Intraoperative electron beam radiation therapy: technique, dosimetry, and dose specification: report of task force 48 of the Radiation Therapy Committee, American Association of Physicists in Medicine.

Intraoperative radiation therapy (IORT) is a treatment modality whereby a large single dose of radiation is delivered to a surgically open, exposed cancer site. Typically, a beam of megavoltage electrons is directed at an exposed tumor or tumor bed through a specially designed applicator system. In the last few years, IORT facilities have proliferated around the world. The IORT technique and the applicator systems used at these facilities vary greatly in sophistication and design philosophy. The IORT beam characteristics vary for different designs of applicator systems. It is necessary to document the existing techniques of IORT, to detail the dosimetry data required for accurate delivery of the prescribed dose, and to have a uniform method of dose specification for cooperative clinical trials. The specific charge to the task group includes the following: (a) identify the multidisciplinary IORT team, (b) outline special considerations that must be addressed by an IORT program, (c) review currently available IORT techniques, (d) describe dosimetric measurements necessary for accurate delivery of prescribed dose, (e) describe dosimetric measurements necessary in documenting doses to the surrounding normal tissues, (f) recommend quality assurance procedures for IORT, (g) review methods of treatment documentation and verification, and (h) recommend methods of dose specification and recording for cooperative clinical trials.

Acceptance testing of an automated scanning water phantom.

The setup of an automated scanning water phantom must be concluded with a methodical functional acceptance test. This test must include mechanical, radiological, and calculational facets. Only after an acceptance test is successfully completed can data collected with a new or repaired system be trusted. An acceptance test outline was written and applied to a newly purchased Wellhofer scanning system. This testing procedure and its results are presented in this paper.

A quality assurance program for monitor unit calculators.

A quality assurance protocol has been developed to assess the relative accuracy of computerized monitor unit calculations. Results from this testing reveal that errors (defined as the difference between the computer results and those obtained using a standard formula with manual calculation) in monitor unit calculators are possible when they are tested over a wide range of clinically relevant field sizes (including blocked fields), source-to-surface distances, and depths. It is suggested that computerized monitor unit calculators be checked against hand calculations and that this be permanently documented both at the time of initial implementation and following any subsequent changes in the program.

Determination of accurate dosimetric parameters for beveled intraoperative electron beam applicators.

Intraoperative electron beam therapy requires accurate dose maximum/monitor unit (Dmax/MU) values for both flat and beveled ended applicators (cones). Measurement of Dmax/MU values with either solid or nontilting scanning water systems may give rise to inaccuracies due to the difficulty in locating an accurate position for dmax, since the ionization chamber usually cannot be scanned along the central axis of the beveled intraoperative applicator. A linear one-dimensional scanner (which permits ionization measurements to be made as a function of water depth) has been modified to provide scanning along a line up to 30 degrees from the perpendicular to the phantom surface. This modification has proven helpful in improving the accuracy of certain dosimetric parameters (e.g., Dmax/MU) of beveled applicators. For example, we found inaccuracies which arose when we measured Dmax/MU of beveled intraoperative radiation therapy cones in either solid or other scanning water systems were greatly reduced, especially for the lower electron beam energies (e.g., 6 and 9 MeV).