A study of minimum segment width parameter on VMAT plan quality, delivery accuracy, and efficiency for cervical cancer using Monaco TPS

Evaluation of the Minimum Segment Width and Fluence Smoothing Tools for Intensity-modulated Techniques in Monaco Treatment Planning System

Purpose

This study aims to minimize monitor units (MUs) of intensity-modulated treatments in the Monaco treatment planning system while preserving plan quality by optimizing the "Minimum Segment Width" (MSW) and "Fluence Smoothing" parameters.

Materials and Methods

We retrospectively analyzed 30 prostate, 30 gynecological, 15 breast cancer, 10 head and neck tumor, 11 radiosurgery, and 10 hypo-fractionated plans. Original prostate plans employed "Fluence Smoothing" = Off and were reoptimized with Low, Medium, and High settings. The remaining pathologies initially used MSW = 0.5 cm and were reoptimized with MSW = 1.0 cm. Plan quality, including total MU, delivery time, and dosimetric constraints, was statistically analyzed with a paired t-test.

Results

Prostate plans exhibited the highest MU variation when changing "Fluence Smoothing" from Off to High (average ΔMU = -5.1%; P < 0.001). However, a High setting may increase overall MU when MSW = 0.5 cm. Gynecological plans changed substantially when MSW increased from 0.5 cm to 1.0 cm (average ΔMU = -29%; P < 0.001). Organs at risk sparing and planning target volumes remained within 1.2% differences. Replanning other pathologies with MSW = 1.0 cm affected breast and head and neck tumor plans (average ΔMU = -168.38, average Δt = -11.74 s, and average ΔMU = -256.56, average Δt = -15.05 s, respectively; all with P < 0.004). Radiosurgery and hypofractioned highly modulated plans did not yield statistically significant results.

Conclusions

In breast, pelvis, head and neck, and prostate plans, starting with MSW = 1.0 cm optimally reduces MU and treatment time without compromising plan quality. MSW has a greater impact on MU than the "Fluence Smoothing" parameter. Plans with high modulation might present divergent behavior, requiring a case-specific analysis with MSW values higher than 0.5 cm.

Influence of minimum segment width on intensity-modulated radiotherapy plan for left-sided breast cancer after breast conserving surgery

To research the influence of the minimum segment width (MSW) on intensity-modulated radiotherapy (IMRT) plan for left breast cancer after breast conserving surgery and provide a reference for plan optimization. Ten patients with left-sided early-stage breast cancer after breast-conserving surgery (BCS) were selected for postoperative radiotherapy. The Monaco 5.11 treatment planning system (TPS) was used, and the optimization parameters of the plan were fixed, while MSW were set to 0.5, 1, 1.5, and 2 cm, and four 5 field-IMRT plans were designed for every patient. The plan quality and execution efficiency of each plan were compared, including planning target volume (PTV) dose, organs at risk (OAR) dose, segments number (SN), monitor units (MU), delivery time (DT), and dose verification results. With the increase of the MSW, the dose indexes of the OAR were similar (P > .05). The D2%, D98%, and D50% of the PTV gradually deviated from the prescription dose, and the uniformity gradually deteriorated (P < .05). The gamma passing rates (GPR) of dose verification and the dose deviation of the PTV were similar (P > .05). The SN, MU, and DT gradually decreased (P < .05), but the amplitude change caused by each increase of 0.5 cm gradually decreased, the decrease of 0.5 to 1 cm was the largest, reaching 16.41%, 21.25%, and 0.35 minutes, respectively, while the 3 values of 1.5 and 2 cm were close. When using Monaco 5.11 TPS to design IMRT plan for left breast cancer after BCS, 1.5 cm MSW could ensure better dose coverage and uniformity of the PTV, as little as possible exposure dose to the OAR, and high dose verification accuracy. The SN, MU, and DT were significantly reduced, and the treatment efficiency was improved.

