Alexandra Quinn

Surface dosimetry for breast radiotherapy in the presence of immobilization cast material

Curative breast radiotherapy typically leaves patients with varying degrees of cosmetic damage. One problem interfering with cosmetically acceptable breast radiotherapy is the external contour for large pendulous breasts which often results in high doses to skin folds. Thermoplastic casts are often employed to secure the breasts to maintain setup reproducibility and limit the presence of skin folds. This paper aims to determine changes in surface dose that can be attributed to the use of thermoplastic immobilization casts. Skin dose for a clinical hybrid conformal/IMRT breast plan was measured using radiochromic film and MOSFET detectors at a range of water equivalent depths representative of the different skin layers. The radiochromic film was used as an integrating dosimeter, while the MOSFETs were used for real-time dosimetry to isolate the contribution of skin dose from individual IMRT segments. Strips of film were placed at various locations on the breast and the MOSFETs were used to measure skin dose at 16 positions spaced along the film strips for comparison of data. The results showed an increase in skin dose in the presence of the immobilization cast of up to 45.7% and 62.3% of the skin dose without the immobilization cast present as measured with Gafchromic EBT film and MOSFETs, respectively. The increase in skin dose due to the immobilization cast varied with the angle of beam incidence and was greatest when the beam was normally incident on the phantom. The increase in surface dose with the immobilization cast was greater under entrance dose conditions compared to exit dose conditions.

Megavoltage cone beam CT near surface dose measurements: potential implications for breast radiotherapy

PURPOSE Cone beam computed tomography (CBCT) is fast becoming standard on modern linear accelerators. CBCT increases the dose to regions within and outside the treatment field, potentially increasing secondary cancer induction and toxicity. This study quantified megavoltage (MV) CBCT skin dose and compared it to skin dose delivered during standard tangential breast radiotherapy. METHOD Dosimetry was performed both in- and out-of-field using thermoluminescent dosimeters (TLDs) and a metal-oxide-semiconductor-field-effect-transistor (MOSFET) detector specifically designed for skin dosimetry; these were placed superficially on a female anthropomorphic phantom. RESULTS The skin dose from a single treatment fraction ranged from 0.5 to 1.4 Gy on the ipsilateral breast, 0.031-0.18 Gy on the contralateral breast, and 0-0.02 Gy in the head and pelvic region. An 8 MU MV CBCT delivered a skin dose that ranged from 0.02 to 0.05 Gy in the chest region and was less than 0.01 Gy in the head and pelvis regions. One MV CBCT per fraction was found to increase the out-of-field skin dose from both the CBCT and the treatment fields by approximately 20%. The imaging dose as a percentage of treatment doses in the ipsilateral breast region was 3% for both dosimeters. CONCLUSION Imaging increases the skin dose to regions outside the treatment field particularly regions immediately adjacent the target volume. This small extra dose to the breasts should be considered when developing clinical protocols and assessing dose for clinical trials.

Normal tissue dose and second cancer risk due to megavoltage fan-beam CT, static tomotherapy and helical tomotherapy in breast radiotherapy.

This study investigates the dose from the 1mm collimator width megavoltage fan-beam CT (fine, normal and coarse pitch) available on tomotherapy as well as for whole-breast tomotherapy treatments. The BEIR VII lifetime attributable risk model was utilised to assess the significance of the imaging dose relative to the treatment dose.

Radiation dose and contralateral breast cancer risk associated with megavoltage cone-beam computed tomographic image verification in breast radiation therapy.

