Two-dimensional mapping of underdosed areas using radiochromic film for patients undergoing total skin electron beam radiotherapy.

PURPOSE: To demonstrate the viability of radiochromic film as an in vivo, two-dimensional dosimeter for the measurement of underdosed areas in patients undergoing total skin electron beam (TSEB) radiotherapy. The results were compared with thermoluminescent dosimeter measurements. METHODS AND MATERIALS: Dosimetry results are reported for an inframammary fold of 2 patients treated using a modified version of the Stanford six-position (i.e., six-field and dual-beam) TSEB technique. The results are presented as contour plots of film optical density and percentage of dose. A linear dose profile measured from film was compared with the thermoluminescent dosimeter measurements. RESULTS: The results showed that the percentage doses as measured by film are in good agreement with those measured by the thermoluminescent dosimeters. The isodose contour plots provided by film can be used as a two-dimensional dose map for a patient when determining the size of the supplemental patch fields. CONCLUSION: Radiochromic film is a viable dosimetry tool that the radiation oncologist can use to understand the surface dose heterogeneity better across complex concave regions of skin to help establish more appropriate margins to patch underdosed areas. Film could be used for patients undergoing TSEB for disorders such as mycosis fungoides or undergoing TSEB or regional skin electron beam for widespread skin metastases from breast cancer and other malignancies.

Composite depth dose measurement for total skin electron (TSE) treatments using radiochromic film.

Total skin electron (TSE) radiotherapy is routinely used to treat cutaneous T-cell lymphomas and can be implemented using a modified Stanford technique. In our centre, the composite depth dose for this technique is achieved by a combination of two patient positions per day over a three-day cycle, and two gantry angles per patient position. Due to patient morphology, underdosed regions typically occur and have historically been measured using multiple thermoluminescent dosimeters (TLDs). We show that radiochromic film can be used as a two-dimensional relative dosimeter to measure the percent depth dose in TSE radiotherapy. Composite depth dose curves were measured in a cylindrical, polystyrene phantom and compared with TLD data. Both multiple films (1 film per day) and a single film were used in order to reproduce a realistic clinical scenario. First, three individual films were used to measure the depth dose, one per treatment day, and then compared with TLD data; this comparison showed a reasonable agreement. Secondly, a single film was used to measure the dose delivered over three daily treatments and then compared with TLD data; this comparison showed good agreement throughout the depth dose, which includes doses well below 1 Gy. It will be shown that one piece of radiochromic film is sufficient to measure the composite percent depth dose for a TSE beam, hence making radiochromic film a suitable candidate for monitoring underdosed patient regions.

Analysis of mechanical sources of patient alignment errors in radiation therapy.

Certain radiation treatments, such as conformal and intensity modulated treatments, involve isocentric treatment fields delivered using multiple angles or continuous angulation of the gantry, collimator and table. At our institution, treatments involving three angles (gantry, collimator, and table) can, if uncorrected, exhibit misalignments of 2 mm or more on premarked field centers and borders on the patient surface during the initial setup on a linear accelerator (linac), even though the linac operates within allowable mechanical tolerances. This paper is an analysis of three principal mechanical sources of patient alignment errors observed on linacs: (i) errors in table and gantry angle, (ii) displacement of gantry rotational axis during gantry rotation, and (iii) displacement between collimator and table rotational axes. On patient surfaces, these small, systematic mechanical errors can each be expected to produce misalignments of up to 1.5 mm, increasing to over 2 mm with nearly horizontal fields delivered at nonzero table angles onto highly oblique patient surfaces. For the underlying target volumes, the mechanical errors can, in combination, be expected to produce target volume misalignments of up to 1 mm on newly installed linacs and 3 mm on older linacs. Thus, 1 mm appears to be a mechanical limit on the positional precision of radiation treatments.