Planning Target Volume D95 Research Articles

Quality assessment for VMAT prostate radiotherapy planning based on data envelopment analysis

The majority of commercial radiotherapy treatment planning systems requires planners to iteratively adjust the plan parameters in order to find a satisfactory plan. This iterative trial-and-error nature of radiotherapy treatment planning results in an inefficient planning process and in order to reduce such inefficiency, plans can be accepted without achieving the best attainable quality. We propose a quality assessment method based on data envelopment analysis (DEA) to address this inefficiency. This method compares a plan of interest to a set of past delivered plans and searches for evidence of potential further improvement. With the assistance of DEA, planners will be able to make informed decisions on whether further planning is required and ensure that a plan is only accepted when the plan quality is close to the best attainable one. We apply the DEA method to 37 prostate plans using two assessment parameters: rectal generalized equivalent uniform dose (gEUD) as the input and D95 (the minimum dose that is received by 95% volume of a structure) of the planning target volume (PTV) as the output. The percentage volume of rectum overlapping PTV is used to account for anatomical variations between patients and is included in the model as a non-discretionary output variable. Five plans that are considered of lesser quality by DEA are re-optimized with the goal to further improve rectal sparing. After re-optimization, all five plans improve in rectal gEUD without clinically considerable deterioration of the PTV D95 value. For the five re-optimized plans, the rectal gEUD is reduced by an average of 1.84 Gray (Gy) with only an average reduction of 0.07 Gy in PTV D95. The results demonstrate that DEA can correctly identify plans with potential improvements in terms of the chosen input and outputs.

Volumetric‐modulated arc therapy for the treatment of a large planning target volume in thoracic esophageal cancer

Recently, volumetric‐modulated arc therapy (VMAT) has demonstrated the ability to deliver radiation dose precisely and accurately with a shorter delivery time compared to conventional intensity‐modulated fixed‐field treatment (IMRT). We applied the hypothesis of VMAT technique for the treatment of thoracic esophageal carcinoma to determine superior or equivalent conformal dose coverage for a large thoracic esophageal planning target volume (PTV) with superior or equivalent sparing of organs‐at‐risk (OARs) doses, and reduce delivery time and monitor units (MUs), in comparison with conventional fixed‐field IMRT plans. We also analyzed and compared some other important metrics of treatment planning and treatment delivery for both IMRT and VMAT techniques. These metrics include: 1) the integral dose and the volume receiving intermediate dose levels between IMRT and VMATI plans; 2) the use of 4D CT to determine the internal motion margin; and 3) evaluating the dosimetry of every plan through patient‐specific QA. These factors may impact the overall treatment plan quality and outcomes from the individual planning technique used. In this study, we also examined the significance of using two arcs vs. a single‐arc VMAT technique for PTV coverage, OARs doses, monitor units and delivery time. Thirteen patients, stage T2‐T3 N0‐N1 (TNM AJCC 7th edn.), PTV volume median 395 cc (range 281–601 cc), median age 69 years (range 53 to 85), were treated from July 2010 to June 2011 with a four‐field (n=4) or five‐field (n=9) step‐and‐shoot IMRT technique using a 6 MV beam to a prescribed dose of 50 Gy in 20 to 25 F. These patients were retrospectively replanned using single arc (VMATI, 91 control points) and two arcs (VMATII, 182 control points). All treatment plans of the 13 study cases were evaluated using various dose‐volume metrics. These included PTV D99, PTV D95, PTV V9547.5Gy(95%), PTV mean dose, Dmax, PTV dose conformity (Van't Riet conformation number (CN)), mean lung dose, lung V20 and V5, liver V30, and Dmax to the spinal canal prv3mm. Also examined were the total plan monitor units (MUs) and the beam delivery time. Equivalent target coverage was observed with both VMAT single and two‐arc plans. The comparison of VMATI with fixed‐field IMRT demonstrated equivalent target coverage; statistically no significant difference were found in PTV D99 (p=0.47), PTV mean (p=0.12), PTV D95 and PTV V9547.5Gy (95%) (p=0.38). However, Dmax in VMATI plans was significantly lower compared to IMRT (p=0.02). The Van't Riet dose conformation number (CN) was also statistically in favor of VMATI plans (p=0.04). VMATI achieved lower lung V20 (p=0.05), whereas lung V5 (p=0.35) and mean lung dose (p=0.62) were not significantly different. The other OARs, including spinal canal, liver, heart, and kidneys showed no statistically significant differences between the two techniques. Treatment time delivery for VMATI plans was reduced by up to 55% (p=5.8E−10) and MUs reduced by up to 16% (p=0.001). Integral dose was not statistically different between the two planning techniques (p=0.99). There were no statistically significant differences found in dose distribution of the two VMAT techniques (VMATI vs. VMATII) Dose statistics for both VMAT techniques were: PTV D99 (p=0.76), PTV D95 (p=0.95), mean PTV dose (p=0.78), conformation number (CN) (p=0.26), and MUs (p=0.1). However, the treatment delivery time for VMATII increased significantly by two‐fold (p=3.0E−11) compared to VMATI. VMAT‐based treatment planning is safe and deliverable for patients with thoracic esophageal cancer with similar planning goals, when compared to standard IMRT. The key benefit for VMATI was the reduction in treatment delivery time and MUs, and improvement in dose conformality. In our study, we found no significant difference in VMATII over single‐arc VMATI for PTV coverage or OARs doses. However, we observed significant increase in delivery time for VMATII compared to VMATI.PACS number: 87.53.Kn, 87.55.‐x

