Planning Target Volume Homogeneity Index Research Articles

Dosimetric evaluation of ultrafractionated dose escalation with simultaneous integrated boost to intraprostatic lesion using 1.5-Tesla MR-Linac in localized prostate cancer.

We analyzed a dose escalation of 36.25 Gy to the entire prostate and a dose increment up to 40 Gy with 1.25 Gy increments to intraprostatic lesion (IPL) using simultaneous integrated boost (SIB) in five fractions. Eighteen low- and intermediate-risk prostate cancer patients treated with 1.5T MR-Linac were retrospectively evaluated. The same planning computed tomography (CT) images generated four plans: no SIB, 37.5 Gy SIB, 38.75 Gy SIB, and 40 Gy SIB. In four plans, planning target volume (PTV) doses, organ at risk (OAR) doses, and PTV-SIB homogeneity index (HI), gradient index (GI) and conformity index (CI) were compared. All plans met the criteria for PTV and PTV-SIB coverage. PTV 40 Gy plan has higher maximum PTV and PTV-SIB doses than other plans. The PTV HI was significantly higher in the SIB 40 Gy plan (0.135 ± 0.007) compared to SIB 38.75 Gy plan (0.099 ± 0.007; p = 0.001), SIB 37.5 Gy (0.067 ± 0.008; p < 0.001), and no SIB plan (0.049 ± 0.010; p < 0.001), while there were no significant differences in HI, GI and CI for PTV-SIB between three plans. Four rectum and bladder plans had similar dosimetric parameters. The urethra D5 was significantly higher in SIB 40 Gy plan compared to no SIB plan (37.7 ± 1.1 Gy vs. 37.0 ± 0.7 Gy; p = 0.009) and SIB 37.5 Gy plan (36.9 ± 0.8 Gy; p = 0.008). There was no significant difference in monitor units between the four consecutive plans. Ultra-hypofractionated dose escalation to IPL up to 40 Gy in 5 fractions with a 1.5-T MR-linac is dosimetrically feasible, potentially paving the way for clinical trials.

Plan Comparison of Left Breast Postmastectomy Radiotherapy: Halcyon IMRT and VMAT Plan vs. Truebeam Fixed-Jaws Plan

The purpose of this work is to provide a Varian Halcyon planning solution for left breast postmastectomy radiotherapy by comparing the plan quality of Varian Truebeam plan with Halcyon plan. Ten patients who previously received left breast postmastectomy intensity-modulated radiotherapy (IMRT) on Truebeam were retrospectively selected and replanned using Halcyon. The planning target volume (PTV) included chest wall and supraclavicular region, and the prescribed dose was 50 Gy in 25 fractions (2 Gy/F). In addition to the original Truebeam fixed-jaws IMRT (IMRT_T), a Halcyon IMRT (IMRT_H) plan and a Halcyon volumetric-modulated arc therapy (VMAT_H) plan were designed for each patient. The conformity index (CI) and homogeneity index (HI) of the PTVs, and the dose of organs at risk were calculated and compared. Statistical significance was performed using the LSD-t test or Wilcoxon signed-rank test with a significance level of 0.05. The V20 and V30 of ipsilateral lung, the V20, V30 and Dmean of heart, and CI of target area in IMRT_H plan were significantly lower than those in IMRT_T plan (P<0.05). No significant difference was found in the V5, V10 and Dmean of ipsilateral lung, the V5 and Dmean of contralateral lung, the V5 and Dmean of contralateral breast, HI of PTV (P>0.05), but the Dmean of left humeral head in IMRT_H plan was significantly higher than that of IMRT_T plan (P<0.05). All the metrics mentioned above showed higher level in VMAT_H plan than the IMRT_T plan and IMRT_H plan (P<0.05), except for HI (P>0.05) and Dmean of left humeral head (P>0.05 when compared to IMRT_H plan). The IMRT_H plan acts better in OAR sparing and PTV conformity than the IMRT_T plan, while the VMAT-H plan can only parallel the other two techniques in HI. The IMRT_H plan is promising in treating left breast postmastectomy patients with dosimetric improvements.

