Session Item

Friday
May 07
14:15 - 15:30
21st century brachytherapy: is it available, affordable and relevant?
There are several reports of declining use of brachytherapy worldwide despite its well-documented advantages. Brachytherapy is often perceived to be resource-intensive and expensive and its role in the era of increasingly sophisticated external beam radiotherapy techniques has been questioned. This session discusses some of the issues that influence availability and utilisation of brachytherapy. Topics include current status of cervix cancer brachytherapy availability worldwide and global initiatives to improve access, socio-economic factors impacting brachytherapy treatment trends in the United States, a literature review of time activity based costing studies of brachytherapy, and brachytherapy training and interest amongst European radiation oncology residents.
Symposium
00:00 - 00:00
Optical surface tracking for non-coplanar SRS treatments verified by film in a RandoAlderson phantom
PO-1340

Abstract

Optical surface tracking for non-coplanar SRS treatments verified by film in a RandoAlderson phantom
Authors: Oellers|, Michel(1)*[michel.oellers@maastro.nl];Swinnen|, Ans(1);Verhaegen|, Frank(1);
(1)MAASTRO Clinic, Medical Physics, Maastricht, The Netherlands;
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Purpose or Objective

To perform a dosimetric verification with EBT-XD film of a non-coplanar plan for multiple brain metastases monitored by a commercial optical surface tracking system (OST).

Material and Methods

A 3-camera OST system was used (Catalyst HDTM, C-RAD, Sweden) on a Varian Truebeam STx linac with a 6DoF couch. The set-up accuracy and agreement between the OST system and cone beam CT (CBCT) and kV-MV imaging at respectively couch angles 0o and 270o were examined. Then dosimetric film measurements at 3 depths in the Rando-Alderson phantom were performed based on a single isocenter non-coplanar VMAT plan containing 4 brain lesions. Initial set-up of the phantom was performed with CBCT at couch 0° and subsequently monitored by Catalyst HDTM at all other couch angles. The absolute measured film doses were compared to the calculated TPS doses.

Results

For repeated tests with the Rando-Alderson phantom, the deviations between rotational and translational isocenter corrections for CBCT and OST system are always within 0.2° (pitch, roll, yaw), and 0.1mm and 0.5mm (longitudinal, lateral, vertical) for couch positions 0° and 270°, respectively. Dose deviations between the absolute measured and calculated doses in the center of the 4 PTVs irradiated with a non-coplanar plan monitored by the OST system are -1.2%, -0.1%, -0.0% and -1.9% for PTV-1, PTV-2, PTV-3 and PTV-4, respectively. The dosimetric agreement between film and TPS presented by a local gamma evaluation criterion of 2%/2mm and 3%/1mm (both with cut-off dose value of 20%) yielded pass rates of 99.2%, 99.2%, 98.6%, 89.9% and 98.8%, 97.5%, 81.7%, 78.1% for PTV-1, PTV-2, PTV-3, and PTV-4 respectively.

Conclusion

Patient positioning for a non-coplanar single isocenter VMAT treatment of multiple brain metastases can be monitored with an OST system with submillimeter and subdegree accuracy.



Figure: A five arc single isocenter non-coplanar VMAT plan with 4 PTVs, verified using film dosimetry with EBT-XD films in the first 3 slices of the Rando-Alderson phantom head.

A five arc single isocenter non-coplanar VMAT plan with 4 PTVs, verified using film dosimetry with EBT-XD films in the first 3 slices of the Rando-Alderson phantom head.

Table:The dosimetric agreement between film and TPS doses is presented by the deviations between measured and calculated doses in the center of the 4 PTVs and by a local gamma evaluation criterion of 2%/2mm as well as 3%/1mm for the 4 PTVs irradiated with single isocenter non-coplanar VMAT and monitored using Catalyst HDTM . To put these numbers into perspective, also the results for PTV-1 irradiated in a co-planar set-up (couch 0˚) are given (highlighted in bold).


PTV
D_film (Gy)D_TPS (gy)Δ(D_film-D_TPS) %Agreement score (2%21mm)Agreement score (3%/1mm)
19.719.82-1.2%99.2%98.8%
19.159.30-1.6%99.8%99.9%
29.869.87-0.1%99.2%97.5%
39.829.82-0.0%98.6%81.7%
49.119.29-1.9%89.9%78.1%