A dose-volume intercomparison of volumetric-modulated arc therapy, 3D static conformal, and rotational conformal techniques for portal vein tumor thrombus in hepatocellular carcinoma

We created volumetric-modulated arc therapy (VMAT) plans for portal vein tumor thrombus (PVTT) in hepatocellular carcinoma, and compared the results with those from three-dimensional conformal radiotherapy (3D-CRT) and rotational conformal radiotherapy (R-CRT) plans. CT scan data from 10 consecutive patients with PVTT treated with 3D-CRT between January 2008 and January 2010 were utilized in the analysis. We analyzed the dosimetric properties of the plans for the 10 patients using the three different techniques with three different isocenter doses of 50, 56 and 60 Gy in 2-Gy fractions. The D95, Dmean, homogeneity index and conformity index were compared for the planning target volume (PTV). The Dmean, V20 and V30 were also compared for normal livers. The monitor units (MUs) and the treatment time were also evaluated. The normal liver V30 for VMAT was significantly less than that for 3D-CRT for the prescribed doses of 56 and 60 Gy (P < 0.05). It was also found that the normal liver V30 resulting from 3D-CRT was prohibitively increased when the prescribed dose was increased in two steps. For PTV D95, we found no significant differences between the three techniques for the 50- and 56-Gy prescriptions, or between VMAT and the other techniques for the 60-Gy prescription. The differences in the MUs and treatment times were not statistically significant between VMAT and 3D-CRT. We have demonstrated that VMAT may be a more advantageous technique for dose escalation reaching 60 Gy in the treatment of PVTT due to the reduced normal liver V30.

Clinical impact of dose overestimation by effective path length calculation in stereotactic ablative radiation therapy of lung tumors

PurposeTo determine the clinical impact of calculated dose differences between effective path length (EPL) and Monte Carlo (MC) algorithms in stereotactic ablative radiation therapy (SABR) of lung tumors. Methods and MaterialsWe retrospectively analyzed the treatment plans and clinical outcomes of 77 consecutive patients treated with SABR for 82 lung tumors between 2003 and 2009 at our institution. Sixty treatments were originally planned using EPL, and 22 using MC. All plans were recalculated for the same beam specifications using MC and EPL, respectively. The doses covering 95%, 50%, and 5% (D95, D50, D5, respectively) of the target volumes were compared between EPL and MC (assumed to be the actual delivered dose), both as physical dose and biologically effective dose. Time to local recurrence was correlated with dose by Cox regression analysis. The relationship between tumor control probability (TCP) and biologically effective dose was determined via logistic regression and used to estimate the TCP decrements due to prescribing by EPL calculations. ResultsEPL overestimated dose compared with MC in all tumor dose-volume histogram parameters in all plans. The difference was >10% of the MC D95 to the planning target volume and gross tumor volume in 60 of 82 (73%) and 52 of 82 plans (63%), respectively. Local recurrence occurred in 13 of 82 tumors. Controlling for gross tumor volume, higher physical and biologically effective planning target volume D95 correlated significantly with local control (P = .007 and P = .045, respectively). Compared with MC, prescribing based on EPL translated to a median TCP decrement of 4.3% (range, 1.2%-37%) and a >5% decrement in 46% of tumors. ConclusionsClinical follow-up for local lung tumor control in a sizable cohort of patients treated with SABR demonstrates that EPL overestimates dose by amounts that substantially decrease TCP in a large proportion. EPL algorithms should be avoided for lung tumor SABR.

