Evaluation of MVCT imaging dose levels during helical IGRT: comparison between ion chamber, TLD, and EBT3 films

Abstract

The purpose of this investigation was to evaluate the dose on megavoltage CT (MVCT) images required for tomotherapy. As imaging possibilities are often used before each treatment and usually used several times before the session, we tried to evaluate the dose delivered during the procedure. For each scanning mode (fine, normal, and coarse), we first established the relative variation of these doses according to different technical parameters (explored length, patient setup). These dose variations measured with the TomoPhant, also known as Cheese phantom, showed the expected variations (due to the variation of scattered radiation) of 15% according to the explored length and ±5% according to the phantom setup (due to the variation of the point of measurement in the bore). In order to estimate patient doses, an anthropomorphic phantom was used for thermoluminescent and film dosimetry. The degree of agreement between the two methods was very satisfactory (the differences correspond to 5 mGy per imaging session) for the three sites studied (head & neck, thorax, and abdomen). These measurements allowed us to estimate the delivered dose of between 1 cGy and 4 cGy according to the site and imaging mode. Finally, we attempted to investigate a way to calculate this delivered dose in our patients from the study conducted on a cylindrical phantom and by taking into account data from the initial kV‐CT scan. The results we obtained were close to our measurements, with discrepancies below 5 mGy per MVCT.PACS numbers: 87.53.Bn, 87.55.km, 87.55.Qr

Patient organ doses from megavoltage computed tomography delivery with a helical tomotherapy unit using a general treatment planning system

“…Our results (the maximum absorbed doses to parotids, lungs and bladder) were compared with the MSADs reported by Mege et al [8], which were delivered to three clinical sites (head and neck, thorax, and pelvis [abdomen]). Our results were 17.2% lower than the average MSADs reported in Mege et al One reason for this difference is the difference in the MVCT dose levels.…”

“…The reason for this is that Jeraj et al reported that the average energy of the MVCT beam is close to that of treatment beam [11]. In a study by Mege et al , the conversion factor for measuring the point dose from MVCT delivery was taken as that for the 6 MV treatment beam due to the small difference between the average energy of the MVCT beam and the treatment beam [8]. In AAPM TG-148, no correction of the conversion factor for the treatment beam to use for the imaging beam was recommended [3].…”

“…In the case of the cheese phantom, the scan length was 18 cm. In the study by Mege et al [8], a scan length of 10 cm was adopted for the measurement of the MVCT dose level. We used a relatively short scan length of 4.8 cm.…”

“…The average organ doses of hips, bladder and rectum from MVCT per fraction for prostate cancer treatment were reported to be 1.02, 1.05 and 1.04 cGy, respectively; however, this method is not available to general clinical practice for routine quantification of patient-specific radiation dose. Mege et al also evaluated MVCT doses in a clinical case using Gafchromic films and an anthropomorphic phantom [8]. In their study, three different sites [head and neck, thorax, and pelvis (abdomen)] were assessed.…”

TomoTherapy treatments of multiple brain lesions: an in-phantom accuracy evaluation

“…Users can choose among three different MVCT slice thickness: fine (2 mm), normal (4 mm) and coarse (6 mm), resulting in variable imaging durations (Yartsev et al 2007), image qualities and absorbed doses (Shah et al 2008, Mege et al 2016. The maximum acquisition length is 150 cm and the Field of View is 39 cm (512 × 512 pixels).…”