Dosimetric considerations for validation of a sequential IMRT process with a commercial treatment planning system

Abstract

Commercial multileaf collimator (MLC) systems can employ leaves with rounded ends. Treatment planning beam modelling should consider the effects of transmission through rounded leaf ends to provide accurate dosimetry for IMRT treatments delivered with segmented MLC. We determined that an MLC leaf gap reduction of 1.4 mm is required to obtain an agreement between calculated and measured profile 50% dose points. A head and neck dosimetry phantom, supplied by the Radiological Physics Center (RPC), was planned and irradiated as a necessary credentialing requirement for the RTOG H-0022 protocol. The agreement between the RPC TLD measurements and treatment planning calculations was within experimental error for the primary and secondary planning target volumes (PTVs); however, the calculated mean dose for the critical structure was approximately 9% lower than the RPC TLD measurements. RPC radiochromic film profile measurements also indicated significant discrepancies (>5%) with calculated values especially in the high dose gradient region in the vicinity of the critical structure. These results substantiate our own in-house phantom measurements, performed with the same IMRT fields as for the RPC phantom experiment, using Kodak EDR2 film to measure absolute dose. Our results indicate a maximum underestimate of calculated dose of 12% with no leaf gap reduction. The discrepancy between measured and calculated phantom values is reduced to +/- 5% when a leaf gap reduction of 1.4 mm is used. A further improvement in the accuracy of dose calculation is not possible without a more accurate modelling of the leaf end transmission by the planning system. In the absence of published dosimetric criteria for IMRT our results stress the need for stringent in-house dosimetric QA and validation for IMRT treatments. We found the dosimetric validation service provided by the RPC to be a valuable component of our IMRT validation efforts.

Measuring output factors of small fields formed by collimator jaws and multileaf collimator using plastic scintillation detectors

“…4 However, with the advent of these new techniques come new problems for performing direct, accurate dosimetry measurements of small radiation fields. Discrepancies have been reported between doses predicted by treatment planning systems and those measured in patients or phantoms, [5][6][7][8] and sophisticated Monte Carlo codes have been used to calculate the dose properties of small fields in lieu of direct measurements, which may be too difficult to perform with typical detectors. [8][9][10][11] Moreover, the rapid advance of small-field radiotherapy technology has precipitated a decrease in the relevance of well-established standards of practice with respect to reference dosimetry.…”

Challenges in Credentialing Institutions and Participants in Advanced Technology Multi-institutional Clinical Trials

“…-Inadequate modeling of the penumbra at MLC leaf ends [17] -Incorrect application of QA calculations or measurements The RPC recently evaluated the deviation rates of several trials that required credentialing for some or all participants [5,18,19]. The results, shown in Table 2, indicated that trials requiring credentialing experienced low deviation rates.…”

Dosimetry tools and techniques for IMRT

“…[36][37][38][39][40] Therefore, it is rec- Another type of small-field dosimeter that has been used in IMRT is the thermoluminescence dosimeter ͑TLD͒. [55][56][57][58][59][60][61][62][63][64][65][66][67][68] TLD dosimetry has been used since the 1970s to quantify megavoltage x-ray dosimetry. [69][70][71][72][73] A TLD is an integrating dosimeter, which is usually small in size, often in the form of a cubical or cylindrical chip, and has a nearly tissueequivalent atomic composition ͑Z = 8.1͒ and a typical physical density of 2.6 g cm −3 .…”

“…In spite of the added difficulty, anthropomorphic phantoms have been effectively used for limited measurements to evaluate the process of patient treatment planning and delivery and to identify treatment planning or dose delivery problems that are not evident in simple homogeneous geometric phantoms. 59,[155][156][157] The phantom setup typically parallels a human simulation and irradiation. For example, a CT simulation of the phantom should be conducted using radiopaque and visible fiducial markers, and when possible, the phantom position should be independently verified, for example using an electronic portal imaging device ͑EPID͒ or film at the treatment unit before delivery.…”