The first investigation of spot-scanning proton arc (SPArc) delivery time and accuracy with different delivery tolerance window settings

Objective. To investigate the impact of various delivery tolerance window settings on the treatment delivery time and dosimetric accuracy of spot-scanning proton arc (SPArc) therapy.Approach. SPArc plans were generated for three representative disease sites (brain, lung, and liver cancer) with an angle sampling frequency of 2.5°. An in-house dynamic arc controller was used to simulate the arc treatment delivery with various tolerance windows (±0.25, ±0.5, ±1, and ±1.25°). The controller generates virtual logfiles during the arc delivery simulation, such as gantry speed, acceleration and deceleration, spot position, and delivery sequence, similar to machine logfiles. The virtual logfile was then imported to the treatment planning system to reconstruct the delivered dose distribution and compare it to the initial SPArc nominal plan. A three-dimensional gamma index was used to quantitatively assess delivery accuracy. Total treatment delivery time and relative lost time (dynamic arc delivery time-fix beam delivery time)/fix beam delivery time) were reported.Main Results. The 3D gamma passing rate (GPR) was greater than 99% for all cases when using 3%/3 mm and 2%/2 mm criteria and the GPR (1%/1 mm criteria) degraded as the tolerance window opens. The total delivery time for dynamic arc delivery increased with the decreasing delivery tolerance window length. The average delivery time and the relative lost time (%) were 630 ± 212 s (253% ± 68%), 322 ± 101 s (81% ± 31%), 225 ± 60 s (27% ± 16%), 196 ± 41 s (11% ± 6%), 187 ± 29 s (6% ± 1%) for tolerance windows ±0.25, ±0.5, ±1, and ±1.25° respectively.Significance. The study quantitatively analyzed the dynamic SPArc delivery time and accuracy with different delivery tolerance window settings, which offer a critical reference in the future SPArc plan optimization and delivery controller design.

Plan quality and treatment efficiency assurance of two VMAT optimization for cervical cancer radiotherapy

To investigate the difference of the fluence map optimization (FMO) and Stochastic platform optimization (SPO) algorithm in a newly-introduced treatment planning system (TPS).

METHODS

34 cervical cancer patients with definitive radiation were retrospectively analyzed. Each patient has four plans: FMO with fixed jaw plans (FMO-FJ) and no fixed jaw plans (FMO-NFJ); SPO with fixed jaw plans (SPO-FJ) and no fixed jaw plans (SPO-NFJ). Dosimetric parameters, Modulation Complexity Score (MCS), Gamma Pass Rate (GPR) and delivery time were analyzed among the four plans.

RESULTS

For target coverage, SPO-FJ plans are the best ones (P ≤ 0.00). FMO plans are better than SPO-NFJ plans (P ≤ 0.00). For OARs sparing, SPO-FJ plans are better than FMO plans for mostly OARs (P ≤ 0.04). Additionally, SPO-FJ plans are better than SPO-NFJ plans (P ≤ 0.02), except for rectum V45Gy. Compared to SPO-NFJ plans, the FMO plans delivered less dose to bladder, rectum, colon V40Gy and pelvic bone V40Gy (P ≤ 0.04). Meanwhile, the SPO-NFJ plans showed superiority in MU, delivery time, MCS and GPR in all plans. In terms of delivery time and MCS, the SPO-FJ plans are better than FMO plans. FMO-FJ plans are better than FMO-NFJ plans in delivery efficiency. MCSs are strongly correlated with PCTV length, which are negatively with PCTV length (P ≤ 0.03). The delivery time and MUs of the four plans are strongly correlated (P ≤ 0.02). Comparing plans with fixed or no fixed jaw in two algorithms, no difference was found in FMO plans in target coverage and minor difference in Kidney_L Dmean, Mu and delivery time between PCTV width≤15.5 cm group and >15.5 cm group. For SPO plans, SPO-FJ plans showed more superiority in target coverage and OARs sparing than the SPO-NFJ plans in the two groups.

CONCLUSIONS

SPO-FJ plans showed superiority in target coverage and OARs sparing, as well as higher delivery efficiency in the four plans.