Evaluation of a hybrid automatic planning solution for rectal cancer

BackgroundScript-based planning and knowledge-based planning are two kinds of automatic planning solutions. Hybrid automatic planning may integrate the advantages of both solutions and provide a more robust automatic planning solution in the clinic. In this study, we evaluated and compared a commercially available hybrid planning solution with manual planning and script-based planning.MethodsIn total, 51 rectal cancer patients in our institution were enrolled in this study. Each patient generated 7 plans: one clinically accepted manual plan ({plan}_{manual}), three script-based plans and three hybrid plans generated with the volumetric-modulated arc therapy technique and 3 different clinical goal settings: easy, moderate and hard ({plan}_{hybrid}^{easy}, {plan}_{hybrid}^{moderate}, {plan}_{hybrid}^{hard}, {plan}_{script}^{easy}, {plan}_{script}^{moderate} and {plan}_{script}^{hard}). Planning goals included planning target volume (PTV) Dmax, bladder Dmean and femur head Dmean. The PTV prescription was the same (50.00 Gy) for the 3 goal settings. The hard setting required a lower PTV Dmax and stricter organ at risk (OAR) dose, while the easy setting was the opposite. Plans were compared using dose metrics and plan quality metric (PQM) scores, including bladder D15 and D50, left and right femur head D25 and D40, PTV D2, D98, CI (conformity index) and HI (homogeneity index).ResultsCompared to manual planning, hybrid planning with all settings significantly reduced the OAR dose (p < 0.05, paired t-test or Wilcoxon signed rank test) for all dose-volume indices, except D25 of the left femur head. For script-based planning, {plan}_{script}^{easy} significantly increased the OAR dose for the femur head and D2 and the PTV homogeneity index (p < 0.05, paired t-test or Wilcoxon signed rank test). Meanwhile, the maximum dose of the PTV was largely increased with hard script-based planning (D2 = 56.06 ± 7.57 Gy). For all three settings, the comparison of PQM between hybrid planning and script-based planning showed significant differences, except for D25 of the left femur head and PTV D2. The total PQM showed that hybrid planning could provide a better and more robust plan quality than script-based planning.ConclusionsThe hybrid planning solution was manual-planning comparable for rectal cancer. Hybrid planning can provide a better and more robust plan quality than script-based planning.

Dosimetric comparative study of 3DCRT, IMRT, VMAT, Ecomp, and Hybrid techniques for breast radiation therapy

PurposeTo assess and compare the dosimetric parameters obtained between three-dimensional conformal radiotherapy (3DCRT), three-dimensional field-in-field (3DFIF), 5-field intensity-modulated radiotherapy (IMRT MF5), tangential IMRT (tIMRT), tangential volumetric modulated arc therapy (tVMAT), electronic tissue compensation (Ecomp), and Hybrid treatment plans.Material and MethodsThirty planning computed tomography datasets obtained from patients previously treated with whole breast radiation therapy (WBRT) were utilized in this study. Treatment plans were created for 3DCRT, 3DFIF, IMRT MF5, tIMRT, tVMAT, Ecomp, and Hybrid techniques using Eclipse Treatment Planning System (version 13.6) with a prescribed dose of 42.5 Gy in 16 fractions.ResultsTechniques with tangential beams produced statistically significantly better organs-at-risk (OARs) dosimetry (p < 0.001). Planning target volume Homogeneity Index (HI) was found to be significantly different among all techniques (p < 0.001), with Ecomp resulting in better HI (1.061 ± 0.029). Ecomp was also observed to require relatively shorter planning time (p < 0.001).ConclusionsTechniques using tangential fields arrangements produced improved OARs dosimetry. Of all the treatment planning techniques employed in this study, Ecomp was found to be relatively easy to plan and produce acceptable dosimetry for WBRT in a short time.

A retrospective evaluation of mixed energy volumetric modulated arc therapy for anal cancers with lymph node involvement

Applying dual, or mixed photon energies during radiation therapy is a common practice in 3-dimensional conformal radiation therapy (3D-CRT). Mixed photon energies are used to provide uniform dose coverage to a planning target volume (PTV) that ranges in depth from the skin surface. Though the application of mixed photon energies in 3D-CRT was once the convention for treating anal cancers with lymph node involvement (AC-LNI), the advantages offered by volumetric modulated arc therapy (VMAT) prove to be the optimal form of therapy for AC-LNI. Recently, multiple researchers have uncovered benefits in employing multiple photon energies in VMAT planning for prostate cancer. A retrospective study was completed to assess the impact of implementing mixed energy VMAT planning in comparison to conventional single energy VMAT planning for AC-LNI. Data from 20 patients with AC-LNI was collected to analyze the dosimetric effects of mixed energy VMAT treatments in terms of PTV conformity index, PTV homogeneity index, monitor unit usage, and organs at risk sparing. For each patient 3 treatment plans were created: a single energy 6 MV plan, a single energy 10 MV plan, and a mixed 6 MV and 10 MV energy plan. Analysis of the resulting dosimetric outcomes showed statistical significance. The current study concluded that mixed energy VMAT plans have some effect on treating AC-LNI when compared to single energy VMAT plans.