PURPOSE To measure and compare organ doses from a standard tangential breast radiation therapy treatment (50 Gy delivered in 25 fractions) and a megavoltage cone-beam computed tomography (MV-CBCT), taken for weekly image verification, and assess the risk of radiation-induced contralateral breast cancer. METHODS AND MATERIALS Organ doses were measured with thermoluminescent dosimeters placed strategically within a female anthropomorphic phantom. The risk of radiation-induced secondary cancer of the contralateral breast was estimated from these values using excess absolute risk and excess relative risk models. RESULTS The effective dose from a MV-CBCT (8-monitor units) was 35.9 ± 0.2 mSv. Weekly MV-CBCT imaging verification contributes 0.5% and 17% to the total ipsilateral and contralateral breast dose, respectively. For a woman irradiated at age 50 years, the 10-year postirradiation excess relative risk was estimated to be 0.8 and 0.9 for treatment alone and treatment plus weekly MV-CBCT imaging, respectively. The 10-year postirradiation excess absolute risk was estimated to be 4.7 and 5.6 per 10,000 women-years. CONCLUSIONS The increased dose and consequent radiation-induced second cancer risk as calculated by this study introduced by the imaging verification protocols utilizing MV-CBCT in breast radiation therapy must be weighed against the benefits of more accurate treatment. As additional image verification becomes more common, it is important that data be collected in regard to long-term malignancy risk.

Imaging dose in breast radiotherapy: does breast size affect the dose to the organs at risk and the risk of secondary cancer to the contralateral breast?

Correct target positioning is crucial for accurate dose delivery in breast radiotherapy resulting in utilisation of daily imaging. However, the radiation dose from daily imaging is associated with increased probability of secondary induced cancer. The aim of this study was to quantify doses associated with three imaging modalities and investigate the correlation of dose and varying breast size in breast radiotherapy.

Kilovoltage cone-beam CT imaging dose during breast radiotherapy: A dose comparison between a left and right breast setup

The purpose of this study was to investigate the delivered dose from a kilovoltage cone-beam computed tomography (kV-CBCT) acquired in breast treatment position for a left and right breast setup. The dose was measured with thermoluminescent dosimeters positioned within a female anthropomorphic phantom at organ locations. Imaging was performed on an Elekta Synergy XVI system with the phantom setup on a breast board. The image protocol involved 120kVp, 140mAs, and a 270° arc rotation clockwise 0° to 270° for the left breast setup and 270° to 180° for the right breast setup (maximum arc rotations possible). The dose delivered to the left breast, right breast, and heart was 5.1mGy, 3.9mGy, and 4.0mGy for the left breast setup kV-CBCT, and 6.4mGy, 6.0mGy, and 4.8mGy for the right breast setup kV-CBCT, respectively. The rotation arc of the kV-CBCT influenced the dose delivered, with the right breast setup kV-CBCT found to deliver a dose of up to 4mGy or 105% higher to the treated breasts surface in comparison with the left breast setup. This is attributed to the kV-CBCT source being more proximal to the anterior of the phantom for a right breast setup, whereas the source is more proximal to the posterior of the patient for a left-side scan.

Image guidance during breast radiotherapy: a phantom dosimetry and radiation-induced second cancer risk study

Imaging procedures utilised for patient position verification during breast radiotherapy can add a considerable dose to organs surrounding the target volume on top of therapeutic scatter dose. This study investigated the dose from a breast kilovoltage cone-beam CT (kV-CBCT), a breast megavoltage fan-beam CT (MV-FBCT), and a TomoDirectTM breast treatment. Thermoluminescent dosimeters placed within a female anthropomorphic phantom were utilised to measure the dose to various organs and tissues. The contralateral breast, lungs and heart received 0.40 cGy, 0.45 cGy and 0.40 cGy from the kV-CBCT and 1.74 cGy, 1.39 cGy and 1.73 cGy from the MV-FBCT. In comparison to treatment alone, daily imaging would increase the contralateral breast, contralateral lung and heart dose by a relative 12%, 24% and 13% for the kV-CBCT, and 52%, 101% and 58% for the MV-FBCT. The impact of the imaging dose relative to the treatment dose was assessed with linear and linear-quadratic radiation-induced secondary cancer risk models for the contralateral breast. The additional imaging dose and risk estimates presented in this study should be taken into account when considering an image modality and frequency for patient position verification protocols in breast radiotherapy.