An effective and optimized patient-specific QA workload reduction for VMAT plans after MLC-modelling optimization

INTRODUCTION

Many complexity metrics characterize modulated plans. First, this study aimed at identify the optimal complexity metrics to reduce workload associated to patient-specific quality assurance (PSQA) for our equipment and processes. Second, it intended to optimize our MLC modelling to improve measurement and calculation agreement with expectation of further reducing PSQA workload.

METHODS

Correlation and sensitivity at specificity equals to 1 were evaluated for PSQA results and different complexity metrics. Thresholds to stop PSQA were determined. After validation of the optimal complexity metric and threshold for our equipment and process, the MLC modelling was reviewed with a recently published methodology. This method is based on measurements with a Farmer-type ionization chamber of synchronous and asynchronous sweeping gap plans. Effect on the PSQA results and the identified threshold was investigated.

RESULTS

In our center, the most appropriate complexity metric for reducing our PSQA workload was the Modulation Complexity Score for VMAT (MCSv). The optimization of the MLC modelling significantly reduced the number of controlled plans, specifically for one of our two Varian Clinac. Any plan with a MCSv >= 0.34 is treated without PSQA.

CONCLUSION

This study rationalized and reduced our PSQA workload by approximately 30%. It is a continuing work with new TPS, machine or PSQA equipment. It encourages centers to re-evaluate their MLC modelling as well as assess the benefit of complexity metrics to streamline their PSQA workflow. An easier access, at least for reporting, at best for optimizing plans, into the TPS would be beneficial for the community.

A study of different minimum segment area parameters on automatic IMRT plans for cervical cancer using Pinnacle3 9.10 TPS

Based on Pinnacle39.10 treatment planning system (TPS) automatic planning module, we investigated the effect of minimum segmentation area (MSA) parameters on Auto-Plan Intensity Modulated Radiotherapy (AP-IMRT) without affecting the dose distribution of the target and the Organ at Risk (OAR). The results provided the basis for the ideal MSA parameters in the design of AP-IMRT plan. Ten patients with cervical cancer in our hospital were selected randomly for AP-IMRT design. Each patient was devised with 10 AP-IMRT plans. The prescription dose of PTV was 50 Gy/25 fractions. The radiotherapy plans of all patients were adopted with 7 field-averaged fixed fields. The MSA was set to 4 cm2, 9 cm2, 14 cm2, 20 cm2, 25 cm2, 40 cm2, 50 cm2, 60 cm2, 80 cm2, and 100 cm2. Plan quality and delivery efficiency were evaluated based on dose-volume histograms (DVHs), control points, monitor units (MUs), dosimetric measurement verification results, and plan delivery time. Except for the small difference in monitor units, the number of segmentations and target dose coverage, there were no statistically significant differences between the other dosimetric parameters in the planning target volumes. With the increase of MSA, the total number of MUs in AP-IMRT decreased from (649 ± 32) MUs to (312 ± 26) MUs, and the total number of segmentations decreased from (69 ± 1) to (28 ± 3). There was no statistical significance in the dose distribution of AP-IMRT target area with the MSA of 4-50 cm2 (P > .05). There was no significant difference in OAR dose between AP-IMRT plans with different MSA (P > .05). The calculated gamma indices using the 3% /3 mm and 2%/2 mm criteria. Both of the gamma pass rate and DTA pass rate all ≥95% under the condition of MSA are greater than 4 cm2, and the difference was no statistically significant (P > .05). The plan delivery times decreased with increasing MSA (P < .05). When using Pinnacle3 9.10 TPS to design AP-IMRT plan for cervical cancer, the parameter of MSA can be increased appropriately. Increasing the MSA allows for improved plan delivery accuracy and efficiency without significantly affecting the AP-IMRT plan quality. The MSA in the range of 14 to 50 cm2 can obtain a more reasonable dose distribution in the target area while the dose of target area and OAR had no significant changes. It is important to improve the plan quality, delivery accuracy, and efficiency for cervical AP-IMRT radiation therapy.

Influence of beamlet width on dynamic IMRT plan quality in nasopharyngeal carcinoma

Objective

This study aimed to identify the effects of beamlet width on dynamic intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma (NPC) and determine the optimal parameters for the most effective radiotherapy plan.