Dose-volume and radiobiological dependence on the calculation grid size in prostate VMAT planning

This study evaluated the effects of dose-volume and radiobiological dependence on the calculation grid size in prostate volumetric-modulated arc therapy (VMAT) planning. Ten patients with prostate cancer were selected for this retrospective treatment planning study. Prostate VMAT plans were created for the patients using the 6 MV photon beam produced by a Varian TrueBEAM linac with the calculation grid size equal to 1, 2, 2.5, 3, 4, and 5 mm. Dose-volume histograms (DVHs) of targets and organs at risk were generated for different grid sizes. We calculated the radiobiological parameters of the tumor control probability (TCP) of clinical target volume (CTV) and planning target volume (PTV), and the normal tissue complication probability (NTCP) of organs at risk (rectal wall, rectum, bladder wall, bladder, left femur, and right femur). The homogeneity, conformity, and gradient indexes of CTV and PTV were calculated for different grid sizes. The TCP of PTV was found decreasing with a rate of 0.06%/mm when the calculation grid size increased from 1 to 5 mm. On the other hand, both NTCPs of rectal wall and rectum were found decreasing with rates of 0.03%/mm and 0.05%/mm, respectively, with an increase of grid size. The homogeneity index of PTV increased with a rate of 0.57/mm of the calculation grid size, whereas the conformity index of PTV decreased with a rate of 0.0075/mm. The gradient index of PTV was found increasing with a rate equal to 0.05/mm. In prostate VMAT planning, variations of dose-volume and radiobiological parameters with calculation grid size on PTV, rectal wall, and rectum were more significant than those of CTV and other organs at risk such as bladder wall, bladder, and femurs. Results in this study are important in the treatment planning quality assurance when the calculation grid size is varied to compromise a shorter dose computing time.

A modified formula for dose calculations of stereotactic ablative body radiotherapy for non–small cell lung cancer

To provide a modified formula consistent with the Monte Carlo (MC) algorithm for dose calculations during stereotactic ablative body radiotherapy for non–small cell lung cancer. Seventy CyberKnife treatment plans were calculated and analyzed by MC and ray-tracing (RT) algorithms, separately. Parameters of treatment plans were compared, and those associated with differences of dose distributions were analyzed to establish a modified formula. Gross tumor volume and tumor tracking volume (TTV) were defined as the evident disease on the sequences of the window width and level of the lung and the mediastinum. Additionally, the formula was validated by another 20 plans. The prescription dose of the 90 patients was 60 Gy/5f. The RT algorithm overestimated the planning target volume (PTV) D95 by an average of 8.59 Gy and the gross tumor volume D99 by an average of 5.84 Gy. The homogeneity index of PTV was underestimated by 0.11 on average, whereas the conformity index and new conformity index was underestimated by 0.05. The RT algorithm overestimated the dose distribution to the spinal cord by 2.23 Gy, the esophagus by 1.96 Gy, the trachea by 1.89 Gy, the left-sided bronchus by 1.77 Gy, the right-sided bronchus by 1.64 Gy, and the heart by 2.16 Gy. The average whole-lung dose volumes of lung tissues and dose volumes of V5 were overestimated by 2.69 Gy and 7.52%, respectively. A power function distribution (R2 = 0.8626) was confirmed between PTV D95 and TTV volumes. PTV D95 calculated by the MC algorithm could be computed easily with TTV and PTV D95 calculated by the RT algorithm based on the formula. The modified equation was more consistent with MC algorithm than with other formula, which could be a reference to those not accessible to the MC algorithm.

Abstract ID: 84 Development of the 2-dimensional MLC movement technique to improve radiation treatment quality

In radiation therapy, leaf width of the multileaf collimator (MLC) determines the resolution of the beam fluence in the vertical direction of the leaf movement. The resolution can be improved by using the MLC with thinner leaves but there is limitation on reducing the width due to spatial constraint of the gantry and increasing leakage dose [1] . The aim of this study was to develop the two-dimensional (2D) MLC movement technique using Monte Carlo (MC) simulation in order to improve the fluence resolution and to minimize unnecessary dose without changing the MLC leaf width. The Varian Trilogy 6MV linear accelerator equipped with the virtual millennium 120 MLC that moves toward 2D direction was modeled by using Geant4 MC tool-kits. The MC-based automatic decision algorithm of the MLC opening was developed to decide the optimal MLC position by comparing the opening at each MLC position. For the patient study to evaluate the developed technique, the patient who received brain 3D conformal radiotherapy was selected. The planning target volume (PTV) was decided with 3-mm-margin and the beam was delivered from 177 directions with the gantry angle from 0 to 360 degree in 2 degree steps. Total 5 × 108 photons were generated in the simulation. By using the 2D optimized MLC, the PTV homogeneity index was improved from 0.123 to 0.125. Mean dose to the PTV was not really changed, from 24.11 Gy to 24.08 Gy when the 2D optimized MLC was used. Meanwhile, the percent volume of the tissue nearby the PTV that was irradiated more than 20.56 Gy (80% of the maximum dose to the tissue with the 1D conventional MLC) was reduced from 5.66% to 0.81% by using the 2D optimized MLC. Mean dose to the normal tissue was also decreased from 11.59 Gy to 9.75 Gy. The results show that the 2D MLC movement technique could improve the radiation treatment quality by reducing normal tissue dose while maintaining the PTV dose.

