An analytical expression for R50% dependent on PTV surface area and volume: a lung SBRT comparison

Radiotherapy dose escalation using pre-treatment diffusion-weighted imaging in locally advanced rectal cancer: a planning study

Objectives

Diffusion-weighted MRI (DWI) may provide biologically relevant target volumes for dose-escalated radiotherapy in locally advanced rectal cancer (LARC). This planning study assessed the dosimetric feasibility of delivering hypofractionated boost treatment to intra-tumoural regions of restricted diffusion prior to conventional long-course radiotherapy.

Methods

Ten patients previously treated with curative-intent standard long-course radiotherapy (50 Gy/25#) were re-planned. Boost target volumes (BTVs) were delineated semi-automatically using 40th centile intra-tumoural apparent diffusion coefficient value with expansions (anteroposterior 11 mm, transverse 7 mm, craniocaudal 13 mm). Biased-dosed combined plans consisted of a single-fraction volumetric modulated arc therapy flattening-filter-free (VMAT-FFF) boost (phase 1) of 5, 7, or 10 Gy before long-course VMAT (phase 2). Phase 1 plans were assessed with reference to stereotactic conformality and deliverability measures. Combined plans were evaluated with reference to standard long-course therapy dose constraints.

Results

Phase 1 BTV dose targets at 5/7/10 Gy were met in all instances. Conformality constraints were met with only 1 minor violation at 5 and 7 Gy. All phase 1 and combined phase 1 + 2 plans passed patient-specific quality assurance. Combined phase 1 + 2 plans generally met organ-at-risk dose constraints. Exceptions included high-dose spillage to bladder and large bowel, predominantly in cases where previously administered, clinically acceptable non-boosted plans also could not meet constraints.

Conclusions

Targeted upfront LARC radiotherapy dose escalation to DWI-defined is feasible with appropriate patient selection and preparation.

Advances in knowledge

This is the first study to evaluate the feasibility of DWI-targeted upfront radiotherapy boost in LARC. This work will inform an upcoming clinical feasibility study.

How Does the Gradient Measure of the Lung SBRT Treatment Plan Depend on the Tumor Volume and Shape?

Purpose

Gradient measure (GM) is a critical index related to normal tissue sparing in radiosurgery. This study aims to describe the dependence of GM on target volume and target shape for lung stereotactic body radiation therapy (SBRT) treatment plans.

Methods

A total of 307 peripheral and 119 central lung SBRT treatment plans were enrolled for this study. A least-squares regression was used for data analysis. First, the equations with different functional forms were established to determine the dependence of GM on a univariaty (V_P or Sp) and bivariaty (V_P and Sp), respectively. Then, the correlation coefficients and p-values of variables for all equations were compared and analyzed to determine the dependence of GM on PTV volume (VP) and sphericity (Sp).

Results

The power equations had the highest coefficient of determination (R²) in the dependence results of GM on univariate V_P. The equations were GM = 0.674 V P 0.178 and GM = 0.660 V P 0.185 for peripheral and central lesions, respectively. On the other hand, the R² of all functional forms were less than 0.25 when the relationship of GM versus univariate Sp was analyzed. Similarly, the power equation also obtained the highest R² in bivariaty V_P and Sp analysis, whether for central or peripheral. However, the R² of the bivariate equations were not improved compared with those of univariate equations. Moreover, the p-values of the variable Sp were greater than 0.05.

Conclusions

The GM of the lung SBRT plan is shape-independent and volume-dependent. The dependence of GM on PTV volume for peripheral and central lung cancer can be described by two different power equations. The results of this study can be used as a potential tool to assist dosimetric quality control during the radiosurgery process.

CT-guided versus MR-guided radiotherapy: Impact on gastrointestinal sparing in adrenal stereotactic body radiotherapy

BACKGROUND AND PURPOSE

To quantify the indication for adaptive, gated breath-hold (BH) MR-guided radiotherapy (MRgRT_BH) versus BH or free-breathing (FB) CT-based image-guided radiotherapy (CT-IGRT) for the ablative treatment of adrenal malignancies.

MATERIALS AND METHODS

Twenty adrenal patients underwent adaptive IMRT MRgRT_BH to a median dose of 50 Gy/5 fractions. Each patient was replanned for VMAT CT-IGRT_BH and CT-IGRT_FB on a c-arm linac. Only CT-IGRT_FB used an ITV, summed from GTVs of all phases of the 4DCT respiratory evaluation. All used the same 5 mm GTV/ITV to PTV expansion. Metrics evaluated included: target volume and coverage, conformality, mean ipsilateral kidney and 0.5 cc gastrointestinal organ-at-risk (OAR) doses (D_0.5cc). Adaptive dose for MRgRT_BH and predicted dose (i.e., initial plan re-calculated on anatomy of the day) was performed for CT-IGRT_BH and MRgRT_BH to assess frequency of OAR violations and coverage reductions for each fraction.

RESULTS

The more common VMAT CT-IGRT_FB, with its significantly larger target volumes, proved inferior to MRgRT_BH in mean PTV and ITV/GTV coverage, as well as small bowel D_0.5cc. Conversely, VMAT CT-IGRT_BH delivered a dosimetrically superior initial plan in terms of statistically significant (p ≤ 0.02) improvements in target coverage, conformality and D_0.5cc to the large bowel, duodenum and mean ipsilateral kidney compared to IMRT MRgRT_BH. However, non-adaptive CT-IGRT_BH had a 71.8% frequency of predicted indications for adaptation and was 2.8 times more likely to experience a coverage reduction in PTV D_95% than predicted for MRgRT_BH.