Methods

This study evaluated 20 patients with NPC were selected for dynamic IMRT. Only the beamlet width in the optimization parameters was changed (set to 2, 4, 6, 8, and 10 mm that were named BL02, BL04, BL06, BL08, and BL10, respectively) to optimize the results of the five groups of plans. Using the plan quality scoring system, the dose results of the planning target volumes (PTVs) and organs at risks (OARs) were analyzed objectively and comprehensively. The lower the quality score, the better the quality of the plan. The efficiency and accuracy of plan execution were evaluated using monitor units (MUs) and plan delivery time (PDT).

Results

The BL04 mm group had the lowest quality score for the targets and OARs (0.087), while the BL10 mm group had the highest total score (1.249). The BL04 mm group had the highest MUs (837 MUs) and longest PDT (358 s). However, the MUs range of each group plan was below 100 MUs, and the PDT range was within 30 s. In the BL02, BL04, BL06, BL08, and BL10 plans, <5 MUs segments accounted for 33%, 16%, 24%, 33%, and 40% of total segments, respectively, with which the lowest was in the BL04 mm group.

Conclusion

Smaller beamlet widths have not only reduced OARs dose while maintaining high dose coverage to the PTVs, but also lead to more MUs that would produce greater PDT. Considering the quality and efficiency of dynamic IMRT, the beamlet width value of the Monaco treatment planning system set to 4 mm would be optimal for NPC.

Optimization of Minimum Segment Width Parameter in the Intensity-Modulated Radiotherapy Plan for Esophageal Cancer

Purpose

This study was designed to explore the optimal minimum segment width (MSW) in the intensity-modulated radiotherapy (IMRT) plan for esophageal cancer.

Patients and Methods

The imaging data of 20 esophageal cancer patients were selected for this study. Four IMRT plans were designed for each patient with MSWs of 0.5, 1.0, 1.5, and 2.0 cm. The conformity index (CI) and homogeneity index (HI) of the planning target volumes (PTV), organs at risk (OARs), control points (CP), monitor units (MU), plan delivery time (DT), and gamma passing rates (GPR) were collected and compared to appraise the treatment plan quality and delivery efficiency.

Results

Lower-MSW plans had larger CI and smaller HI values, and lower dose parameters of OARs and PTVs. The HI, CI, and dose parameter of OARs in the 0.5 and 1.0 cm MSW groups were similar and much better than those of the 1.5 and 2.0 cm MSW groups. Meanwhile, the plan in the 0.5 cm MSW group had significantly higher MUs, CPs, and DTs, and a significantly lower relative dose of GPR with a 3% dose difference and 3 mm distance to agreement criteria than the other three groups.

Conclusion

The 0.5 and 1 cm MSW groups had better dosimetric parameters and IMRT plan quality than the other groups. However, plans with 0.5 cm MSW had worse delivery accuracy and efficiency than the other three groups. Thus, MSW of 1.0 cm was the optimal choice to ensure good quality, delivery accuracy, and treatment efficiency in IMRT plans for esophageal cancer.

Correlation Between Average Segment Width and Gamma Passing Rate as a Function of MLC Position Error in Volumetric Modulated Arc Therapy

Objective:

This study analyzed the correlation between the average segment width (ASW) and gamma passing rate according to the multi-leaf collimator (MLC) position error.

Method:

To evaluate the changes in the gamma passing rate according to the MLC position error, 21 volumetric modulated arc therapy (VMAT) plans were generated using pelvic lymph node metastatic prostate cancer patient's data which is sensitive to MLC position errors as they involve several long, narrow, irregular fields. The ASW for each VMAT plan was calculated using our own code developed using Visual Basic for Applications (VBA). The gamma passing rate of the VMAT plan according to the MLC position error was evaluated using ArcCHECK (Sun Nuclear, Melbourne, FL, USA) while inducing symmetric MLC position errors in 0.25 mm intervals from −1 mm to +1 mm in the infinity medical linear accelerator (Elekta AB, Stockholm, Sweden). Finally, we examined the correlation between the change in the passing rate (γgradient) due to the MLC position error and the ASW in VMAT through linear regression analysis using the least squares method.

