Peripheral dose around a mobile linac for intraoperative radiotherapy: radiation protection aspects

Verification of electron beam parameters in an intraoperative linear accelerator using dosimetric and radiobiological response methods

BACKGROUND

The availability of linear accelerators (linac) for research purposes is often limited and therefore alternative radiation sources are needed to conduct radiobiological research. The National Centre for Radiation Research in Poland recently developed an intraoperative mobile linac that enables electron irradiation at energies ranging from 4 to 12 MeV and dose rates of 5 or 10 Gy/min. The present study was conducted to evaluate the electron beam parameters of this intraoperative linac and to verify the set-up to evaluate out-of-field doses in a water phantom, which were determined through dosimetric and biological response measurements.

MATERIALS AND METHODS

The distribution of radiation doses along and across the radiation beam were measured in a water phantom using a semiconductor detector and absolute doses using an ionisation chamber. Two luminal breast cancer cell lines (T-47D and HER2 positive SK-BR-3) were placed in the phantom to study radiation response at doses ranging from 2 to 10 Gy. Cell response was measured by clonogenic assays.

RESULTS AND CONCLUSION

The electron beam properties, including depth doses and profiles, were within expected range for the stated energies. These results confirm the viability of this device and set-up as a source of megavoltage electrons to evaluate the radiobiological response of tumour cells.

A Network Meta-Analysis of Surgical Treatment in Patients With Early Breast Cancer

BACKGROUND

In early breast cancer treatment, the preferred surgical regimen remains a topic of controversy, and conventional pairwise meta-analysis cannot provide a hierarchy based on clinical trial evidence. Therefore, a network meta-analysis was performed both for direct and indirect comparisons and to assess the survival outcomes of surgical regimens.

METHODS

Randomized clinical trials comparing different surgical regimens for the treatment of early breast cancer were identified. Overall survival (OS) and disease-free-survival (DFS) were analyzed using random-effects network meta-analysis on the hazard ratio (HR) scale and calculated as combined HRs and 95% confidence intervals (CIs). All statistical tests were two-sided.

RESULTS

The network meta-analysis compared 11 different surgical regimens that consisted of 13 and 17 direct comparisons between strategies for OS (34 trials; n = 23 587 patients) and DFS (32 trials; n = 22 552 patients), respectively. The values of surface under the cumulative ranking for OS and DFS after mastectomy (M)+radiotherapy (RT) were observed to be the largest. Breast-conserving surgery (BCS)+axillary node sampling+RT almost achieved the threshold for inferiority compared with the other surgical treatment arms and was statistically significantly associated with worse OS (HR = 0.51, 95% CI = 0.24 to 0.94; HR = 0.48, 95% CI = 0.22 to 0.92; HR = 0.51, 95% CI = 0.23 to 0.96). No statistically significant difference between BCS+sentinel lymph node biopsy (SLNB)+RT vs BCS+SLNB+intraoperative RT was observed in carrying out network meta-analysis (HR = 0.95, 95% CI = 0.64 to 1.36).

CONCLUSIONS

M+RT has the most favorable survival outcomes among the various surgical regimens for the treatment of early breast cancer patients. For patients who receive BCS, SNLB has more favorable outcomes than axillary node sampling. Intraoperative RT and postoperative RT have similar outcomes in patients who receive SLNB.

Electron radiotherapy (IOERT) for applications outside of the breast: Dosimetry and influence of tissue inhomogeneities

PURPOSE

The purpose of study is to investigate the dosimetry of electron intraoperative radiotherapy (IOERT) of the Intraop Mobetron 2000 mobile LINAC in treatments outside of the breast. After commissioning and external validation of dosimetry, we report in vivo results of measurements for treatments outside the breast in a large patient cohort, and investigate if the presence of inhomogeneities can affect in vivo measurements.

METHODS AND MATERIALS

Applicator factors and profile curves were measured with a stereotactic diode. The applicators factors of the 6 cm flat and beveled applicators were also confirmed with radiochromic films, parallel-plate ion chamber and by an external audit performed with ThermoLuminescent Dosimeters (TLDs). The influence of bone on dose was investigated by using radiochromic films attached to an insert equivalent to cortical bone, immersed in the water phantom. In vivo dosimetry was performed on 126 patients treated with IOERT using metal oxide-silicon semiconductor field effect transistors (MOSFETs) placed on the tumor bed.

RESULTS

Relatively small differences were found among different detectors for measurements of applicator factors. In the external audit, the agreement with the TLD was mostly within ±0.2%. The largest increase of dose due to the presence of cortical bone insert was +6.0% with energy 12 MeV and 3 cm applicator. On average, in vivo dose was significantly (+3.1%) larger than prescribed dose.

CONCLUSION

IOERT in applications outside the breast results in low discrepancies between in vivo and prescribed doses, which can be also explained with the presence of tissue inhomogeneity.