Safety of adjuvant intensity-modulated postoperative radiation therapy in endometrial cancer: Clinical data and dosimetric parameters according to the International Commission on Radiation Units (ICRU) 83 report

Application of a linear interpolation algorithm in radiation therapy dosimetry for 3D dose point acquisition

Air-vented ion chambers are generally used in radiation therapy dosimetry to determine the absorbed radiation dose with superior precision. However, in ion chamber detector arrays, the number of array elements and their spacing do not provide sufficient spatial sampling, which can be overcome by interpolating measured data. Herein, we investigated the potential principle of the linear interpolation algorithm in volumetric dose reconstruction based on computed tomography images in the volumetric modulated arc therapy (VMAT) technique and evaluated how the ion chamber spacing and anatomical mass density affect the accuracy of interpolating new data points. Plane measurement doses on 83 VMAT treatment plans at different anatomical sites were acquired using Octavius 729, Octavius1500, and MatriXX ion chamber detector arrays, followed by the linear interpolation to reconstruct volumetric doses. Dosimetric differences in planning target volumes (PTVs) and organs at risk (OARs) between treatment planning system and reconstruction were evaluated by dose volume histogram metrics. The average percentage dose deviations in the mean dose (D_mean) of PTVs reconstructed by 729 and 1500 arrays ranged from 4.7 to 7.3% and from 1.5 to 2.3%, while the maximum dose (D_max) counterparts ranged from 2.3 to 5.5% and from 1.6 to 7.6%, respectively. The average percentage dose/volume deviations of mixed PTVs and OARs in the abdomen/gastric and pelvic sites were 7.6%, 3.5%, and 7.2%, while mediastinum and lung plans showed slightly larger values of 8.7%, 5.1%, and 8.9% for 729, 1500, and MatriXX detector arrays, respectively. Our findings indicated that the smaller the spacing between neighbouring detectors and the more ion chambers present, the smaller the error in interpolating new data points. Anatomical regions with small local mass density inhomogeneity were associated with superior dose reconstruction. Given a large mass density difference in the various human anatomical structures and the characteristics of the linear interpolation algorithm, we suggest that an alternative data interpolation method should be used in radiotherapy dosimetry.

Should we customize PTV expansions for BMI? Daily cone beam computerized tomography to assess organ motion in postoperative endometrial and cervical cancer patients

AIM

A single-institution review assessing patient characteristics contributing to daily organ motion in postoperative endometrial and cervical cancer patients treated with intensity-modulated radiotherapy (IMRT).

BACKGROUND

The Radiation Therapy Oncology Group has established consensus guidelines for postoperative pelvic IMRT, recommending a 7 mm margin on all three axes of the target volume.

MATERIALS AND METHODS

Daily shifts on 457 radiation setups for 18 patients were recorded in the x axis (lateral), y axis (superior-inferior) and z axis (anterior-posterior); daily positions of the planning tumor volume were referenced with the initial planning scan to quantify variations.

RESULTS

Of the 457 sessions, 85 (18.6%) had plan shifts of at least 7 mm in one of the three dimensions. For obese patients (body mass index [BMI] ≥ 30), 75/306 (24.5%) sessions had plan shifts ≥7 mm. Odds of having a shift ≥7 mm in any direction was greater for obese patients under both univariate (OR 4.227, 95% CI 1.235-14.466, p = 0.021) and multivariate (OR 5.000, 95% CI 1.341-18.646, p = 0.016) analyses (MVA). Under MVA, having a BMI ≥ 30 was associated with increased odds of shifts in the anterior-posterior (1.173 mm, 95% CI 0.281-2.065, p = 0.001) and lateral (2.074 mm, 95% CI 1.284-2.864, p < 0.000) directions but not in the superior-inferior axis (0.298 mm, 95% CI -0.880 to 1.475, p = 0.619) exceeding 7 mm.

CONCLUSIONS

Based on these findings, the standard planned tumor volume expansion of 7 mm is less likely to account for daily treatment changes in obese patients.