Results:

The ASW and γgradient were found to have a linear correlation according to the MLC position error, and the coefficient of determination was 0.88. For a 1 mm position error of MLC in VMAT, the gamma passing rate improved by approximately 11.9% as the ASW increased by 10 mm.

Conclusion:

These results are expected to be employed as guidelines to minimize the dose uncertainty due to MLC position error in VMAT.

Comparison of Moderate Hypofractionated Volumetric-Modulated Arc Therapy Plans With and Without Flattening Filter for Localized Prostate Cancer

Background/Aim The aim of this study was to compare volumetric-modulated arc therapy (VMAT) radiation plans between conventional VMAT with flattening filter (cFF-VMAT) and flattening filter-free VMAT (FFF-VMAT) for localized prostate cancer. Materials and methods Ten patients with localized prostate cancer who underwent cFF-VMAT at Yokosuka General Hospital Uwamachi, Yokosuka, Japan, from July 2020 to October 2020 were enrolled. Dose-volume histogram (DVH) parameters of the target volume, normal organs, monitor units (MU), and beam-on time (BOT) were compared between cFF-VMAT and FFF-VMAT plans. Results No significant difference was observed for DVH parameters for the target volume. No significant difference was observed in all parameters for the bladder and rectum between the cFF-VMAT and FFF-VMAT groups. The mean values of MU were 686 ± 52 and 784 ± 80 in cFF-VMAT and FFF-VMAT, respectively (p < 0.001). The mean BOT was 97.0 ± 6.6 s and 72.9 ± 1.4 s for cFF-VMAT and FFF-VMAT, respectively (p < 0.001). Conclusion DVH parameters of the target volume and normal organs were not significantly different between the cFF-VMAT and FFF-VMAT plans. In FFF-VMAT, MU was significantly higher, and the BOT was significantly shorter than those in cFF-VMAT.

Investigation of optimum minimum segment width on VMAT plan quality and deliverability: A comprehensive dosimetric and clinical evaluation using DVH analysis

Purpose

Minimum segment width (MSW) plays a fundamental role in the shaping of optimized apertures and creation of segments of varying sizes and shapes in complex radiotherapy treatment plans. The purpose of this work was to study the effect of MSW on dose distribution in patients planned with VMAT for various treatment sites using dose volume histogram (DVH) analysis.

Materials and methods

For the validation of optimum MSW, 125 clinical treatment plans were evaluated. Five groups were identified (brain, head and neck, thorax, pelvis, and extremity), and five cases were chosen from each group. For each case, five plans were created with different MSW (0.5, 0.8, 1.0, 1.25, and 1.5 cm). The quality of treatment plans created using different MSW were compared using dosimetric indicators such as target coverage (D₉₈—dose to 98% of the planning target volume (PTV), maximum dose (D₂—maximum dose to 2% of the PTV), monitor units (MU), and DVH parameters related to organs at risk (OAR). The effect of the MSW on delivery accuracy was quantitatively analyzed using the measured fluence utilizing ionization chamber‐based transmission detector and model‐based dose verification system. Traditional global gamma analysis (2%, 2 mm) and dose volume information was gathered for the PTV and organs at risk and compared for different MSWs.

Results

A total of 125 plans were created and compared across five groups. In terms of treatment plan quality, the plans using MSW of 0.5 cm was found to be superior in all groups. PTV coverage (D₉₈) decreased significantly (p < 0.05) as the MSW increased. Similarly, the maximum dose (D₂) was found to be increased significantly (p < 0.05) as the MSW increased from 0.5 cm, with MSW of 1.5 cm being the least in terms of plan quality for both PTVs and OARs. In terms of plan deliverability using DVH analysis, treatment planning system (TPS) compared to measured fluence, VMAT plans produced with MSW of 0.5 cm showed a better dosimetric index and a smaller deviation for both PTVs and OARs. The deliverability of the plans deteriorated as the MSW increased.

