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Small fields measurements with radiochromic films.

Gonzalez-Lopez A, Vera-Sanchez JA, Lago-Martin JD - J Med Phys (2015 Apr-Jun)

Bottom Line: The results obtained for the OFs measurements show a good agreement with the values obtained from RC films and the PinPoint and Semiflex chambers when the field size is greater or equal than 2 × 2 cm(2).These agreements give confidence on the accuracy of the method as well as on the results obtained for smaller fields.Also, good agreement was found between the measured profiles and the Monte Carlo calculated profiles for the field size of 1 × 1 cm(2).

View Article: PubMed Central - PubMed

Affiliation: Hospital Universitario Virgen de la Arrixaca, Carretera Madrid-Cartagena, Murcia, Spain.

ABSTRACT
The small fields in radiotherapy are widely used due to the development of techniques such as intensity-modulated radiotherapy and stereotactic radio surgery. The measurement of the dose distributions for small fields is a challenge. A perfect dosimeter should be independent of the radiation energy and the dose rate and should have a negligible volume effect. The radiochromic (RC) film characteristics fit well to these requirements. However, the response of RC films and their digitizing processes present a significant spatial inhomogeneity problem. The present work uses a method for two-dimensional (2D) measurement with RC films based on the reduction of the spatial inhomogeneity of both the film and the film digitizing process. By means of registering and averaging several measurements of the same field, the inhomogeneities are mostly canceled. Measurements of output factors (OFs), dose profiles (in-plane and cross-plane), and 2D dose distributions are presented. The field sizes investigated are 0.5 × 0.5 cm(2), 0.7 × 0.7 cm(2), 1 × 1 cm(2), 2 × 2 cm(2), 3 × 3 cm(2), 6 × 6 cm(2), and 10 × 10 cm(2) for 6 and 15 MV photon beams. The OFs measured with the RC film are compared with the measurements carried out with a PinPoint ionization chamber (IC) and a Semiflex IC, while the measured transversal dose profiles were compared with Monte Carlo simulations. The results obtained for the OFs measurements show a good agreement with the values obtained from RC films and the PinPoint and Semiflex chambers when the field size is greater or equal than 2 × 2 cm(2). These agreements give confidence on the accuracy of the method as well as on the results obtained for smaller fields. Also, good agreement was found between the measured profiles and the Monte Carlo calculated profiles for the field size of 1 × 1 cm(2). We expect, therefore, that the presented method can be used to perform accurate measurements of small fields.

No MeSH data available.


Calibration curves using one piece of film for dose values (solid lines) and averaging 10film pieces for each dose value (dashed line)
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Figure 3: Calibration curves using one piece of film for dose values (solid lines) and averaging 10film pieces for each dose value (dashed line)

Mentions: Figure 3 shows the reduction of uncertainty when applying the method of averaging of readings. This figure shows 100 curves (solid lines), randomly chosen from 100,000 possibilities. Each curve is obtained by fitting the readings of five pieces of film to the doses 520, 400, 320, 170, and 50 cGy. The curve obtained by fitting the averaged readings is also shown (bold and dashed line). Using this curve, the dose estimation for the films irradiated to 250 cGy (averaged readings) is 249.6 cGy. On the other hand, the standard deviation for the estimation based on one single piece of film for each dose value is 19 cGy.


Small fields measurements with radiochromic films.

Gonzalez-Lopez A, Vera-Sanchez JA, Lago-Martin JD - J Med Phys (2015 Apr-Jun)

Calibration curves using one piece of film for dose values (solid lines) and averaging 10film pieces for each dose value (dashed line)
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4478646&req=5

Figure 3: Calibration curves using one piece of film for dose values (solid lines) and averaging 10film pieces for each dose value (dashed line)
Mentions: Figure 3 shows the reduction of uncertainty when applying the method of averaging of readings. This figure shows 100 curves (solid lines), randomly chosen from 100,000 possibilities. Each curve is obtained by fitting the readings of five pieces of film to the doses 520, 400, 320, 170, and 50 cGy. The curve obtained by fitting the averaged readings is also shown (bold and dashed line). Using this curve, the dose estimation for the films irradiated to 250 cGy (averaged readings) is 249.6 cGy. On the other hand, the standard deviation for the estimation based on one single piece of film for each dose value is 19 cGy.

Bottom Line: The results obtained for the OFs measurements show a good agreement with the values obtained from RC films and the PinPoint and Semiflex chambers when the field size is greater or equal than 2 × 2 cm(2).These agreements give confidence on the accuracy of the method as well as on the results obtained for smaller fields.Also, good agreement was found between the measured profiles and the Monte Carlo calculated profiles for the field size of 1 × 1 cm(2).

View Article: PubMed Central - PubMed

Affiliation: Hospital Universitario Virgen de la Arrixaca, Carretera Madrid-Cartagena, Murcia, Spain.

ABSTRACT
The small fields in radiotherapy are widely used due to the development of techniques such as intensity-modulated radiotherapy and stereotactic radio surgery. The measurement of the dose distributions for small fields is a challenge. A perfect dosimeter should be independent of the radiation energy and the dose rate and should have a negligible volume effect. The radiochromic (RC) film characteristics fit well to these requirements. However, the response of RC films and their digitizing processes present a significant spatial inhomogeneity problem. The present work uses a method for two-dimensional (2D) measurement with RC films based on the reduction of the spatial inhomogeneity of both the film and the film digitizing process. By means of registering and averaging several measurements of the same field, the inhomogeneities are mostly canceled. Measurements of output factors (OFs), dose profiles (in-plane and cross-plane), and 2D dose distributions are presented. The field sizes investigated are 0.5 × 0.5 cm(2), 0.7 × 0.7 cm(2), 1 × 1 cm(2), 2 × 2 cm(2), 3 × 3 cm(2), 6 × 6 cm(2), and 10 × 10 cm(2) for 6 and 15 MV photon beams. The OFs measured with the RC film are compared with the measurements carried out with a PinPoint ionization chamber (IC) and a Semiflex IC, while the measured transversal dose profiles were compared with Monte Carlo simulations. The results obtained for the OFs measurements show a good agreement with the values obtained from RC films and the PinPoint and Semiflex chambers when the field size is greater or equal than 2 × 2 cm(2). These agreements give confidence on the accuracy of the method as well as on the results obtained for smaller fields. Also, good agreement was found between the measured profiles and the Monte Carlo calculated profiles for the field size of 1 × 1 cm(2). We expect, therefore, that the presented method can be used to perform accurate measurements of small fields.

No MeSH data available.