Comparison of ionization chambers of various volumes for IMRT absolute dose verification

Abstract

IMRT plans are usually verified by phantom measurements: dose distributions are measured using film and the absolute dose using an ionization chamber. The measured and calculated doses are compared and planned MUs are modified if necessary. To achieve a conformal dose distribution, IMRT fields are composed of small subfields, or "beamlets." The size of beamlets is on the order of 1 x 1 cm2. Therefore, small chambers with sensitive volumes < or = 0.1 cm3 are generally used for absolute dose verification. A dosimetry system consisting of an electrometer, an ion chamber, and connecting cables may exhibit charge leakage. Since chamber sensitivity is proportional to volume, the effect of leakage on the measured charge is relatively greater for small chambers. Furthermore, the charge contribution from beamlets located at significant distances from the point of measurement may be below the small chambers threshold and hence not detected. On the other hand, large (0.6 cm3) chambers used for the dosimetry of conventional external fields are quite sensitive. Since these chambers are long, the electron fluence through them may not be uniform ("temporal" uniformity may not exist in the chamber volume). However, the cumulative, or "spatial" fluence distribution (as indicated by calculated IMRT dose distribution) may become uniform at the chamber location when the delivery of all IMRT fields is completed. Under the condition of "spatial" fluence uniformity, the charge collected by the large chamber may accurately represent the absolute dose delivered by IMRT to the point of measurement. In this work, 0.6, 0.125, and 0.009 cm3 chambers were used for the absolute dose verification for tomographic and step-and-shoot IMRT plans. With the largest, 0.6 cm3 chamber, the measured dose was equal to calculated within 0.5%, when no leakage corrections were made. Without leakage corrections, the error of measurement with a 0.125 cm3 chamber was 2.6% (tomographic IMRT) and 1.5% (step-and-shoot IMRT). When doses measured by a 0.125 cm3 chamber were corrected for leakage, the difference between the calculated and measured doses reduced to 0.5%. Leakage corrected doses obtained with the 0.009 cm3 chamber were within 1.5%-1.7% of calculated doses. Without leakage corrections, the measurement error was 16% (tomographic IMRT) and 7% (step-and-shoot IMRT).

Dosimetric characterization and behaviour in small X-ray fields of a microchamber and a plastic scintillator detector

“…1 However, the required dose accuracy within small photon fields is difficult to achieve, as lateral electronic equilibrium breaks down and traditional detectors have limitations to represent the unperturbed dose distribution in water as accurately as possible. [2][3][4][5][6][7][8] As reported in literature, 9,10 this is mostly due to dose averaging over the finite size of the active volume and the nonwater equivalence of the materials that surround the active volume. With regard to volume-averaging effects, the commercial availability of detectors suitable for measurements of small fields down to the small field sizes mostly used in linear accelerators with multileaf collimator (MLC) reduces the importance of the effect.…”

A study to establish reasonable action limits for patient‐specific quality assurance in intensity‐modulated radiation therapy

“…A three‐dimensional (3D) water scanning system (Wellhofer Dosimetrie, Schwarzenbruck, Germany) with a cylindrical ion chamber (Model IC‐15: Wellhofer Dosimetrie, Schwarzenbruck, Germany) of 0.125 cm 3 volume was used to collect the beam model data in accordance with published data. ( 10 , 11 ) The dose verification for absolute and relative modes was performed using the 2D diode array (MapCheck Model 1175: Sun Nuclear, Melbourne, FL), which consists of 445 N‐type diodes in a variable spacing arrangement across a $22\times 22$‐cm field. The $10\times 10$‐cm center portion of the octagonal grid contains 221 diodes with 7‐mm spacing, and the outer area surrounding the central grid contains 224 diodes with 14‐mm spacing.…”

Dosimetric Verification for Primary Focal Hypermetabolism of Nasopharyngeal Carcinoma Patients Treated with Dynamic Intensity-modulated Radiation Therapy

“…Therefore, measurements must be our push the quantity and the flat area dose. We use the ionization chamber is 0.6 cm 3 , high metabolic area can use 0.15 cm 3 ionization chamber (Leybovich et al, 2003). In this lab, ionization chamber verification of 11 patients absolute error within -5.80% -5.23%, average error of plus or minus 2.39% ± 0.66, absolute doses of the maximum error is 5.80%, now at home and abroad, the standard is 5% (Hu, 1999), then part of the plan is not through the verification.…”