Conclusion

Dose volume histogram (DVH) analysis demonstrated that treatment plans with minimal MSW showed better plan quality and deliverability and provided clinical relevance as compared to gamma index analysis.

Risk for second bladder and rectal malignancies from cervical cancer irradiation

The objective of this study was to estimate the risk of developing second malignancies to partially in‐field organs from volumetric modulated arc therapy (VMAT) of cervical cancer and to compare the above risks with those from the conventional three‐dimensional conformal radiotherapy (3D‐CRT). Seventeen consecutive patients with uterine cervix carcinoma were selected. VMAT and 3D‐CRT plans were generated with 6 and 10 MV photons, respectively. The prescribed tumor dose was 45 Gy given in 25 fractions. Differential dose‐volume histogram data from the treatment plans were obtained for the partially in‐field organs such as bladder and rectum. These data were used to estimate the patient‐specific lifetime attributable risk (LAR) for bladder and rectal cancer induction with a non‐linear model based on a mixture of plateau and bell‐shaped dose–response relationships. The estimated risks per 10000 people were compared with the baseline risks for unexposed population. The patient‐specific rectal cancer risk estimates from VMAT were significantly lower than those from 3D‐CRT (P = 0.0144). The LARs for developing bladder malignancies from VMAT were significantly high compared to those from conventional irradiation (P = 0.0003). The mean difference between the patient‐specific LARs for radiation‐induced bladder and rectal malignancies as derived from 3D‐CRT and VMAT plans was 6.6% and 2.0%, respectively. The average LAR for developing bladder and rectal malignant diseases due to VMAT was 9.2 × 10^‐4 and 43.7 × 10^‐4, respectively. The corresponding risks following 3D‐CRT were 8.6 × 10^‐4 and 44.6 × 10^‐4. These average risks showed that pelvic irradiation increases the baseline probability for cancer induction by 12.6‐19.1%. The differences in the second cancer risks associated with the VMAT and 3D‐CRT for cervical cancer were found to be small. Both treatment techniques resulted in considerable increased probabilities for developing bladder and rectal malignancies relative to those of unirradiated population.

Dosimetric characteristics of VMAT plans with respect to a different increment of gantry angle size for Ca cervix

Aim:

We have investigated the influence in volumetric-modulated arc therapy (VMAT) plans by a sequence of increment of gantry angle (IGA) in definitive radiotherapy treatment for cervical cancer. The plans are quantitatively analysed in terms of conformity index (CI), heterogeneity index (HI), dose–gradient index (DGI), target coverage (TC) by prescription dose, monitor unit (MU) usage, control points (CPs) and dose to organs.

Materials and Methods:

In this retrospective study, we selected 27 patients with cervical cancer having aged between 54 and 69. All the patients enrolled in this study were at T3N1M0 stage of cervical cancer. The prescription dose to planning target volume (PTV) was 50 Gy and was administered in 2 Gy/fraction through VMAT technique. VMAT plans were optimised by varying the parameter ‘IGA’ as 10, 20, 30 and 40°.

Results:

Homogenous dose distribution within PTV and TC by prescription dose was significantly enhanced (p < 0·05) with larger IGA. The difference between volume receiving 15 Gy (V15Gy) in bowel was up to 10% with larger IGA (30 and 40°) and V25Gy in femoral head was up to 3% with smaller IGA (10 and 20°). CPs were enhanced and MU usage was reduced with larger IGA (30 and 40°). IGA 40° had reduced the MU usage than IGA 30° but the CI and DGI were compromised due to large MLC field segments.

Conclusion:

This study recommends that the larger IGA could yield better results when the number of sectors is even, for a cervical cancer patient. However, more data from more patients need to be obtained and analysed to make this an evidence-based hypothesis.

Study on the ability of 3D gamma analysis and bio-mathematical model in detecting dose changes caused by dose-calculation-grid-size (DCGS)

Objective

To explore the efficacy and sensitivity of 3D gamma analysis and bio-mathematical model for cervical cancer in detecting dose changes caused by dose-calculation-grid-size (DCGS).

