Download Pre-treatment and in-vivo dosimetry of Helical Tomotherapy (HT)

Pre-treatment and in-vivo dosimetry of Helical Tomotherapy (HT) treatment
plans by using the Dosimetry Check system coupled to HT detectors
1
E. Mezzenga, 1E. Cagni, 1A. Botti, 1M. Orlandi, 2W.D. Renner, 1M. Iori
1
Medical Physics Unit, ASMN-IRCCS of Reggio Emilia, Italy; 2MathResolution LLC, Columbia, MD, USA
Purpose: In radiotherapy, the proper way to ensure that the dose delivered to a patient is correct
and strongly agree with that simulated by his treatment plan currently requires a pre-treatment
dosimetric verification. Although this procedure can be realised by using a combined detectorphantom system to monitor the patient plan dose delivered on a phantom, the quality of the entire
treatment can not be completely certified since the dose is not directly monitored during each daily
treatment. The availability of a system that could monitor the dose in real time during the patient
treatment would strongly increase the patient radiotherapy safety. The Dosimetry Check software
(Math Resolution, LLC, Maryland) is one of these medical device which is capable of evaluating
either pre-treatment or in-vivo patient plan dosimetry by using the on-board imaging detectors of
the treatment unit. The purpose of the study is to assess the proper functioning of the DC software,
coupled to the mega-voltage (MV) detectors of an helical Tomotherapy (HT) unit, to verify
different patient plans in either pre-treatment and in-vivo modality.
Materials & methods: to evaluate HT pre-treatment and in-vivo dosimetry, 10 different plans were
selected where brain, head & neck, thorax and prostate tumour patients were treated. For the pretreatment dosimetry, each plan was delivered without the presence of the treatment couch and the
delivered fluence fields were measured by the HT on-board MV detectors. For the in-vivo
dosimetry, instead, the fluence transmitted through the patient and treatment couch was measured.
All the detector acquisitions were performed during the first treatment and the pre-clinical patient
computer-tomography (CT) datasets were used for the DC dose computation using a ray-tracing in
backprojection algorithm. Using the Tomotherapy software for simulating the treatment plan, the
CT scans, the structures, the plans and the dose files were imported as DICOM RT data into the
DC software. In the same software the recorded MV detector signals acquired during the plan
delivery were also imported. All this was used to calculate the absolute doses on the slices and the
dose-volume histograms of the reference structures defined onto the patient CT anatomy. The doses
of the plans simulated and reconstructed from measurements were compared and analysed. The
dosimetric verification was conducted in terms of gamma-index analysis with a tolerance of 3% in
dose and 3mm for the distance to agreement. A dose threshold of 10% on the calculated dose was
used.
Results: the gamma-index values between the planned dose and that calculated by the DC tool
ranged from 88% to 100% for the pre-treatment dose verification method, while for the in-vivo
dosimetry the same agreement ranged between 88% and 99.61%. The lowest values have been
observed for the thorax treatment, where the in-homogeneities were more present than in all the
other treated anatomical sites. On the other hand, this effect was much less strong in prostate and
brain tumours where the results were the best in terms of dose agreement reaching values around 97
– 99%.
Conclusions: The Dosimetry Check software coupled with the MV Tomotherapy detectors have
proved to be an invaluable tool for the volumetric pre-treatment and in-vivo dosimetry verification
of the HT treatments. The dose agreement reached for brain and head & neck treatments is very
high also for the in-vivo verification methods. However, because the DC tool is still based on a
pencil beam algorithm, which is fast but that over-estimate the dose values where in-homogeneities
are present, cautions should still be used for thorax and prostate treatments where the pre-clinical
method is still more safe and reliable.