Defining Action Levels for In Vivo Dosimetry in Intraoperative Electron Radiotherapy

Abstract

In vivo dosimetry is recommended in intraoperative electron radiotherapy (IOERT). To perform real-time treatment monitoring, action levels (ALs) have to be calculated. Empirical approaches based on observation of samples have been reported previously, however, our aim is to present a predictive model for calculating ALs and to verify their validity with our experimental data. We considered the range of absorbed doses delivered to our detector by means of the percentage depth dose for the electron beams used. Then, we calculated the absorbed dose histograms and convoluted them with detector responses to obtain probability density functions in order to find ALs as certain probability levels. Our in vivo dosimeters were reinforced TN-502RDM-H mobile metal-oxide-semiconductor field-effect transistors (MOSFETs). Our experimental data came from 30 measurements carried out in patients undergoing IOERT for rectal, breast, sarcoma, and pancreas cancers, among others. The prescribed dose to the tumor bed was 90%, and the maximum absorbed dose was 100%. The theoretical mean absorbed dose was 90.3% and the measured mean was 93.9%. Associated confidence intervals at P ¼ .05 were 89.2% and 91.4% and 91.6% and 96.4%, respectively. With regard to individual comparisons between the model and the experiment, 37% of MOSFET measurements lay outside particular ranges defined by the derived ALs. Calculated confidence intervals at P ¼ .05 ranged from 8.6% to 14.7%. The model can describe global results successfully but cannot match all the experimental data reported. In terms of accuracy, this suggests an eventual underestimation of tumor bed bleeding or detector alignment. In terms of precision, it will be necessary to reduce positioning uncertainties for a wide set of location and treatment postures, and more precise detectors will be required. Planning and imaging tools currently under development will play a fundamental role.

Eficacia, efectividad y seguridad de la radioterapia intraoperatoria en carcinoma de mama

“…En profesionales sanitarios (110) australianos que trabajan con mujeres diagnosticadas de cáncer de mama, se investigó el incremento máximo de riesgo de recurrencia local que estaban dispuestos a aceptar de la RIO para optar por la RIO en lugar de la EBRT, asumiendo que la decisión no tenía impacto en la supervivencia global. Casi un 60% optaría por RIO asumiendo un riesgo de recurrencia entre un 1% y 3%, mientras que algo más del 3% nunca optaría por la RIO.…”

Present state and issues in IORT Physics

“…For both IORT modalities, electrons and kV-Xrays, a large number of examinations have been published dealing with in-vivo dosimetry, possibly indicating a need for better information in IORT dose and dose distribution. As detectors, MOSFETs [ 86 , 113 – 118 ], radiochromic film [ 106 , 107 , 115 – 118 , 119 ], optically stimulated luminescence detectors (OSLDs) [ 96 ] and TLDs [ 118 , 120 ] have been used, in one publication the use of semi-conductor diodes is reported [ 121 ]. Additional file 1 gives a compilation of the publications showing the detectors and their uncertainties as stated by the authors, the investigated treatment sites and the measured quantity, and highlights the detected deviations from the expected dose.…”

“…With radiochromic film, uncertainties of around 3% are reported for electrons [ 106 , 117 , 118 ]. For kV X-rays one would expect larger uncertainties due to the energy dependence of film and TLDs discussed in the section on kV dosimetry which depends on type of film .…”

“…The assumption of errors in set-up or detector positioning is supported by two publications which report smaller deviations for entrance dose than for dose in depth [ 115 , 119 ]). López -Tarjuelo et al [ 118 ] develop a model to calculate probability density distributions and confidence intervals for the dose measured with MOSFET and radiochromic film detectors from the convolution of a probability distribution of detector response with the expected dose distribution (similar to the convolutions used in small field dosimetry). A measured dose deviation exceeding the confidence interval could then be considered to lie beyond an action level.…”

Practical issues regarding angular and energy response in in vivo intraoperative electron radiotherapy dosimetry

“…Then, if the setup remains the same, the rest of the treatment can be modified to achieve the intended absorbed dose. To the authors' knowledge, action levels have been proposed 11,12,34 ; however, no experience with treatment compensation has been reported.…”