Preclinical studies for implementing flattening filter-free beams in hypofractionated volumetric-modulated arc therapy (VMAT) for prostate cancer radiotherapy

AbstractBackgroundVolumetric-modulated arc therapy (VMAT) has emerged as one of the most favourable techniques for radiotherapy treatment in recent years because of its conformal dose distribution to the planning target volume (PTV), lower doses to adjacent normal organs at risk (OARs) and faster and easier dose delivery. A typical conventional VMAT protocol for low-intermediate risk prostate cancer uses a flattened 6 MV photon beam to deliver 78 Gy in 39 fractions, however, a recent Radiation Therapy Oncology Group study investigated prostate cancer radiotherapy with a hypofractionated dose scheme of 36·25 Gy in 5 fractions. One advantage of flattening filter-free (FFF) beams in radiotherapy is the higher doses in the central region on the dose profile and much higher dose delivery rates.Methods and materialsThis paper reports the investigation of preclinical studies for implementing FFF beams in hypofractionated VMAT for prostate cancer radiotherapy. All treatment planning were accomplished using Varian EclipseTM treatment planning system version 11 and delivered on Varian Truebeam linear accelerators. The studies compared the biological-effective dose–volume histograms and dose–volume histograms of PTV and OARs for 20 patients using conventional and hypofractionated dose schemes. The study also evaluated the 6 and 10 MV FFF by comparing 6 and 10 MV VMAT plans with the FFF beams. The treatment time was investigated using plans with 6 MV beams and doses of 2, 4, 5, 6, 7·25 Gy/fraction and plans with 10 MV FFF with a dose of 7·25 Gy/fraction. We also investigated an angular monitor unit (MU) quantity (MU/deg) and its threshold value for RapidArcTM plans, beyond which FFF beams can be considered superior to flattened beams in terms of treatment time increased caused by higher dose per fraction.ResultsThe results show that the hypofractionated plans resulted in greater biological equivalent doses to PTV and lower doses to OARs. The 10 MV FFF plans have statistically lower mean doses to all the OARs, whereas PTV homogeneity index remains the same compared with other beam energies. The mean body integral dose for the 20 patients is 8·7% lower using 10 MV FFF compared with 6 MV FFF mainly because of the higher energy and less required MUs with the 10 MV FFF beam. The hypofractionated scheme with 10 MV FFF plan has the same treatment time as that of the 6 MV plan at 2 Gy/fraction, as the higher dose delivery rates at 10 MV FFF can compensate for the higher prescribed dose per fraction without the need of extra treatment time.ConclusionIn this study, we observed that the 10 MV FFF beam is better for hypofractionated prostate cancer VMAT plan delivery. The threshold value of MU/deg is found to be 2·083 MU/deg based on our machine configurations.

Immune priming by stereotactic body radiation: Dosimetric characteristics.

85 Background: A systemic immune response to tumors after stereotactic body radiation therapy (SBRT) has been observed. The role of radiation dosimetry variables in priming an immune response is not well characterized. Methods: We analyzed radiation dosimetry variables in a patient with metastatic melanoma receiving pembrolizumab for about 1 year prior to concurrent use with Cyberknife SBRT for a symptomatic lesion in the right lower lobe of the lung. The patient received 50 Gy in 5 fractions prescribed to the 74% isodose line (using a Monte Carlo dose calculation) and developed a systemic immune response three months after this intervention. Results: We observed priming of the immune system as man ifested by radiological and clinical responses in non-irradiated cervical lymph nodes and a new persistent FDG-avid lesion on PET scan located in the base of the tongue. Given the concern for a new malignancy, the patient underwent hemi tongue base glossectomy; interestingly, this surgical specimen showed tonsillar mucosa with reactive lymphoid hyperplasia without evidence of tumor. Upon review of the CyberKnife SBRT plan, we identified that the planning target volume (PTV) was atypically under covered to 89.9% and also had a relatively high value of gross tumor volume (GTV) conformity index (CI) of 2.1. In our experience with CyberKnife plans for lung lesions, usually the PTV coverage is more than 95% and the GTV CI is less than 1.5. Other dosimetry variables had more typical values, including GTV coverage of 99.9%, PTV CI of 1.1, and both GTV and PTV homogeneity index of 1.4. Conclusions: Relatively low PTV coverage and high GTV CI were associated with immune system priming in a well-documented case of systemic responses in non-irradiated pathological and normal tissues, in the setting of concurrent immunotherapy and SBRT. Further studies are warranted to confirm this association.