Differences in the dose-volume metrics with heterogeneity correction status and its influence on local control in stereotactic body radiation therapy for lung cancer.

The purpose of this study is to evaluate the dose–volume metrics under different heterogeneity corrections and the factors associated with local recurrence (LR) after stereotactic body radiation therapy (SBRT) for non-small-cell lung cancer (NSCLC). Eighty-three patients who underwent SBRT for pathologically proven stage I NSCLC were reviewed retrospectively. The prescribed dose was 48 Gy in four fractions at the isocenter (IC) under heterogeneity correction with the Batho power law (BPL). The clinical plans were recalculated with Eclipse (Varian) for the same monitor units under the BPL and anisotropic analytical algorithm (AAA) and with no heterogeneity correction (NC). The dose at the IC, dose that covers 95% of the volume (D95), minimum dose (Min), and mean dose (Mean) of the planning target volume (PTV) were compared under each algorithm and between patients with local lesion control (LC) and LR. The IC doses under NC were significantly lower than those under the BPL and AAA. Under the BPL, the mean PTV D95, Min and Mean were 8.0, 9.4 and 7.4% higher than those under the AAA, and 9.6, 9.2 and 4.6% higher than those under NC, respectively. Under the AAA, all dose–volumetric parameters were significantly lower in T1a patients than in those with T1b and T2a. With a median follow-up of 35.9 months, LR occurred in 18 patients. Between the LC and LR groups, no significant differences were observed for any of the metrics. Even after stratification according to T-stage, no significant difference was observed between LC and LR.

The preliminary study of setup errors' impact on dose distribution of image guide radiation therapy for head and neck cancer

Objective To measure the set-up errors of patients with head and neck (HN then to further investigate the necessity of adjustment online for H&N cancer during IMRT treatment.Methods Cone-beam CT (CBCT) scanning of thirty patients with H&N cancer were acquired by once weekly with a total of 6 times during IMRT treatment.The CBCT images and the original planning CT images were matched by the bony structure and worked out the translational errors of the x,y,z axis,as well as rotational errors.The dose distributions were recalculated based on the data of each setup error.The dose of planning target volume (PTV) and organs at risk were calculated in the replanning,and than compared with the original plan by paired t-test.Results The mean value of x,y,z axis translational set-up errors were ( 1.06 ± 0.95 ) mm,( 0.95 ± 0.77 ) mm and ( 1.31 ± 1.07 ) mm,respectively.The rotational error of x,y,z axis were ( 1.04 ±0.791 ),( 1.06 ±0.89) and (0.81 ±0.61 ),respectively.PTV 95％ volume dose ( D95 ) and PTV minimal dose of replanning for 6 times set-up were lower than original plan (6526.6 cGy:6630.3 cGy,t =3.98,P =0.000 and 5632.6 cGy:5792.5 cGy,t =- 2.89,P =0.007).Brain stem received 45 Gydose volume ( V45 ) and 1％ brain stem volume dose ( D01 )were higher than original plan ( 3.54％:2.75％,t =3.84,P =0.001 and 5129.7 cGy:4919.3 cGy,t =4.36,P =0.000).Conclusions The set-up errors led to the dose of PTV D95 obviously insufficient and significantly increased V45,D01 of the brainstem.So,adjustment online is necessary for H&N cancer during IMRT treatment. Key words: Set-up error; Dosimetry; Head and neck neoplasms/image guide radiotherapy

Characterisation of dose distribution in linear accelerator-based intracranial stereotactic radiosurgery with the dynamic conformal arc technique: consideration of the optimal method for dose prescription and evaluation

The purpose of this study was to characterise dose distribution in linear accelerator-based intracranial stereotactic radiosurgery using the dynamic conformal arc technique, and to validate the pertinence of dose prescription to the specific percentage isodose surface (IDS). 73 plans for brain metastases were reviewed and replanned with a uniform method for target definition and treatment planning. In all cases except 1 the dose prescription to the 80% IDS satisfied the criteria of the standardised prescription IDS as previously proposed. However, both of the planning target volume (PTV) coverage values for the 80% and 90% IDSs and the PTV D99 and D95 (IDS receiving at least 99% or 95% of the PTV) were inconsistent and significantly increased as a function of the PTV size. The 80% IDS for a PTV of more than 5 cm(3) achieved adequate PTV coverage without a leaf margin. The dose conformity for 80% IDS gradually worsened as the PTV increased, whereas that for the PTV D99 or D95 improved as a function of the PTV size. The addition of a leaf margin attained 100% PTV coverage for 80% IDS, while leading to a poorer dose conformity. The dose prescription to the specific percentage IDS does not necessarily guarantee consistent target coverage, D99 and D95, and desirable dose conformity in proportion to the target volume. The dose prescription and evaluation at the specific target coverage would therefore be preferable as an objective method in order to report the "marginal dose" and to clearly compare the planning parameters with those from other modalities.