CONCLUSION

Breath-hold VMAT radiotherapy provides superior target coverage and conformality over MRgRT_BH, but the ability of MRgRT_BH to safely provide ablative doses to adrenal lesions near mobile luminal OAR through adaptation and direct, real-time motion tracking is unmatched.

Analysis of R50% location dependence on LINAC-based VMAT cranial stereotactic treatments

Stereotactic radiosurgery (SRS) and stereotactic radiation therapy (SRT) techniques are used to deliver high doses per fraction to various types of intra-cranial targets. LINAC-based solutions are growing in prevalence due to recent advances in technologies such as high-definition multi-leaf collimators and volumetric arc therapy radiation delivery. A wide variety of clinical pathologies including intracranial metastases, meningioma, glioblastoma, arteriovenous malformation, acoustic neuroma, and trigeminal neuralgia have been successfully treated using SRS/SRT techniques. These lesions can be in virtually at any location within the cranium. Several publications have shown a wide dispersion of intermediate dose conformality (intermediate dose spill) indices such as the Paddick Gradient Index or R50% for lesions of a specific volume. A complete explanation of this dispersion is lacking but location has been suggested as a contributing factor. While prior studies of PTV location in SRS/SRT are retrospective in nature, we have conducted a prospective study to ascertain the potential effects of location within the cranium on plan intermediate dose conformality as measured by R50% while controlling for lesion volume, lesion shape, prescription (Rx) dose, and Rx isodose surface. Lesion volumes utilized in this study are consistent with metastatic disease presentation. Results indicate only a weak relationship between intermediate dose conformality as measured by R50% and the lesion location when considering nine different, strategically placed lesions. Close proximity to critical structures can reduce the degree of conformality, but the effect appears to be minimal. Single isocenter multiple target cases were studied in addition to single target plans. All critical structure doses observed in this study were found to be within the recommendations of AAPM Task Group report 101. Lesion location does not appear to be a significant contributing factor to the observed variation of dose conformality seen in several SRS/SRT publications.

The surface area effect: How the intermediate dose spill depends on the PTV surface area in SRS

Purpose

Stereotactic radiosurgery (SRS) is rapidly becoming the standard of care for many intracranial targets. The characteristics of the planning target volume (PTV) can affect the intermediate dose spill and thus normal brain volume dose which is correlated with brain toxicity. R50% (volume receiving 50% of prescription dose divided by PTV volume) is a useful metric to quantify the intermediate dose spill. We propose a novel understanding of how the PTV surface area (SA_PTV) affects the intermediate dose spill of SRS treatments.

Methods

Using a phantom model provided by a computed tomography (CT) of the IROC Head Phantom® and Eclipse® Treatment Planning System, we investigate the relationship of R50% and SA_PTV in single‐target SRS treatments. The planning studies are conducted for SRS treatments on a Varian TrueBeam® linear accelerator with high‐definition MLC and a 6 MVFFF beam mode. These data are analyzed to ascertain trends in R50% related to SA_PTV. Since SA_PTV is not available as a structure property in the Eclipse RTPS, we introduce an Eclipse script to extract PTV surface area of arbitrary‐shaped PTVs. We compare a physically reasonable theoretical prediction of R50%, R50%_Analytic, to the R50% achieved in treatment planning studies.

Results

The SRS phantom study indicates good correlation between the plan R50% and SA_PTV. A near‐linear relationship of plan R50% vs SA_PTV is observed as predicted by the R50%_Analytic model. Agreement between plan R50% values and R50%_Analytic predictions is good for all but the very smallest PTV volumes.

Conclusions

We demonstrate dependence of the intermediate dose spill measured by R50% on the SA_PTV. We call that dependence the surface area effect. This dependence is explicit in the R50%_Analytic prediction model. The predicted value of R50%_Analytic for a given PTV could be used for guidance during SRS treatment plan optimization, and plan evaluation for that PTV.

An analytical expression for R50% dependent on PTV surface area and volume: A cranial SRS comparison

The intermediate dose spill for a stereotactic radiosurgery (SRS) plan can be quantified with the metric R50%, defined as the 50% isodose cloud volume (V_IDC50% ) divided by the volume of the planning target volume (PTV). By coupling sound physical principles with the basic definition of R50%, we derive an analytical expression for R50% for a spherical PTV. Our analytical expression depends on three quantities: the surface area of PTV (SA_PTV ), the volume of PTV (V_PTV ), and the distance of dose drop-off to 50% (Δr). The value of ∆r was obtained from a simple set of cranial phantom plan calculations. We generate values from our analytical expression for R50% (R50%_Analytic ) and compare the values to clinical R50% values (R50%_Clinical ) extracted from a previously published SRS data set that spans the V_PTV range from 0.15 to 50.1 cm³ . R50%_Analytic is smaller than R50%_Clinical in all cases by an average of 15% ± 7%, and the general trend of R50%_Clinical vs V_PTV is reflected in the same trend of R50%_Analytic . This comparison suggests that R50%_Analytic could represent a theoretical lower limit for the clinical SRS data; further investigation is required to confirm this. R50%_Analytic could provide useful guidance for what might be achievable in SRS planning.