Functional image guided radiation therapy planning in volumetric modulated arc therapy for patients with malignant pleural mesothelioma

PurposeTo investigate the incorporation of functional lung image-derived low-attenuation area (LAA) based on 4-dimensional computed tomography (4D-CT) in volumetric modulated arc therapy (VMAT) planning for patients with malignant pleural mesothelioma (MPM) after extrapleural pneumonectomy. Methods and materialsTwelve patients with MPM after extrapleural pneumonectomy were included. The primarily affected side was the right in 6 patients and the left in 6 patients. LAA was generated from 4D-CT data according to CT values with a threshold of less than –860 Hounsfield units (HU). Functional lung image was defined as the area where LAA was excluded from contralateral lung image. Two radiation therapy plans were designed: (1) Plan C, conventional VMAT and (2) Plan F, functional VMAT plan based on the functional lung. Both plans were compared in each patient with respect to the following dosimetric parameters: fV20, V20, fV10, V10, fV5, and V5, the percentages of functional or contralateral lung volumes irradiated with >20 Gy, 10 Gy, or 5 Gy, respectively; functional mean lung dose (fMLD) and mean lung dose (MLD), the mean dose to the functional or contralateral lung, respectively; maximum dose to the cord; mean doses to the liver and heart; and planning target volume homogeneity index. ResultsfV5 and MLD were significantly lower in Plan F (fV5, median 57.5% in Plan C vs 38.5% in Plan F, P < .01; MLD, median 7.0 Gy in Plan C vs 6.4 Gy in Plan F, P = .04). fV10, V5, and fMLD were also significantly lower in Plan F. Compared with Plan C, planning target volume homogeneity index and liver, heart, and cord doses were not significantly elevated in Plan F. ConclusionsSignificant reductions in fV5, fV10, fMLD, V5, and MLD were achieved with the functional image guided VMAT plan without negative effects on other factors. LAA-based functional image guided radiation therapy planning in VMAT is a feasible method to spare the functional lung in patients with MPM.

High Tangent Radiation Therapy With Field-in-Field Technique for Breast Cancer.

Purpose:We evaluated whether the field-in-field (FIF) technique improves the homogeneity of the target in high tangent radiation therapy (HTRT).Materials and Methods:This study included 30 patients. In total, 3 HTRT plans were created: 1 with conventional opposed fields (Conv-p), 1 with the FIF technique (FIF-p), and 1 with FIF technique using lung-blocked subfields (FIF-LB-p).Results:The maximum dose of the breast and planning target volume (PTV) was significantly lower for FIF-p and FIF-LB-p than Conv-p. Homogeneity index of PTV was also significantly lower for FIF-p and FIF-LB-p than Conv-p. Homogeneity index of the breast or PTV was significantly better for FIF-p than FIF-LB-p. The volumes of the breast or the PTV receiving 95% and 90% of the prescribed dose were also significantly better for FIF-p, indicating the advantages of FIF-p.Conclusions:The FIF technique was useful in HTRT and improved homogeneity in the target.

OPTIMUM TREATMENT MODE APPLIED TO POST-OPERATIVE CERVICAL CANCER FOR 5F-IMRT PLAN BASED ON FOUR VARIABLES IN VARIAN ECLIPSE TPS

Purpose: This study aimed to determine the dosimetric effect on the target volume, organs at risk (OARs) and normal tissues based on the different choice for four types of mechanical variables, i.e., treatment position, dose calculation algorithm, mulitleaf collimator (MLC) motion mode and X-ray energy; and to investigate the optimum treatment mode applied to post-operative cervical cancer for 5-field intensity-modulated radiation therapy (5F-IMRT) technique. Methods: The dosimetric difference on the target volume and OARs under the influence of four types of variables were initially compared by changing one variable at a time. Then, based on the above compared results, we compared the dosimetric difference on planning target volume (PTV) and OARs between group A composed of the superior four variables and group B composed of the relatively inferior four variables. The dosimetric parameters included dose distribution of the target volume, OARs and normal tissues, conformal index (CI), homogeneity index (HI), monitor units (MU) and beam-on time ([Formula: see text]. The independent and paired t-tests were used for statistical analysis, and the threshold for statistical significance was [Formula: see text]. Results: Compared with the supine position, the maximum dose of PTV ([Formula: see text]), the maximum dose of small intestine ([Formula: see text]) and [Formula: see text] of bladder ([Formula: see text] were all lower in prone position. In contrast with the pencil beam convolution (PBC), CI of PTV (CI[Formula: see text]) was larger while HI of PTV (HI[Formula: see text]) was lower, both [Formula: see text] and the maximum dose of rectum ([Formula: see text]) were lower using anisotropic analytical algorithm (AAA). Moreover, the same results were obtained using sliding window (SW) compared with multiple static segments (MSS). The mean dose of PTV ([Formula: see text] and CI[Formula: see text] was larger while the maximum dose of the spinal cord ([Formula: see text]), [Formula: see text] and the maximum dose of femoral heads were lower with 15 MV X-rays compared with 6 MV X-rays. In comparison with group B comprising the supine position, PBC, MSS and 6 MV X-rays, [Formula: see text] and HI[Formula: see text] decreased 1.4% and 53.4% respectively, CI[Formula: see text] increased 5.8% medially, while [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] all decreased in group A comprising of prone position, AAA, SW and 15 MV X-rays. Conclusion: The treatment mode composed of prone position, AAA algorithm, SW and 15 MV X-rays is chosen for the post-operative cervical cancer of 5F-IMRT technique, which is more capable of meeting the target volume constraints and maximal protection of OARs.