Methods

17 patients’ plans for cervical cancer were enrolled (Pinnacle TPS, VMAT), and the DCGS was changed from 2.0 mm to 5.0 mm to calculate the planned dose respectively. The dose distribution calculated by DCGS = 2.0 mm as the “reference” data set (RDS), the dose distribution calculated by the rest DCGS as the“measurement”data set (MDS), the 3D gamma passing rates and the (N) TCPs of the all structures under different DCGS were obtained, and then analyze the ability of 3D gamma analysis and (N) TCP model in detecting dose changes and what factors affect this ability.

Results

The effect of DCGS on planned dose was obvious. When the gamma standard was 1.0 mm, 1.0 and 10.0%, the difference of the results of the DCGS on dose-effect could be detected by 3D gamma analysis (all p value < 0.05). With the decline of the standard, 3D gamma analysis’ ability to detect this difference shows weaker. When the standard was 1.0 mm, 3.0 and 10.0%, the p value of > 0.05 accounted for the majority. With DCGS = 2.0 mm being RDS, ∆gamma-passing-rate presented the same trend with ∆(N) TCPs of all structures except for the femurs only when the 1.0 mm, 1.0 and 10.0% standards were adopted for the 3D gamma analysis.

Conclusions

The 3D gamma analysis and bio-mathematical model can be used to analyze the effect of DCGS on the planned dose. For comparison, the former’s detection ability has a lot to do with the designed standard, and the latter’s capability is related to the parameters and calculated accuracy instrinsically.

Dosimetric comparison of two treatment planning systems for spine SBRT

Two commonly used treatment planning systems (TPS) are compared for the planning of spine stereotactic body radiotherapy (SBRT). The main purpose is to highlight relative advantages and disadvantages of each system and propose a methodologic approach for comparisons. Twenty clinical plans were inversely planned with step-and-shoot intensity-modulated radiotherapy (IMRT) each using 9 to 11 beams, referred to as IMRT_P. The prescription dose was 24 Gy in 2 fractions, and the plans were generated following our institutional protocol using the Pinnacle³ v9.2. Each case was replanned using a 2-arc volumetric modulated arc therapy (VMAT) approach, referred as VMAT_P. CT and structure sets were DICOM exported to Monaco v5.10 and planned in 2 different ways: IMRT (IMRT_M) and VMAT (VMAT_M) using the same prescription dose. Dose volume histograms (DVH) and other dose statistics of planning target volumes (PTV) and organ-at-risk (OAR) were analyzed and compared between plans. The gradient index (GI = ratio of 50% isodose volume to prescribed isodose volume) was used to measure dose fall-off outside of the PTV. Another metric - Gradient Index Inner (GI_inner = the rate (in Gy/mm) - at which the dose changes from the level of the spinal cord/thecal sac toward the prescription dose) was developed and compared. All plans were considered clinically acceptable by institutional guidelines and achieved all of the OAR dose constraints. VMAT_M and IMRT_M showed comparable dose statistics for the PTV when compared to VMAT_P and IMRT_P, respectively. For IMRT plans, the median GI_inner was 1.88 Gy/mm vs 1.52 Gy/mm for IMRT_M and IMRT_P respectively (p< 0.001). All other IMRT metrics were statistically similar except for the PTV maximum dose (D_max), which was higher for IMRT_M than IMRT_P (median 30.7 Gy vs 29.0 Gy, p< 0.001). For VMAT plans, only PTV D_min showed a statistical different between VMAT_M and VMAT_P of median 12.7 Gy vs 9.7 Gy (p< 0.001). In terms of beam sequencing parameters, the number of monitor units was statistically higher for VMAT_P compared to VMAT_M (median = 6764 vs 5376) whereas the number of segments for IMRT_M was statistically greater than IMRT_P (median = 155 vs 73). We were able to generate clinically acceptable plans for different types of spine SBRT using 2 different TPS. We used an evaluation strategy involving coverage, conformity, and dose gradient that can compared between TPS.