ESTRO 36 Abstract Book

S517 ESTRO 36 2017 _______________________________________________________________________________________________

The number of needles varied from 0 to 8 (average 3.8 needles per application). The rescaled TRAK and mean volume of the HR-CTV was 0.37 cGy m 2 (range: 0.23-0.50 cGy m 2 ) and 26.8 cc (range: 8.0-59.1 cc), respectively. In general, the TRAK value increased with volume. In figure 1a the TRAK values are plotted against the HR-CTV. The relation between these parameters can be described by a linear equation (see figure 1b). When setting an upper and lower limit of two standard deviations a 95% confidence interval can be derived and outliers can be identified. The higher TRAK value of these outliers suggest the volume that received the prescribed dose is much larger than the HR-CTV. This was true for these plans: due to excessive reduction of the HR-CTV, a higher dose in the IR-CTV was desired and planned in the direction of the uterus top.

A decommissioned rectal retractor was modified by drilling a small hole to allow a microMOSFET to be inserted. The MOSFET was commissioned measuring energy dependence and angular dependence of response for the range of source-MOSFET positions expected in cervix brachytherapy treatments. Standard and conformal cervix plans covering the range of applicator sizes and geometries used in clinical treatments were delivered in a water phantom. The MOSFET was monitored during treatment delivery and measured doses compared to treatment planning system (TPS) calculated doses for the total plan and for ring and inter-uterine tube (IUT) individually. Results Corrections were applied for energy dependence response (6% variation between 1 and 8 cm source-MOSFET positions) and angular dependence of response (up to 8% under response for the largest polar angle of 170°). Total plan measurements agreed with TPS calculated doses within 3.1% - 7.8% for 30° and 60° applicators but measured 16% -24% high for 45° applicators (k=2 uncertainty was estimated as 14% for total plan measurements). Separate analysis of ring and IUT measurements similarly showed good agreement for all cases except the 45° IUT for which measurements were on average 55.3% higher than expected. For the 45° IUT the MOSFET position is directly in line with the source cable and longitudinal source axis based on the source positions assumed by the TPS (see figure). A combination of a small rotation of the source relative to the IUT axis and deviation of the actual source position from the centre of the IUT could explain the measurement difference. To verify this, treatments for the 45° applicator were re- measured with the MOSFET taped to the outside of the rectal retractor in a position that was not aligned to the IUT and measured doses agreed within 8%.

Conclusion The HR-CTV can be used to predict the TRAK value. Outliers may indicate abnormalities in treatment planning and further inspection of their dose distributions is required. In this study, the deviations in the dose distributions of the outliers were accepted, since they resulted in an improved individualized treatment plan. Using this relationship, the quality assurance of the treatment plan can be improved. PO-0942 Real time in vivo dosimetry for cervix HDR brachytherapy - feasibility study using a MOSFET J. Mason 1 , P. Bownes 1 1 Leeds Cancer Centre, Medical Physics & Engineering, Leeds, United Kingdom Purpose or Objective Implementation of in vivo dosimetry (IVD) in brachytherapy is partly limited by lack of c ommercially available devices that support IVD. In this study a modified rectal retractor and MOSFET were used to investigate the feasibility of real time IVD for cervix brachytherapy with simulated treatment plans delivered Poster: Brachytherapy: Physics

Conclusion In vivo dosimetry for cervix brachytherapy would be feasible if commercial rectal retractors were designed to allow a dosimeter to be inserted. However it is important to avoid dosimeter positions aligned with the source longitudinal axis as this is a region of high dose uncertainty. PO-0943 Evaluation of a recent in vivo dosimetry methodology for HDR prostate BT using MOSFET detectors R. Fabregat Borrás 1 , S. Ruiz-Arrebola 1 , E. Rodriguez Serafín 1 , M. Fernández Montes 1 , A. García Blanco 2 , J.

in a water phantom. Material and Methods