Clinical implications of Eclipse analytical anisotropic algorithm and Acuros XB algorithm for the treatment of lung cancer.

The aim of the present study was to investigate the dose-volume variations of planning target volume (PTV) and organs at risks (OARs) in 15 left lung cancer patients comparing analytical anisotropic algorithm (AAA) versus Acuros XB algorithm. Originally, all plans were created using AAA with a template of dose constraints and optimization parameters, and the patients were treated using intensity modulated radiotherapy. In addition, another set of plans was created by performing only dose calculations using Acuros algorithm without doing any reoptimization. Thereby, in both set of plans, the entire plan parameters, namely, beam angle, beam weight, number of beams, prescribed dose, normalization point, region of interest constraints, number of monitor units, and plan optimization were kept constant. The evaluated plan parameters were PTV coverage at dose at 95% volume (TV95) of PTV (D95), the dose at 5% of PTV (D5), maximum dose (Dmax), the mean dose (Dmean), the percent volume receiving 5 Gy (V5), 20 Gy (V20), 30 Gy (V30) of normal lung at risk (left lung- gross target volume [GTV], the dose at 33% volume (D33), at 67% volume (D67), and the Dmean (Gy) of the heart, the Dmax of the spinal cord. Furthermore, homogeneity index (HI) and conformity index were evaluated to check the quality of the plans. Significant statistical differences between the two algorithms, P < 0.05, were found in D95, Dmax, TV95, and HI of PTV. Furthermore, significant statistical differences were found in the dose parameters for the OARs, namely, V5, V20, and V30 of left lung-GTV, right lung (Dmean), D33, and Dmean of the heart, and Dmax of the spine, respectively. Although statistical differences do exist, the magnitude of the differences is too small to cause any clinically observable effect.

Dosimetric study of different radiotherapy planning approaches for hippocampal avoidance whole-brain radiation therapy (HA-WBRT) based on fused CT and MRI imaging.

The purpose of this study was to compare the dosimetric characteristics for hippocampal avoidance (HA) between the treatment plans based on fused CT and MRI imaging during whole brain radiotherapy (WBRT) pertaining to: (1) 3-dimensional conformal radiotherapy (3D-CRT), (2) dynamic intensity modulated radiation therapy (dIMRT), and (3) RapidArc for patients with brain metastases. In our study, HA was defined as hippocampus beyond 5 mm, and planning target volume (PTV) was obtained subtracting HA volume from the volume of whole brain. There were 10 selected patients diagnosed with brain metastases receiving WBRT. These patients received plans for 3D-CRT (two fields), dIMRT (seven non-coplanar fields) and RapidArc (dual arc). The prescribed dose 30 Gy in 10 fractions was delivered to the whole-brain clinical target volume of patients. On the premise of meeting the clinical requirements, we compared target dose distribution, target coverage (TC), homogeneity index (HI), dose of organs at risk (OARs), monitor units (MU) and treatment time between the above three radiotherapy plans. V90 %, V95 % and TC of PTV for 3D-CRT plan were lowest of the three plans. V90 %, V95 % and HI of PTV in RapidArc plan were superior to the other two plans. TC of PTV in RapidArc plan was similar with dIMRT plan (P > 0.05). 3D-CRT was the optimal plan in the three plans for hippocampal protection. The median dose (Dmedian) and the maximum doses (Dmax) of hippocampus in 3D-CRT were 4.95, 10.87 Gy, which were lowest among the three planning approaches (P < 0.05). Dmedian and Dmax of hippocampus in dIMRT were 10.68, 14.11 Gy. Dmedian and Dmax of hippocampus in RapidArc were 10.30 gGy, 13.92 Gy. These parameters of the last two plans pertain to no significant difference (P > 0.05). When WBRT (30 Gy,10F) was equivalent to single dose 2 Gy,NTDmean of hippocampus in 3D-CRT, dIMRT and RapidArc were reduced to 3.60, 8.47, 8.20 Gy2, respectively. In addition, compared with dIMRT, MU of RapidArc was reduced and the treatment time was shortened by nearly 25 %. All three radiotherapy planning approaches in our study can meet the clinical requirements of HA. Although TC in 3D-CRT was lowest, hippocampus was protected best by this plan. So many radiation fields and the design of non-coplanar fields lead to the complication of dIMRT. TC and HI in RapidArc were superior to the other two plans with the precise of meeting the clinical requirements. The difference in protection for hippocampus between dIMRT and RapidArc was statistically significant. In addition, RapidArc can remarkably reduce MU and the treatment time.

