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Purpose CT-scanning for treatment planning in radiotherapy is often hampered by the fact that scan parameter optimization may be limited due to treatment planning system constraints. The current work reviews the exposure levels for a selected group of treatment locations, i.e. head, head&neck, thorax, breast, abdomen and prostate. Methods Equal scan protocols, based on filtered back projection, are set on the two scanners (Siemens Definition AS+). Scan parameters from the CT scanners are automatically transferred, using the DICOM SR protocol, after each scanning sequence to a dose monitoring system (Sectra DoseTrack). These data include, but are not limited to CTDI, DLP, weight and age. For adult patients with weights between 60 and 80 kg the median CTDI (mGy) and DLP (mGycm) values as well as standard deviation (1 SD) have been calculated for the treatment locations. 20 or more patients are included for each location. Results The exposure levels for the selected sites are more or less equal between the two scanners. CTDI and DLP values (CTDI/DLP) for CT 1 are: Head 41 ± 4/1125 ± 145, Head&neck 11 ± 1/452 ± 61, Thorax 9 ± 3/390 ± 161, Breast ca 9 ± 2/355 ± 61, Abdomen 17 ± 3/815 ± 239, Prostate ca respectively. Corresponding values for CT 2 are: Head 42 ± 6/1168 ± 222, Head&neck 11 ± 1/458 ± 89, Thorax 9 ± 2/412 ± 136, Breast ca 9 ± 1/357 ± 60, Abdomen 17 ± 3/801 ± 177, Prostate ca 17 ± 2/813 ± 110 for each scan site. CTDI values are well below current national dose reference levels (DRL) in diagnostic radiology were these are given. DLP values are however slightly outside the diagnostic DRL values were these exist. Increased scan length requirements due to treatment planning flexibility, dose calculation accuracy and DVH evaluation of complete organs add to higher DLP values. Conclusions Exposure levels have been evaluated for the head, head&neck, thorax, breast, abdomen and prostate regions. These exposure levels turn out to be lower for CTDI than current national diagnostic radiology dose reference levels indicating well adapted scan protocols. Scan length requirements for treatment planning do however add to DLP making them slightly higher than current DRL levels.

The approach in treatment planning of applying beam quality correction factors to model compensator-induced depth-hardening effects is investigated and the present work comprises a dosimetric verification of the model for a common compensator material. Lead sheet modulators for four different phantom shapes were designed using a treatment planning system based on the model. The modulators were designed to yield homogeneous dose in a plane. The calculated modulation created by the lead sheets was re-imported into the treatment planning system and applied to a water phantom geometry for verification purposes. Comparing measurements, a total of 31 different geometries were measured, with calculations in this geometry showing good agreement for depth doses, dose profiles and output data with a maximum deviation of 4% except locally in the penumbra region and close to the edges of the cut lead sheets.