Accuracy of TomoTherapy treatments for superficial target volumes

Abstract

Helical tomotherapy is a technique for delivering intensity modulated radiation therapy treatments using a continuously rotating linac. In this approach, fan beams exiting the linac are dynamically modulated in synchrony with the motion of the gantry and couch. Helical IMRT deliveries have been applied to treating surface lesions, and the purpose of this study was to evaluate the accuracy of dose calculated by the TomoTherapy HiArt treatment planning system for superficial planning target volumes (PTVs). TomoTherapy treatment plans were developed for three superficial PTVs (2-, 4-, and 6-cm deep radially by 90 degrees azimuthally by 4-cm longitudinally) contoured on a 27-cm diameter cylindrical white opaque, high-impact polystyrene phantom. The phantom included removable transverse and sagittal film cassettes that contained bare Kodak EDR2 films cut such that their edges matched the phantom surface (+/-0.05 cm). The phantom was aligned to the machine's isocenter (+/-0.05 cm) and was irradiated according to the treatment plans. Films were scanned with a Vidar film digitizer, and optical densities were converted to dose using a calibration determined from a 6 MV perpendicular film exposure. This perpendicular calibration required that axial film doses (parallel irradiation) be scaled by 1.02 so that mid-arc depth doses matched those measured in the sagittal plane (perpendicular irradiation). All film readings were scaled by 0.935 to correct for over-response due to phantom Cerenkov light. Measured dose distributions were registered to calculated ones and compared. Calculated doses overpredicted measured doses by as much as 9.5% of the prescribed dose at depths less than 1 cm. At depths greater than 1 cm, calculated dose distributions showed agreement to measurement within 5% in the high-dose region and within 0.2 cm distance-to-agreement in the dose falloff regions. In the low-dose region posterior to the PTVs (<10% of the prescribed dose), the dose algorithm underpredicted the dose by 1%-2% of the prescribed dose. Clinically, it is recommended that 1 cm of bolus be used on the surface to ensure that cancerous tissues less than 1 cm depth are not underdosed.

Feasibility of Postmastectomy Treatment With Helical TomoTherapy

“…The accuracy of superficial dose in IMRT cases has been documented by Chung et al (25) for the Pinnacle 3 treatment-planning system and by Mutic and Low (26) for the Peacock treatment-planning system (NOMOS, Sewickley, PA). Similarly, we have documented the accuracy of superficial dose calculated by the TomoTherapy treatment-planning system for the CW geometry in phantom (27)(28)(29) and are currently assessing skin dose in the CW in vivo (30). As in the case of conventional tangent photon beams, the use of a bolus on the CW for all or part of the TomoTherapy treatment can ensure a sufficiently high skin dose and greater accuracy of calculated dose near the patient surface.…”

Skin dose study of chest wall treatment with tomotherapy

“…However, bolus has been suggested as desirable for tomotherapy to reduce the effect of daily setup error and potential underdosing of the surface 11 and to correct for shallow depth dose overestimation in the TomoTherapy treatment planning algorithm. 12 Because of the angular dependence of MOSFET dosimeters (3.0%-3.5%), their measurement uncertainties (2%-3%), and other possible setup errors, the estimated accuracy of MOSFET dosimetry is about ±6%. 24 The estimated 12% overestimation of TomoTherapy treatment planning system may include the MOSFET measurement uncertainty.…”

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.…”