SU‐E‐T‐765: Treatment Planning Comparison of SFUD Proton and 4Ï€ Radiotherapy for Prostate Cases

Purpose:Single‐Field Uniform Dose (SFUD) proton scanning beams and non‐coplanar 4π intensity‐modulated radiation therapy (IMRT) represent the most advanced treatment methods based on heavy ion and X‐rays, respectively. Here we compare their performance for prostate treatment.Methods:Five prostate patients were planned using 4π radiotherapy and SFUD to an initial dose of 54Gy to a planning target volume (PTV) that encompassed the prostate and seminal vesicles, then a boost prescription dose of 25.2Gy to the prostate for a total dose of 79.2 Gy. 4π plans were created by inversely selecting and optimizing 30 beams from 1162 candidate non‐coplanar beams using a greedy column generation algorithm. The SFUD plans utilized two coplanar, parallel‐opposing lateral scanning beams. The SFUD plan PTV was modified to account for range uncertainties while keeping an evaluation PTV identical to that of the X‐ray plans for comparison. PTV doses, bladder and rectum dose volumes (V40, V45, V60, V70, V75.6, and V80), R50, and PTV homogeneity index (D95/D5) were evaluated.Results:Compared to SFUD, 4π resulted in 6.8% lower high dose spillage as indicated by R50. Bladder and rectum mean doses were 38.3% and 28.2% lower for SFUD, respectively. However, bladder and rectum volumes receiving &gt;70Gy were 13.1% and 12% greater using proton SFUD. Due to the parallel‐opposing beam arrangement, SFUD resulted in greater femoral head (87.8%) and penile bulb doses (43.7%). 4π PTV doses were slightly more homogeneous (HI 0.99 vs. 0.98) than the SFUD dose.Conclusion:Proton is physically advantageous to reduce the irradiated normal volume and mean doses to the rectum and bladder but it is also limited in the beam orientations and entrance dose, which resulted in greater doses to the femoral heads and penile bulb, and larger volumes of rectum and bladder exposed to high dose due to the required robust PTV definition.This project is supported by Varian Medical Systems.

SU‐E‐T‐807: VMAT Vs. DIMRT Vs. SsIMRT Assessing the Dosimetric Parameters of Cervical Carcinoma Treatment with a 20‐Patient Sample

Purpose:The purpose of this study is to assess the dosimetric parameters of cervical carcinoma treatment using 3 different radiation therapy deliveryMethods:volumetric‐modulated arc therapy (VMAT), the static‐field dynamic multileaf collimator intensity‐modulated radiation therapy (dIMRT) and the static‐field step‐and‐shoot intensity‐modulated radiotherapy (ssIMRT).Methods:Twenty patients with cervical carcinoma were selected to be planned with dual arc VMAT, dIMRT and ssIMRT using Monaco 3.3 TPS on the Axesse™ linear accelerator in this investigation. The total dose of the planning target volume (PTV) is 60Gy. The homogeneity index (HI), conformity index (CI), dose volume histograms (DVHs), delivery efficiency, dose of organs at risks (bladder, rectum, and femoral heads), were all measured.Results:Dose distribution in 3 different radiation therapy delivery methods satisfied clinical requirements. Mean HI of PTV with VMAT, dIMRT and ssIMRT is 1.08, 1.10, and 1.09 (p&gt;0.05). Mean CI of PTV with VMAT, dIMRT and ssIMRT is 0.82, 0.8 and 0.8 (p&gt;0.05). For the DVH of V10, V20 and V30 in bladder, there was a significant difference: VMAT&gt;dIMRT=ssIMRT (p &lt;0.05). For the DVH of V40 and V50, there was a significant difference: VMAT&lt;dIMRT=ssIMRT (p &lt;0.05). The DVHs of rectum and femoral heads also reflected a similar characteristic with bladder that VMAT gave a higher dose than dIMRT and ssIMRT in low‐dose regions (p &lt;0.05), but gave a lower dose than dIMRT and ssIMRT in high‐dose regions (p &lt;0.05). For the delivery efficiency, there was a significant difference: VMAT &gt; dIMRT &gt;ssIMRT (p &lt;0.05).Conclusion:The results show that VMAT has a great advantage in delivery efficiency than dIMRT and ssIMRT, without compromise to the PTV coverage, HI and CI. The delivery methods should be considered under the actual cervical carcinoma radiotherapy situation.

