Commissioning of a System to Correlate Dose Measurements to Patients Anatomy

Abstract

Purpose/Objective(s)Complex treatment plans, especially for intensity modulated radiotherapy (IMRT), need to be verified with at least one independent method. For this quality assurance a variety of measurement methods exist, like ionization chamber, film, or 2D arrays. No matter which method is chosen, it is difficult to correlate a potentially occurring discrepancy between the calculated and measured dose distribution in the phantom to patients anatomy and to estimate its impact on the patient's organs.Materials/MethodsA 2D ionization chamber array (MatriXX, IBA-dosimetry) and the Compass software (IBA dosimetry) are used for this correlation. The MatriXX detector can either be used for 2D absolute dose measurement in a slab phantom or it can be used in combination with the Compass software, attached to the gantry. The Compass software is then able to reconstruct the dose in the patient and in the phantom, respectively, using the MatriXX measurements and an inherent beam modeling. This kind of measurement-based recalculation needs to be commissioned similar to a treatment planning system. For the commissioning four different quantities were compared: 1) the calculated dose from the treatment planning system (TPS), 2) the reconstructed dose from Compass, 3) the measured dose in a slab phantom using the MatriXX detector, and 4) the measured dose in a homogenous pelvis phantom using EDR2 films. The Gamma Index (4% delta dose, 4mm distance to agreement) was used as criteria to compare those four different dose distributions in a region of interest, which was selected to be completely inside the phantom, covering as much as possible from the field. The comparison of the four above mentioned quantities was performed for 3D-conformal as well as IMRT plans created for typical target shapes in the pelvis as well as in the head and neck region.ResultsThe agreement between the measurements and the dose reconstruction from Compass depends on the treatment modality (3D or IMRT) as well as on the target shape. For the investigated IMRT plan the agreement between Compass and measurements (film as well as MatriXX) was slightly better than the agreement between the TPS and measurements: The amount of pixels which passed the above mentioned Gamma criteria in a region of interest was 97.21% for Compass-film, 94.32% for TPS-film, 84.87% for Compass-MatriXX, and 80.67% for TPS-MatriXX. The amount of passing pixels for a 3D prostate plan was 99.83% for TPS-MatriXX and 100% for Compass-MatriXX. However, for a non-IMRT head and neck plan with large fields, local discrepancies between -5.1% and +8.3% were detected for Compass-film, whereas the TPS agreed well within 2% with the measurement.ConclusionsThe Compass software seems to be well suited for IMRT verification, but the beam model has to be improved for large open fields. Purpose/Objective(s)Complex treatment plans, especially for intensity modulated radiotherapy (IMRT), need to be verified with at least one independent method. For this quality assurance a variety of measurement methods exist, like ionization chamber, film, or 2D arrays. No matter which method is chosen, it is difficult to correlate a potentially occurring discrepancy between the calculated and measured dose distribution in the phantom to patients anatomy and to estimate its impact on the patient's organs. Complex treatment plans, especially for intensity modulated radiotherapy (IMRT), need to be verified with at least one independent method. For this quality assurance a variety of measurement methods exist, like ionization chamber, film, or 2D arrays. No matter which method is chosen, it is difficult to correlate a potentially occurring discrepancy between the calculated and measured dose distribution in the phantom to patients anatomy and to estimate its impact on the patient's organs. Materials/MethodsA 2D ionization chamber array (MatriXX, IBA-dosimetry) and the Compass software (IBA dosimetry) are used for this correlation. The MatriXX detector can either be used for 2D absolute dose measurement in a slab phantom or it can be used in combination with the Compass software, attached to the gantry. The Compass software is then able to reconstruct the dose in the patient and in the phantom, respectively, using the MatriXX measurements and an inherent beam modeling. This kind of measurement-based recalculation needs to be commissioned similar to a treatment planning system. For the commissioning four different quantities were compared: 1) the calculated dose from the treatment planning system (TPS), 2) the reconstructed dose from Compass, 3) the measured dose in a slab phantom using the MatriXX detector, and 4) the measured dose in a homogenous pelvis phantom using EDR2 films. The Gamma Index (4% delta dose, 4mm distance to agreement) was used as criteria to compare those four different dose distributions in a region of interest, which was selected to be completely inside the phantom, covering as much as possible from the field. The comparison of the four above mentioned quantities was performed for 3D-conformal as well as IMRT plans created for typical target shapes in the pelvis as well as in the head and neck region. A 2D ionization chamber array (MatriXX, IBA-dosimetry) and the Compass software (IBA dosimetry) are used for this correlation. The MatriXX detector can either be used for 2D absolute dose measurement in a slab phantom or it can be used in combination with the Compass software, attached to the gantry. The Compass software is then able to reconstruct the dose in the patient and in the phantom, respectively, using the MatriXX measurements and an inherent beam modeling. This kind of measurement-based recalculation needs to be commissioned similar to a treatment planning system. For the commissioning four different quantities were compared: 1) the calculated dose from the treatment planning system (TPS), 2) the reconstructed dose from Compass, 3) the measured dose in a slab phantom using the MatriXX detector, and 4) the measured dose in a homogenous pelvis phantom using EDR2 films. The Gamma Index (4% delta dose, 4mm distance to agreement) was used as criteria to compare those four different dose distributions in a region of interest, which was selected to be completely inside the phantom, covering as much as possible from the field. The comparison of the four above mentioned quantities was performed for 3D-conformal as well as IMRT plans created for typical target shapes in the pelvis as well as in the head and neck region. ResultsThe agreement between the measurements and the dose reconstruction from Compass depends on the treatment modality (3D or IMRT) as well as on the target shape. For the investigated IMRT plan the agreement between Compass and measurements (film as well as MatriXX) was slightly better than the agreement between the TPS and measurements: The amount of pixels which passed the above mentioned Gamma criteria in a region of interest was 97.21% for Compass-film, 94.32% for TPS-film, 84.87% for Compass-MatriXX, and 80.67% for TPS-MatriXX. The amount of passing pixels for a 3D prostate plan was 99.83% for TPS-MatriXX and 100% for Compass-MatriXX. However, for a non-IMRT head and neck plan with large fields, local discrepancies between -5.1% and +8.3% were detected for Compass-film, whereas the TPS agreed well within 2% with the measurement. The agreement between the measurements and the dose reconstruction from Compass depends on the treatment modality (3D or IMRT) as well as on the target shape. For the investigated IMRT plan the agreement between Compass and measurements (film as well as MatriXX) was slightly better than the agreement between the TPS and measurements: The amount of pixels which passed the above mentioned Gamma criteria in a region of interest was 97.21% for Compass-film, 94.32% for TPS-film, 84.87% for Compass-MatriXX, and 80.67% for TPS-MatriXX. The amount of passing pixels for a 3D prostate plan was 99.83% for TPS-MatriXX and 100% for Compass-MatriXX. However, for a non-IMRT head and neck plan with large fields, local discrepancies between -5.1% and +8.3% were detected for Compass-film, whereas the TPS agreed well within 2% with the measurement. ConclusionsThe Compass software seems to be well suited for IMRT verification, but the beam model has to be improved for large open fields. The Compass software seems to be well suited for IMRT verification, but the beam model has to be improved for large open fields.