The aim of this article is to derive and verify a mathematical formulation for the reduction of the six-dimensional (6D) positional inaccuracies of patients (lateral, longitudinal, vertical, pitch, roll and yaw) to three-dimensional (3D) linear shifts. The formulation was mathematically and experimentally tested and verified for 169 stereotactic radiotherapy patients. The mathematical verification involves the comparison of any (one) of the calculated rotational coordinates with the corresponding value from the 6D shifts obtained by cone beam computed tomography (CBCT). The experimental verification involves three sets of measurements using an ArcCHECK phantom, when (i) the phantom was not moved (neutral position: 0MES), (ii) the position of the phantom shifted by 6D shifts obtained from CBCT (6DMES) from neutral position and (iii) the phantom shifted from its neutral position by 3D shifts reduced from 6D shifts (3DMES). Dose volume histogram and statistical comparisons were made between [Formula: see text] and [Formula: see text]. The mathematical verification was performed by a comparison of the calculated and measured yaw (γ°) rotation values, which gave a straight line, Y  =  1X with a goodness of fit as R =  0.9982. The verification, based on measurements, gave a planning target volume receiving 100% of the dose (V100%) as 99.1  ±  1.9%, 96.3  ±  1.8%, 74.3  ±  1.9% and 72.6  ±  2.8% for the calculated treatment planning system values TPSCAL, 0MES, 3DMES and 6DMES, respectively. The statistical significance (p-values: paired sample t-test) of V100% were found to be 0.03 for the paired sample [Formula: see text] and 0.01 for [Formula: see text]. In this paper, a mathematical method to reduce 6D shifts to 3D shifts is presented. The mathematical method is verified by using well-matched values between the measured and calculated γ°. Measurements done on the ArcCHECK phantom also proved that the proposed methodology is correct. The post-correction of the table position condition introduces a minimal spatial dose delivery error in the frameless stereotactic system, using a 6D motion enabled robotic couch. This formulation enables the reduction of 6D positional inaccuracies to 3D linear shifts, and hence allows the treatment of patients with frameless stereotactic radiosurgery by using only a 3D linear motion enabled couch.

ABSTRACT:In 204

In a retrospective computer dosimetry analysis of 58 patients with carcinoma of the oral tongue treated with interstitial radium implants alone or in combination with external irradiation, dose and volume appear to be the most important factors in both local control and the incidence of necrosis; in the dose rate range commonly used in clinical interstitial radiotherapy, dose rate has no significant effect. The optimal minimum tumor doses for local control vary with the size of the primary lesion: 6,000 rads for T1 lesions and 6,500 rads for T2 lesions treated with interstitial radium implants alone and 7,500 rads for lesions treated with interstitial radium implants in combination with external irradiation. For lesions treated with implants plus external irradiation, greater local control was achieved when most of the dose was delivered through the interstitial implants. For a given volume, the incidence of necrosis was directly proportional to the degree of overdosage; for a given dose, the incidence of necrosis was directly proportional to the volume receiving the dose.

Purpose: To select a reference point in a low dose gradient region of an IMRT treatment plan to enhance the MU and point dose agreement between Pinnacle and RadCalc. Method and Materials: After generating an IMRT plan within Pinnacle, we export it to RadCalc for a second check of the MU's. Frequently, the MU difference is significant for a plan with split beams or isocenter out of the field. In contrast to Pinnacle, RadCalc displays a coordinate grid over its BEV fluence. By utilizing this feature for the problematic beams, we selected reference points in low gradient regions of each beam's fluence map. In RadCalc's BEV, we identified the coordinate shifts relative to isocenter of the preferred points. We generated an Excel spreadsheet to calculate the updated coordinates in Pinnacle's 3‐D CT‐based coordinate system to reflect the desired point shift in RadCalc. These new coordinates were then entered in Pinnacle for the patient plan and re‐exported to RadCalc. The modified MU and dose comparisons within RadCalc generally fell within 5% per beam. Results: While this method adds a few extra steps to the planning process, it provides a way to choose reference points whereby the MU's and point doses between Pinnacle and RadCalc are likely to agree within a few percent, and it makes determining the coordinates of such points a reasonably efficient process. Conclusion: RadCalc is a useful program for verifying IMRT MU's and point doses generated by Pinnacle. However because Pinnacle exports the user selected reference point (typically isocenter), there are common conditions in which RadCalc understandably determines large percent differences in calculations. Our method uses RadCalc's fluence map along with a spreadsheet to determine the Pinnacle coordinates of a preferred calculation point, rather than “guessing” where to place a POI to bring about better calculation agreement.

No abstract