Pre-treatment and in-vivo dosimetry ofHelical Tomotherapy (HT) treatment plans by using the Dosimetry Check system coupled to HT detectors
1E. Mezzenga, 1E. Cagni, 1A. Botti, 1M. Orlandi, 2W.D. Renner, 1M.Iori
1Medical 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 detector-phantom 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,10different plans were selected where brain,headneck, thoraxand prostate tumour patients were treated. For the pre-treatment dosimetry, each plan was delivered without the presence of the treatment couch and the delivered fluence fields were measured by the HTon-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 softwarefor 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 recordedMV detector signals acquired during the plan delivery were also imported. All this was used to calculate the absolute doseson the slices and the dose-volume histogramsof 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 doseand 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.