE

For x-ray beams in the low and medium energy range, reference dosimetry is established in terms of air kerma. Fricke dosimetry has shown great potential in the absolute measurements of the absorbed dose to water for high-energy ranges. Therefore, the main purpose of this work was to compare the absorbed dose to water for medium-energy x-ray beams obtained through Fricke dosimetry with that obtained from the air kerma rate.

METHODS

To determine the absorbed dose to water using Fricke dosimetry, the polyethylene bags methodology was chosen. Fricke solution was irradiated at four different beam qualities. The absorbed dose to water values obtained using Fricke dosimetry were compared to those obtained using the standard protocol, using the Z-score.

RESULTS

Values of the Z-score were <2 for all measurements of absorbed dose to water, which means that the values obtained using Fricke dosimetry are equivalent to those obtained using the reference protocol. The combined standard uncertainty for the absorbed dose to water obtained by Fricke dosimetry was lower than that obtained with the ionization chamber.

CONCLUSIONS

Chemical dosimetry using a standard FeSO₄ solution has been demonstrated to be a potential option as a standard for the quantity absorbed dose to water for medium kV x-ray qualities.