SU‐GG‐T‐537: Dosimetric Effect of Residual Tumor Motion in Phase and Amplitude‐Based Gated Lung Stereotactic Body Radiotherapy

Purpose To evaluate dosimetric effect of residual tumor motion in lung Stereotactic Body Radiotherapy(SBRT) with two gating methods using external markers: amplitude‐(AG) and phase‐based(PG). Methods and Materials The 3D tumor motion data were obtained from the treatment logs of 6 lung patients with various target size, each treated with 3 fractions of SBRT using Cyberknife Robotic Radiosurgical System and Synchrony™(Accuray, Sunnyvale CA). Amplitude and phase‐based gating treatments were simulated with a gating window of 5 mm. Planning target volume(PTV) was defined as physician‐contoured clinical target volume(CTV) surrounded by isotropic 5 mm margin. Each patient was prescribed with 60Gy/3fractions prescribed to 80% isodose line which covered at least 95% of the PTV. In order to reconstruct the dose with motion, the multileaf collimator(MLC) leave positions at each segment were resorted according to the residual tumor motion. The newly created MLC file was imported back to treatment planning system(Eclipse™; Varian Medical Systems, Palo Alto, CA) for dose recalculation. Results In either AG or PG, with a 5‐mm margin, CTV coverage is not affected as much as PTV. With residual tumor motion constrained within gating window(5 mm), both gating methods result in an insignificant decrease(&lt;1%) in dose coverage on PTVs and CTVs. With baseline shift(2–10 mm), the residual motion during PG has higher probability of excursion outside gating window. The PTV D95 decreases by 9–14% with PG and 5–9% with AG. Overall, AG appears to offer greater target dose coverage compared to PG, however at the expense of reduction of the duty cycle. Compared with PG, AG increases the fractional treatment time by up to 46%, which may be undesirable in gated lung SBRT treatments. Conclusion The ideal gating method for lung SBRT requires further investigation and optimization but may be individualized dependent on the empirical pattern of motion observed in each case.

Volumetric Modulated Arc Therapy: Planning and Evaluation for Prostate Cancer Cases

To develop an optimization method using volumetric modulated arc therapy (VMAT) and evaluate VMAT plans relative to the standard intensity-modulated radiotherapy (IMRT) approach in prostate cancer. A single gantry rotation was modeled using 177 equispaced beams. Multileaf collimator apertures and dose rates were optimized with respect to gantry angle subject to dose-volume-based objectives. Our VMAT implementation used conjugate gradient descent to optimize dose rate, and stochastic sampling to find optimal multileaf collimator leaf positions. A treatment planning study of 11 prostate cancer patients with a prescription dose of 86.4 Gy was performed to compare VMAT with a standard five-field IMRT approach. Plan evaluation statistics included the percentage of planning target volume (PTV) receiving 95% of prescribed dose (V95), dose to 95% of PTV (D95), mean PTV dose, tumor control probability, and dosimetric endpoints of normal organs, whereas monitor unit (MU) and delivery time were used to assess delivery efficiency. Patient-averaged PTV V95, D95, mean dose, and tumor control probability in VMAT plans were 96%, 82.6 Gy, 88.5 Gy, and 0.920, respectively, vs. 97%, 84.0 Gy, 88.9 Gy, and 0.929 in IMRT plans. All critical structure dose requirements were met. The VMAT plans presented better rectal wall sparing, with a reduction of 1.5% in normal tissue complication probability. An advantage of VMAT plans was that the average number of MUs (290 MU) was less than for IMRT plans (642 MU). The VMAT technique can reduce beam on time by up to 55% while maintaining dosimetric quality comparable to that of the standard IMRT approach.

