Sujatha Pai

TG-69 : Radiographic film for megavoltage beam dosimetry

TG-69 is a task group report of the AAPM on the use of radiographic film for dosimetry. Radiographic films have been used for radiation dosimetry since the discovery of x-rays and have become an integral part of dose verification for both routine quality assurance and for complex treatments such as soft wedges (dynamic and virtual), intensity modulated radiation therapy (IMRT), image guided radiation therapy (IGRT), and small field dosimetry like stereotactic radiosurgery. Film is convenient to use, spatially accurate, and provides a permanent record of the integrated two dimensional dose distributions. However, there are several challenges to obtaining high quality dosimetric results with film, namely, the dependence of optical density on photon energy, field size, depth, film batch sensitivity differences, film orientation, processing conditions, and scanner performance. Prior to the clinical implementation of a film dosimetry program, the film, processor, and scanner need to be tested to characterize them with respect to these variables. Also, the physicist must understand the basic characteristics of all components of film dosimetry systems. The primary mission of this task group report is to provide guidelines for film selection, irradiation, processing, scanning, and interpretation to allow the physicist to accurately and precisely measure dose with film. Additionally, we present the basic principles and characteristics of film, processors, and scanners. Procedural recommendations are made for each of the steps required for film dosimetry and guidance is given regarding expected levels of accuracy. Finally, some clinical applications of film dosimetry are discussed.

SU‐FF‐T‐506: Patterns of Care in the Era of ICRU‐50 for 3D Conformal Radiation Therapy: A Multi‐Institutional Study

Purpose:Radiation outcomes can be compared meaningfully only if the target dose prescription and specification is uniform with the disease site and type. ICRU‐50 recommended specific guidelines in 3DCRT for target volume definitions and dose reporting. This study evaluates the pattern of care retrospectively among institutions in the era of ICRU‐50. Materials & Methods:Dosimetric information of 1204 patients with 3DCRT was collected retrospectively from 10 participating institutions. The dose‐volume histogram data for the target volume was evaluated. Standard dose parameters such as minimum, maximum, median, and mean doses to the target volume along with V90, V95, V100, V105, V110, V115, Dmax i.e. volume (%) receiving 90%, 95%, 100%, 105%, 115% and maximum dose respectively were collected. The normalization dose is also reported. Results: Significant dosimetric variations from 0% to 138% were observed in disease sites and institutions. The minimum target dose reflective of poor quality of treatment planning, intertwining structures and structure closed to the surface has wide variation. The number of patients with doses beyond 10% and +10% was 41% and 22% respectively. When −5% and +5% dose window was used, 55% and 69% patients failed to meet the dose criterion respectively. For a small subset of patients the minimum dose in the target was higher than 100% and maximum dose was lower than 100%. The variation in mean target dose was 102.3 ± 3.7%. The diversity in normalization was also significant. Conclusion: Even with the implementation of ICRU‐50 guidelines, there is a large variation in dose delivery in 3DCRT. The variation is institution and site specific. It is shown that mean target dose is very close to the prescribed dose that can be used as a surrogate for the prescribed dose. For any meaningful comparison of the 3DCRT outcome, strict guidelines for dose reporting should be maintained.

Semianalytical expressions for (L̄/p)airmed and Prepl for electron beams

The tables of the mean restricted collision mass stopping power ratios for water, polystyrene and acrylic relative to air given in the AAPM TG-21 protocol have been fitted to an expression with 20 coefficients using the depth in the phantom and the mean incident electron energy as two independent variables. Using these expressions, the calculated values agree with the tabulated values within +0.5% in 85% of the cases and within +/- 1.0% in 95% of the cases. For each of the four cylindrical chamber inner diameters, given in the protocol, the electron fluence correction Prepl has been fitted to an expression with four coefficients using the mean electron energy at depth z as an independent variable.

Semianalytical expressions for (L̄/p)airmed and Prepl for electron beams: Technical Report: Semianalytical expressions for (L ̄/ρ)medair

The tables of the mean restricted collision mass stopping power ratios for water, polystyrene and acrylic relative to air given in the AAPM TG-21 protocol have been fitted to an expression with 20 coefficients using the depth in the phantom and the mean incident electron energy as two independent variables. Using these expressions, the calculated values agree with the tabulated values within +0.5% in 85% of the cases and within +/- 1.0% in 95% of the cases. For each of the four cylindrical chamber inner diameters, given in the protocol, the electron fluence correction Prepl has been fitted to an expression with four coefficients using the mean electron energy at depth z as an independent variable.