Investigation of photon beam output factors for conformal radiation therapy$mdash$Monte Carlo simulations and measurements

Abstract

The purpose of this study was to investigate beam output factors (OFs) for conformal radiation therapy and to compare the OFs measured with different detectors with those simulated with Monte Carlo methods. Four different detectors (diode, diamond, pinpoint and ionization chamber) were used to measure photon beam OFs in a water phantom at a depth of 10 cm with a source-surface distance (SSD) of 100 cm. Square fields with widths ranging from 1 cm to 15 cm were observed; the OF for the different field sizes was normalized to that measured at a 5 cm x 5 cm field size at a depth of 10 cm. The BEAM/EGS4 program was used to simulate the exact geometry of a 6 MV photon beam generated by the linear accelerator, and the DOSXYZ-code was implemented to calculate the OFs for all field sizes. Two resolutions (0.1 cm and 0.5 cm voxel size) were chosen here. In addition, to model the detector four kinds of material, water, air, graphite or silicon, were placed in the corresponding voxels. Profiles and depth dose distributions resulting from the simulation show good agreement with the measurements. Deviations of less than 2% can be observed. The OF measured with different detectors in water vary by more than 35% for 1 cm x 1 cm fields. This result can also be found for the simulated OF with different voxel sizes and materials. For field sizes of at least 2 cm x 2 cm the deviations between all measurements and simulations are below 3%. This demonstrates that very small fields have a bad effect on dosimetric accuracy and precision. Finally, Monte Carlo methods can be significant in determining the OF for small fields.

The Radiological Physics Center's standard dataset for small field size output factors

“…Furthermore, as the air cavity makes up an increasing amount of the radiation field, there is an increase in the lateral electronic disequilibrium, resulting in less dose deposition than if water existed in place of the air. Combined, these effects result in pinpoint chambers underestimating the true output for small fields by several percent (absolute difference), particularly for field sizes at or below $1\times 1{ \text{cm}}^2$ 6 , 7 , 10 …”

“…Figure 2 illustrates that the microchamber is sufficiently small enough to fall within the flat portion of the dose distribution, allowing it to make an appropriate ionization measurement without any loss of signal because of dose falloff near the edge of the field. Moreover, in comparisons between detectors, pinpoint ion chambers with double the active volume have been shown to be accurate within 1%, compared to other small field detectors (including diamond and diodes), down to $2\times 2{ \text{cm}}^2$ fields 6 , 7 , 10 . The Exradin A16 was always equilibrated to the bias placed on it before accumulating ionization readings by pre‐irradiating it with 500 mu.…”

“…The challenges of penumbra size versus detector size 6 , 7 and the impact of changes in the energy spectrum on detector response 6 , 7 , 8 all complicate the measurement process. Consequently, there have been multiple incidents recently of incorrect small field size output factors being measured resulting in the mistreatment of patients (9) …”

Total scatter factors of small beams: A multidetector and Monte Carlo study

“…[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] It is known that the main problems with the detectors are retraceable to their finite size compared to the small size of the beams and to the nonwater equivalence of the materials. Moreover, the dosimetry of small beams is complicated by the lack of lateral electronic equilibrium.…”

Dosimetric Verifications of the Output Factors in the Small Field Less Than 3 cm2Using the Gafchromic EBT2 Films and the Various Detectors

“…Nonetheless, there are the issues to solve the problems in radiotherapy, such as 4D calculations, 1) tumor tracking on a moving tumors, 2,3) image quality and absorbed dose during cone beam computed tomography (CBCT) acquisition to verify patient set-up, 4) calculation-accuracy of radiation treatment planning system (RTPs) in an inhomogeneous area on low density, 5,6) and dosimetry of a small field (＜3×3 cm 2 ). [7][8][9][10][11][12][13][14][15][16][17][18][19][20] These issues have been great challenges in modern radiotherapy. Especially, small field dosimetry is the most important in modern radiotherapy because it has been frequently used to remove the tumor with high dose hypo-fractionated stereotactic radiosurgery (SRS) or high dose single fraction SRS with small size target.…”