Dosimetry and Acute Toxicity in IG-IMRT using a Consensus Prostate Bed CTV Following Prostatectomy

Recently published institutional guidelines for anatomic boundaries of the prostate bed clinical target volume (CTV) have raised concerns regarding the feasibility of high-dose delivery to the planning target volume (PTV) while respecting rectal and bladder dose-volume constraints. This study describes the prospective use of daily image guided intensity modulated radiation therapy (IG-IMRT), and reports the preliminary measures of acute toxicity in this group of patients. Between August 2007 and March 2008, 25 patients were enrolled on a prospective research ethics board approved clinical trial. All were CT-simulated with a full bladder and empty rectum, immobilized in a customized vacuum device. Seven field step-and-shoot IMRT plans were generated, prescribing 6600cGy in 33 fractions to the ICRU reference point. Dose planning objectives were as follows, in the order of priority: PTV D99 ≥ 5400cGy, posterior half of rectal wall (PostRW) 1cm3 ≤ 5500cGy, rectal wall 1cm3 ≤ 6600cGy, bladder wall 2 cm3 ≤ 6730cGy, penile bulb 0.5 cm3 ≤ 6600cGy, femur 1 cm3 ≤ 5500cGy and PTV V95 ≥ 95%. Dosimetry endpoints were measured using dose-volume histograms. Patient-reported acute radiation-induced toxicity was assessed weekly during treatment, and graded using the CTCAE v3.0 toxicity scale. PTV D99 ≥ 5400cGy was achievable in all cases with a mean dose of 5827cGy. PTV coverage was compromised to achieve the PostRW constraint, resulting an average V95 of 95.3% (range, 92 - 99% and SD = 2.0). Among the 25 patients, 60% had PTV V95 ≥ 95%. Dose constraints for other normal tissue were met. Grade 2 GU and GI toxicities were experienced by 8% and 32% of patients, respectively. One patient reported grade 3 GU toxicity due to voiding more frequently than once per hour, which resolved during the course of treatment. For patients who reported experiencing GI 2 toxicity, 4 of them required the use of medication for symptom management. The use of IG-IMRT technique is feasible in delivering high-dose to the PTV with minimal compromise to coverage while respecting normal tissue dose constraints. Preliminary acute toxicity data is considered acceptable. Ongoing investigation will continue to evaluate biochemical control and late toxicity.

Dose calculation validation of VMC++ for photon beams

The radiation therapy specific Voxel Monte Carlo (VMC+ +) dose calculation algorithm achieves a dramatic improvement in MC dose calculation efficiency for radiation therapy treatment planning dose evaluation compared with other MC algorithms. This work aims to validate VMC+ + for radiation therapy photon beam planning. VMC++ was validated with respect to the well-benchmarked EGS-based DOSXYZnrc by comparing depth dose and lateral profiles for field sizes ranging from 1 X 1 to 40 x 40 cm(2) for 6 and 18 MV beams in a homogeneous water phantom and in a simulated bone-lung-bone phantom. Patient treatment plan dose distributions were compared for five prostate plans and five head-and-neck (H/N) plans, all using intensity-modulated radiotherapy beams. For all tests, the same incident particles were used in both codes to isolate differences due to modeling of the radiation source. Voxel-by-voxel observed differences were analyzed to distinguish between systematic and purely statistical differences. Dose-volume-histogram-derived dose indices were compared for the patient plans. For the homogeneous water phantom and the bone-lung-bone phantom, the depth dose curve predicted by VMC+ + agreed with that predicted by DOSXYZnrc within expected statistical uncertainty in all voxels except the surface voxel of the water phantom, where VMC+ + predicted a lower dose. When the electron cutoff parameter was decreased for both codes, the surface voxel agreed within expected statistical uncertainty. For prostate plans, the most severe difference between the codes resulted in 55% of the voxels showing a systematic difference of 0.32% of maximum dose. For H/N plans, the largest difference observed resulted in 2% of the voxels showing a systematic difference of 0.98% of maximum dose. For the prostate plans, the most severe difference in the planning target volume D95 was 0.4%, the rectum D35 was 0.2%, the rectum DI7 was 0.2%, the bladder D50 was 0.3% and the bladder D25 was 0.3%. For the H/N plans, the most severe difference in the gross tumor volume D98 was 0.4%, the clinical target volume D90 was 0.2%, the nodes D90 was 0.2%, the parotids D95 was 0.8%, and the cord D2 was 0.8%. All of these differences are clinically insignificant. VMC++ showed an average efficiency gain over DOSXYZnrc of at least an order of magnitude without introducing significant systematic bias. VMC + + can be used for photon beam MC patient dose computations without a clinically significant loss in accuracy.