The effect of different lung densities on the accuracy of various radiotherapy dose calculation methods: Implications for tumour coverage

Since its inception, doses applied using Gamma Knife Radiosurgery (GKR) have been calculated using a simple TMR algorithm, which assumes the patient's head is of even density, the same as water. This results in a significant approximation of the dose delivered by the Gamma Knife. We investigated how GKR dose calculations varied when using a new convolution algorithm clinically available for GKR planning that takes into account density variations in the head compared with the established calculation algorithm. Fifty‐five patients undergoing GKR and harboring 85 lesions were voluntarily and prospectively enrolled into the study. Their clinical treatment plans were created and delivered using TMR 10, but were then recalculated using the density correction algorithm. Dosimetric differences between the planning algorithms were noted. Beam on time (BOT), which is directly proportional to dose, was the main value investigated. Changes of mean and maximum dose to organs at risk (OAR) were also assessed. Phantom studies were performed to investigate the effect of frame and pin materials on dose calculation using the convolution algorithm. Convolution yielded a mean increase in BOT of 7.4% (3.6%–11.6%). However, approximately 1.5% of this amount was due to the head contour being derived from the CT scans, as opposed to measurements using the Skull Scaling Instrument with TMR. Dose to the cochlea calculated with the convolution algorithm was approximately 7% lower than with the TMR 10 algorithm. No significant difference in relative dose distribution was noted and CT artifact typically caused by the stereotactic frame, glue embolization material or different fixation pin materials did not systematically affect convolution isodoses. Nonetheless, substantial error was introduced to the convolution calculation in one target located exactly in the area of major CT artifact caused by a fixation pin. Inhomogeneity correction using the convolution algorithm results in a considerable, but consistent, dose shift compared to the TMR 10 algorithm traditionally used for GKR. A reduction of the prescription dose may be necessary to obtain the same clinical effect with the convolution algorithm. Head shape definition using CT outlining can reduce treatment uncertainty from head shape approximations.PACS number(s): 87.53.‐j; 87.55.D; 87.55.kd

Section: Discussionmentioning

confidence: 53%

Investigation of dosimetric differences between the TMR 10 and convolution algorithm for Gamma Knife stereotactic radiosurgery

Rojas-Villabona

Kitchen

Paddick

2016

“…This study design is similar to a previously published phantom study by Aarup et al, (5) who investigated the effect of various lung densities on target coverage differential. With these approaches, all other confounding factors were “controlled,” and therefore a large number of cases were not required for statistical power in the comparison.…”

Section: Discussionmentioning

confidence: 85%

“…Previous studies have shown that target dose variations between various algorithms are dependent on a multitude of factors: tumor size, tumor location, lung density, beam energy, and delivery technique 2 , 3 , 5 , 6 , 7 , 8 , 9 , 10 , 11 . Specifically, greater target dose deficiencies are present for smaller tumors, 2 , 3 , 10 peripheral tumors, 2 , 10 and lower lung densities 3 , 5 . Other factors, such as beam energy and delivery technique, were also shown to affect the target dose deficiency in more complex patterns 6 , 7 , 8 …”

Section: Introductionmentioning

confidence: 99%

Effect of the normalized prescription isodose line on the magnitude of Monte Carlo vs. pencil beam target dose differences for lung stereotactic body radiotherapy

