Abstract
Delivery of accurate intensity‐modulated radiation therapy (IMRT) or stereotactic radiotherapy depends on a multitude of steps in the treatment delivery process. These steps range from imaging of the patient to dose calculation to machine delivery of the treatment plan. Within the treatment planning system's (TPS) dose calculation algorithm, various unique small field dosimetry parameters are essential, such as multileaf collimator modeling and field size dependence of the output. One of the largest challenges in this process is determining accurate small field size output factors. The Radiological Physics Center (RPC), as part of its mission to ensure that institutions deliver comparable and consistent radiation doses to their patients, conducts on‐site dosimetry review visits to institutions. As a part of the on‐site audit, the RPC measures the small field size output factors as might be used in IMRT treatments, and compares the resulting field size dependent output factors to values calculated by the institution's treatment planning system (TPS). The RPC has gathered multiple small field size output factor datasets for X‐ray energies ranging from 6 to 18 MV from Varian, Siemens and Elekta linear accelerators. These datasets were measured at 10 cm depth and ranged from 10×10 cm2 to 2×2 cm2. The field sizes were defined by the MLC and for the Varian machines the secondary jaws were maintained at a 10×10 cm2. The RPC measurements were made with a micro‐ion chamber whose volume was small enough to gather a full ionization reading even for the 2×2 cm2 field size. The RPC measured output factors are tabulated and are reproducible with standard deviations (SD) ranging from 0.1% to 2.4%, while the institutions' calculated values had a much larger SD range, ranging up to 7.9%. The absolute average percent differences were greater for the 2×2 cm2 than for the other field sizes. The RPC's measured small field output factors provide institutions with a standard dataset against which to compare their TPS calculated values. Any discrepancies noted between the standard dataset and calculated values should be investigated with careful measurements and with attention to the specific beam model.PACS number: 87.53.Bn
Highlights
Modern radiotherapy routinely involves the use of small radiation fields, either for the delivery of stereotactic treatments, or as components of intensity-modulated radiation therapy (IMRT)
The Radiological Physics Center (RPC) measured and institution treatment planning system (TPS) calculated small field size dependence output factors for Varian, Elekta, and Siemens accelerators are found in Tables 1–3, respectively
The RPC measured output factors decreased with decreasing multileaf collimator (MLC) field size, as expected
Summary
Modern radiotherapy routinely involves the use of small radiation fields, either for the delivery of stereotactic treatments, or as components of intensity-modulated radiation therapy (IMRT). Quantifying and accounting for the associated field size dependent output factors for such small fields poses several challenges.[1]. Commissioning a treatment planning system for such small fields poses many unique challenges. Source size[2,3,4] and MLC modeling[5] impact dose calculation for small fields more so than for larger fields. As IMRT and stereotactic radiotherapy typically use many small segments or fields to achieve the desired dose distribution and target coverage, accurate small field size dependent output factors are required to be modeled within the TPS. Complicating the issue of commissioning small radiotherapy fields is the challenge in making accurate small field dose measurements. There have been multiple incidents recently of incorrect small field size output factors being measured resulting in the mistreatment of patients.[9]
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