A conformation number to quantify the degree of conformality in brachytherapy and external beam irradiation: Application to the prostate

Even with advanced inverse‐planning techniques, radiation treatment plan optimization remains a very time‐consuming task with great output variability, which prompted the development of more automated approaches. One commercially available technique mimics the actions of experienced human operators to progressively guide the traditional optimization process with automatically created regions of interest and associated dose‐volume objectives. We report on the initial evaluation of this algorithm on 10 challenging cases of locoreginally advanced head and neck cancer. All patients were treated with VMAT to 70 Gy to the gross disease and 56 Gy to the elective bilateral nodes. The results of post‐treatment autoplanning (AP) were compared to the original human‐driven plans (HDP). We used an objective scoring system based on defining a collection of specific dosimetric metrics and corresponding numeric score functions for each. Five AP techniques with different input dose goals were applied to all patients. The best of them averaged the composite score 8% lower than the HDP, across the patient population. The difference in median values was statistically significant at the 95% confidence level (Wilcoxon paired signed‐rank test p=0.027). This result reflects the premium the institution places on dose homogeneity, which was consistently higher with the HDPs. The OAR sparing was consistently better with the APs, the differences reaching statistical significance for the mean doses to the parotid glands (p<0.001) and the inferior pharyngeal constrictor (p=0.016), as well as for the maximum doses to the spinal cord (p=0.018) and brainstem (p=0.040). If one is prepared to accept less stringent dose homogeneity criteria from the RTOG 1016 protocol, nine APs would comply with the protocol, while providing lower OAR doses than the HDPs. Overall, AP is a promising clinical tool, but it could benefit from a better process for shifting the balance between the target dose coverage/homogeneity and OAR sparing.PACS number(s): 87.55.D

C N = \frac{V_{T, r e f}}{V_{T}} \times \frac{V_{T, r e f}}{V_{r e f}}

where

V_{T, r e f}

is the volume of target receiving a dose equal to or greater than the reference dose,

V_{T}

is the target volume, and

V_{r e f}

is the volume receiving a dose equal to or greater than the reference dose.…”

Section: Methodsmentioning

confidence: 99%

Initial evaluation of automated treatment planning software

Gintz

Latifi

Caudell

et al. 2016

“…Thus, CTV target coverage ( V 95% ), overdose ( V 107% ) and homogeneity index (HI = ( D 2% − D 98% )/ D p )31 were evaluated, where D 2% and D 98% denote the minimum dose to 2% and 98% of the CTV and D p is the prescribed dose. Conformity number with reference isodose (RI) of 95% was determined for the PTV as CN = (PTV RI /PTV)×(PTV RI /V RI ),32 where PTV RI denotes the volume of the PTV covered by the RI and V RI the volume of the RI. For the healthy tissue outside the PTV, the integral dose (defined as the mean dose times the volume) and the size of hotspot volumes > 107% were calculated.…”

Section: Methodsmentioning

confidence: 99%

Potential proton and photon dose degradation in advanced head and neck cancer patients by intratherapy changes

Stützer

Jakobi

Bandurska-Luque

et al. 2017

PurposeEvaluation of dose degradation by anatomic changes for head‐and‐neck cancer (HNC) intensity‐modulated proton therapy (IMPT) relative to intensity‐modulated photon therapy (IMRT) and identification of potential indicators for IMPT treatment plan adaptation.MethodsFor 31 advanced HNC datasets, IMPT and IMRT plans were recalculated on a computed tomography scan (CT) taken after about 4 weeks of therapy. Dose parameter changes were determined for the organs at risk (OARs) spinal cord, brain stem, parotid glands, brachial plexus, and mandible, for the clinical target volume (CTV) and the healthy tissue outside planning target volume (PTV). Correlation of dose degradation with target volume changes and quality of rigid CT matching was investigated.ResultsRecalculated IMPT dose distributions showed stronger degradation than the IMRT doses. OAR analysis revealed significant changes in parotid median dose (IMPT) and near maximum dose (D 1ml) of spinal cord (IMPT, IMRT) and mandible (IMPT). OAR dose parameters remained lower in IMPT cases. CTV coverage (V 95%) and overdose (V 107%) deteriorated for IMPT plans to (93.4 ± 5.4)% and (10.6 ± 12.5)%, while those for IMRT plans remained acceptable. Recalculated plans showed similarly decreased PTV conformity, but considerable hotspots, also outside the PTV, emerged in IMPT cases. Lower CT matching quality was significantly correlated with loss of PTV conformity (IMPT, IMRT), CTV homogeneity and coverage (IMPT). Target shrinkage correlated with increased dose in brachial plexus (IMRT, IMPT), hotspot generation outside the PTV (IMPT) and lower PTV conformity (IMRT).ConclusionsThe study underlines the necessity of precise positioning and monitoring of anatomy changes, especially in IMPT which might require adaptation more often. Since OAR doses remained typically below constraints, IMPT plan adaptation will be indicated by target dose degradations.

“…The gradient index (GI) (6) and Paddick conformity index (PCI) 7 , 8 were also determined and compared for the GK and VMAT plans.…”

Section: Methodsmentioning

confidence: 99%

Fractionated SRT using VMAT and Gamma Knife for brain metastases and gliomas — a planning study

Huss

Barsoum

Dodoo

et al. 2015

Stereotactic radiosurgery using Gamma Knife (GK) or linear accelerators has been used for decades to treat brain tumors in one fraction. A new positioning system, Extend™, was introduced by Elekta AB for fractionated stereotactic radiotherapy (SRT) with GK. Another option for fractionated SRT is advanced planning and delivery using linacs and volumetric modulated arc therapy (VMAT). This project aims to assess the performance of GK Extend™ for delivering fractionated SRT by comparing GK treatments plans for brain targets performed using Leksell GammaPlan (LGP) with VMAT treatment plans. Several targets were considered for the planning: simulated metastasis‐ and glioma‐like targets surrounding an organ at risk (OAR), as well as three clinical cases of metastases. Physical parameters such as conformity, gradient index, dose to OARs, and brain volume receiving doses above the threshold associated with risk of damaging healthy tissue, were determined and compared for the treatment plans. The results showed that GK produced better dose distributions for target volumes below 15 cm3, while VMAT results in better dose conformity to the target and lower doses to the OARs in case of fractionated treatments for large or irregular volumes. The volume receiving doses above a threshold associated with increased risk of damage to normal brain tissue was also smaller for VMAT. The GK consistently performed better than VMAT in producing a lower dose‐bath to the brain. The above is subjected only to margin‐dependent fractionated radiotherapy (CTV/PTV). The results of this study could lead to clinically significant decisions regarding the choice of the radiotherapy technique for brain targets.PACS numbers: 87.53.Ly, 87.55.D‐