Zheng

Liang

Zhu

et al. 2016

In lung stereotactic body radiotherapy (SBRT) cases, the pencil beam (PB) dose calculation algorithm is known to overestimate target dose as compared to the more accurate Monte Carlo (MC) algorithm. We investigated whether changing the normalized prescription isodose line affected the magnitude of MC vs. PB target dose differences. Forty‐eight patient plans and twenty virtual‐tumor phantom plans were studied. For patient plans, four alternative plans prescribed to 60%, 70%, 80%, and 90% isodose lines were each created for 12 patients who previously received lung SBRT treatments. Using 6 MV dynamic conformal arcs, the plans were individually optimized to achieve similar dose coverage and conformity for all plans of the same patient, albeit at the different prescription levels. These plans, having used a PB algorithm, were all recalculated with MC to compare the target dose differences. The relative MC vs. PB target dose variations were investigated by comparing PTV D95, Dmean, and D5 loss at the four prescription levels. The MC‐to‐PB ratio of the plan heterogeneity index (HI) was also evaluated and compared among different isodose levels. To definitively demonstrate the cause of the isodose line dependence, a simulated phantom study was conducted using simple, spherical virtual tumors planned with uniform block margins. The tumor size and beam energy were also altered in the phantom study to investigate the interplay between these confounding factors and the isodose line effect. The magnitude of the target dose overestimation by PB was greater for higher prescription isodose levels. The MC vs. PB reduction in the target dose coverage indices, D95 and V100 of PTV, were found to monotonically increase with increasing isodose lines from 60% to 90%, resulting in more pronounced target dose coverage deficiency at higher isodose prescription levels. No isodose level‐dependent trend was observed for the dose errors in the target mean or high dose indices, Dmean or D5. The phantom study demonstrated that the observed isodose level dependence was caused by different beam margins used for the different isodose levels: a higher prescription line required a larger beam margin, leading to more low‐density lung tissues in the field and, therefore, larger dose errors at the target periphery (when calculated with PB). The phantom study also found that the observed isodose level dependence was greater for smaller targets and for higher beam energies. We hereby characterized the effect of normalized prescription isodose line on magnitude of PTV dose coverage as calculated by MC vs. PB. When comparing reported MC dose deficiency values for different patients, the selection of prescription isodose line should be considered in addition to other factors known to affect differences in calculated doses between various algorithms.PACS number(s): 87.55.kh, 87.55.dk, 87.55.de

“…Another limitation of our study was that only one Type‐A and one Type‐B algorithm were investigated; therefore, the results and trends found may or may not be generalizable to other algorithms. Herein, the Type‐B algorithm utilized was AAA, which may be less accurate than another Type‐B algorithm, CCC 24, 25, 26. However, it should also be noted that AAA is still quite prevalent in the current clinical treatment planning, which makes this study quite relatable to contemporary practice.…”

Section: Discussionmentioning

confidence: 99%

Does intensity modulation increase target dose calculation errors of conventional algorithms for lung SBRT?

Zheng

Verma

Wang

et al. 2018

PurposeConventional dose algorithms (Type A and Type B) for lung SBRT can display considerable target dose errors compared to Type‐C algorithms. Intensity‐modulated techniques (IMRT/VMAT) are increasingly being utilized for lung SBRT. Therefore, our study aimed to assess whether intensity modulation increased target dose calculation errors by conventional algorithms over conformal techniques.MethodsTwenty lung SBRT patients were parallely planned with both IMRT and dynamic conformal arc (DCA) techniques using a Type‐A algorithm, and another 20 patients were parallely planned with IMRT, VMAT, and DCA using a Type‐B algorithm. All 100 plans were recalculated with Type‐C algorithms using identical beam and monitor unit settings, with the Type‐A/Type‐B algorithm dose errors defined using Type‐C recalculation as the ground truth. Target dose errors for PTV and GTV were calculated for a variety of dosimetric end points. Using Wilcoxon signed‐rank tests (p < 0.05 for statistical significance), target dose errors were compared between corresponding IMRT/VMAT and DCA plans for the two conventional algorithms. The levels of intensity modulation were also evaluated using the ratios of MUs in the IMRT/VMAT plans to those in the corresponding DCA plans. Linear regression was used to study the correlation between intensity modulation and relative dose error magnitudes.ResultsOverall, larger errors were found for the Type‐A algorithm than for the Type‐B algorithm. However, the IMRT/VMAT plans were not found to have statistically larger dose errors from their corresponding DCA plans. Linear regression did not identify a significant correlation between the intensity modulation level and the relative dose error.ConclusionIntensity modulation did not appear to increase target dose calculation errors for lung SBRT plans calculated with conventional algorithms.