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ESTRO 2020

Session Item

Physics track: Dose measurement and dose calculation
9319
Poster
Physics
00:00 - 00:00
In-vivo surface dose measurements in breast cancer patient treated with helical radiation therapy
Marco Valenti, Italy
PO-1346

Abstract

In-vivo surface dose measurements in breast cancer patient treated with helical radiation therapy
Authors: Francesca Angeletti.(Polytechnic University of Marche, Degree course for medical radiology technician, Ancona, Italy), Francesca Cucciarelli.(Azienda Ospedaliero Universitaria Ospedali Riuniti, Radiotherapy, Ancona, Italy), Stefania Maggi.(Azienda Ospedaliero Universitaria Ospedali Riuniti, Medical Physics, Ancona, Italy), Giovanna Mantello.(Azienda Ospedaliero Universitaria Ospedali Riuniti, Radiotherapy, Ancona, Italy), Marco Parisotto.(Azienda Ospedaliero Universitaria Ospedali Riuniti, Medical Physics, Ancona, Italy), Marco Valenti.(Azienda Ospedaliero Universitaria Ospedali Riuniti, Medical Physics, Ancona, Italy)
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Purpose or Objective

In-vivo assessment of Thomotherapy Treatment Planning System (TPS) dose estimation accuracy in breast cancer patient treated with helical volumetric radiation therapy when skin is part of the target.

Material and Methods

In vivo dosimetry was performed weekly, as part of our quality assurance program, on 7 breast patient where skin is part of the volume to treat. For superficial dose measurements we used EBT3 gaf chromic. Small strips (1,5X4 cm2) of film were positioned on patient skin in a specific point, individuated on CT scan images, where TPS estimated dose in near to 200 cGy (fraction prescription dose). EBT3 film strips were scanned 24 hour after irradiation in RGB mode using an EPSON 1000 scanner with a resolution of 72 dpi. Red channel was used and, for optical density to dose conversion, we used the procedure described in [1]. In each measurement day, to check dosimetric system stability, a reference strip of EBT3 was irradiated in virtual water phantom with a known dose.

Results

In figure 1 are reported the in vivo measured dose value for each patient. The blue dots represent the single fraction measures and the red squares the patient average value. Near each average dose point is reported the indication of single patient percentage deviation from calculated dose. All the measurements are summarized in the box plot of figure 2.  Measured surface dose is always lower than prescribed dose (ranging from -45% to -6%, with a media value of -31%). Another work [2], performed on 17 patients, reports e higher surface dose value (-10% respect to prescription dose) but in this case TLD dosimeters have been used and the surface dose is referred to a deeper point (about 0,5mm) and a “virtual bolus” like approach is used for planning. Several factors concur to the observed dose deviation: TPS surface dose overestimation [3], patient breathing and patient set up inaccuracy. The latter factor is highlighted by the observed inter-fraction dose variation.

Conclusion

Conclusion

The mean measured surface dose is 141±22 cGy, with a deviation of -30% from prescribed dose. If the clinical goal is to have a better dose coverage at the evaluated depth (0,15mm for EBT3) alternative approaches must be considered, as the use of bolus material [4,5] or the use of a virtual bolus strategy for planning [2,6].

Bibliography

[1] Lewis et al. - Med Phys. 2012 Oct;39(10):6339-50

[2] Zibold et al. - Strahlenther Onkol. 2009 Sep;185(9):574-81

[3] Ramsey et al. - Med Phys. 2007 Aug;34(8):3286-93

[4] Cheek et al. - Med Phys. 2008 Aug;35(8):3565-73

[5] Galle et al. - Technol Cancer Res Treat. 2016 Jun;15(3):411-5

[6] Moliner er al. - J Appl Clin Med Phys. 2015 Nov 8;16(6):164–176