Evaluation of surface and superficial dose for head and neck treatments using conventional or intensity-modulated techniques

Abstract

With increased use of intensity-modulated radiation therapy (IMRT) for head and neck treatment questions have arisen as to selection of an optimum treatment approach when either superficial sparing or treatment is desired. Other work has pointed out the increased superficial dose resulting from obliquity effects when multiple tangential beams are applied to head and neck treatment, as is the general case in IMRT planning. Helical tomotherapy might be expected to result in even further enhanced superficial dose compared with conventional bilateral field treatment. We have designed a typical right oropharynx target volume in an anthropomorphic head and neck phantom. Three different treatment techniques have been used to optimally treat this target, including bilateral static fields, eight-field IMRT and helical tomotherapy. The phantom was immobilized in a standard treatment position and treated on a Varian 2300cd linear accelerator and on a Hi-Art Helical Tomotherapy unit. 1 mm3 lithium-fluoride thermoluminescent dosimeters (TLDs) were placed on the surface of the phantom at a number of axial test positions. Film strips (Kodak EDR2) were either wrapped around the surface or sandwiched within the phantom. Measured doses at the surface and as a function of depth are compared with the planning system predictions for each treatment technique. The maximum surface doses on the proximal treatment side, averaged from TLDs and films, were measured to be 69-82% of the target dose with the bilateral fields yielding the lowest surface doses (69%), tomotherapy about 2% more than that (71%) and IMRT 13% more (82%). Anterior to the target volume, doses are always low for bilateral treatment. In this case the minimum anterior surface dose (chin area) was 6% of the prescription dose from that technique as compared with 26% and 35% from the IMRT and tomotherapy methods, respectively. The Eclipse and Tomotherapy planning systems both modelled deep and superficial doses well. Surface doses were better modelled by Eclipse at the test points, while the tomotherapy plans consistently overestimated the measured doses by 10% or more. Depth dose measurements, extracted from embedded films, indicated the depth of dose build-up to >99% to be the shallowest for IMRT (2-5 mm) followed by tomotherapy (5-8 mm) and bilateral fields (10-15 mm). The amount of surface dose is clearly technique dependent and should be taken into account in the planning stage.

Clinical and Dosimetric Implications of Air Gaps between Bolus and Skin Surface during Radiation Therapy

“…They also investigated that with thermoplastic shell D Surf was 84% and 100% of the prescribed dose for parallel opposed (POP) and IMRT treatments respectively. Higgins et al [9] demonstrated that D Surf using POP, tomotherapy and IMRT were 69%, 71% and 82% respectively. Dogan and Glasgow [10] reported that D Surf with 6MV photon beam IMRT were 8% and 6% lower than those of the open field for zero and 75 0 gantry angles respectively.…”

Tolerance of Superficial Dose to Setup Error With Tomotherapy

“…These doses are substantially less than the predictions of the TPS (full coverage at 3 mm depth, 83% at skin surface), consistent with the documented overestimation of superficial dose by the tomotherapy TPS. [8][9][10][11][12] At a lateral displacement of 2.5 mm, the dose at 3 mm depth remained at 99.2% + 3.3% of the nondisplaced measurement (control). With 5 mm displacement, this ratio remained relatively well preserved at 92.4% + 4%.…”

“…10 While the dose at 3 mm depth was relatively unaffected by displacements less than 5 mm, it remains the case that delivered dose at superficial depths is less than that calculated by the current planning system. [9][10][11][12] If the skin is truly at risk, or it is clinically essential that structures within the buildup region for 6 MV photons receive the prescribed dose, despite increased skin toxicity, treatment with bolus or the use of a more accurate algorithm (Monte Carlo) would be required. Our results would be expected to carry over to other rotational IMRT approaches, such as intensity-modulated arc therapy, although that remains a topic of future investigation.…”

“…Several investigators have noted that the tomotherapy planning system (TPS) tends to overestimate superficial dose. [8][9][10][11][12] However, incidental surface dose tends to be higher with tomotherapy than with conventional IMRT or opposed lateral fields. 11 It has been noted that the tomotherapy system tends to produce fluence up to 7 mm beyond the skin surface.…”

“…[8][9][10][11][12] However, incidental surface dose tends to be higher with tomotherapy than with conventional IMRT or opposed lateral fields. 11 It has been noted that the tomotherapy system tends to produce fluence up to 7 mm beyond the skin surface. 10 We performed head and neck phantom experiments to evaluate tomotherapy's robustness with regard to superficial coverage.…”