Carotid-Sparing TomoHelical 3-Dimensional Conformal Radiotherapy for Early Glottic Cancer.

PurposeThe purpose of this study was to investigate the dosimetric benefits and treatment efficiency of carotid-sparing TomoHelical 3-dimensional conformal radiotherapy (TH-3DCRT) for early glottic cancer. Materials and MethodsTen early-stage (T1N0M0) glottic squamous cell carcinoma patients were simulated, based on computed tomography scans. Two-field 3DCRT (2F-3DCRT), 3-field intensity-modulated radiation therapy (3F-IMRT), TomoHelical-IMRT (TH-IMRT), and TH-3DCRT plans were generated with a 67.5-Gy total prescription dose to the planning target volume (PTV) for each patient. In order to evaluate the plan quality, dosimetric characteristics were compared in terms of conformity index (CI) and homogeneity index (HI) for PTV, dose to the carotid arteries, and maximum dose to the spinal cord. Treatment planning and delivery times were compared to evaluate treatment efficiency. ResultsThe median CI was substantially better for the 3F-IMRT (0.65), TH-IMRT (0.64), and TH-3DCRT (0.63) plans, compared to the 2F-3DCRT plan (0.32). PTV HI was slightly better for TH-3DCRT and TH-IMRT (1.05) compared to 2F-3DCRT (1.06) and 3F-IMRT (1.09). TH-3DCRT, 3F-IMRT, and TH-IMRT showed an excellent carotid sparing capability compared to 2F-3DCRT (p < 0.05). For all plans, the maximum dose to the spinal cord was < 45 Gy. The median treatment planning times for 2F-3DCRT (5.85 minutes) and TH-3DCRT (7.10 minutes) were much lower than those for 3F-IMRT (45.48 minutes) and TH-IMRT (35.30 minutes). The delivery times for 2F-3DCRT (2.06 minutes) and 3F-IMRT (2.48 minutes) were slightly lower than those for TH-IMRT (2.90 minutes) and TH-3DCRT (2.86 minutes). ConclusionTH-3DCRT showed excellent carotid-sparing capability, while offering high efficiency and maintaining good PTV coverage.

Evaluation of a Knowledge-Based Planning Solution for Head and Neck Cancer

Automated and knowledge-based planning techniques aim to reduce variations in plan quality. RapidPlan uses a library consisting of different patient plans to make a model that can predict achievable dose-volume histograms (DVHs) for new patients and uses those models for setting optimization objectives. We benchmarked RapidPlan versus clinical plans for 2 patient groups, using 3 different libraries. Volumetric modulated arc therapy plans of 60 recent head and neck cancer patients that included sparing of the salivary glands, swallowing muscles, and oral cavity were evenly divided between 2 models, Model(30A) and Model(30B), and were combined in a third model, Model60. Knowledge-based plans were created for 2 evaluation groups: evaluation group 1 (EG1), consisting of 15 recent patients, and evaluation group 2 (EG2), consisting of 15 older patients in whom only the salivary glands were spared. RapidPlan results were compared with clinical plans (CP) for boost and/or elective planning target volume homogeneity index, using HI(B)/HI(E) = 100 × (D2% - D98%)/D50%, and mean dose to composite salivary glands, swallowing muscles, and oral cavity (D(sal), D(swal), and D(oc), respectively). For EG1, RapidPlan improved HI(B) and HI(E) values compared with CP by 1.0% to 1.3% and 1.0% to 0.6%, respectively. Comparable D(sal) and D(swal) values were seen in Model(30A), Model(30B), and Model60, decreasing by an average of 0.1, 1.0, and 0.8 Gy and 4.8, 3.7, and 4.4 Gy, respectively. However, differences were noted between individual organs at risk (OARs), with Model(30B) increasing D(oc) by 0.1, 3.2, and 2.8 Gy compared with CP, Model(30A), and Model60. Plan quality was less consistent when the patient was flagged as an outlier. For EG2, RapidPlan decreased D(sal) by 4.1 to 4.9 Gy on average, whereas HI(B) and HI(E) decreased by 1.1% to 1.5% and 2.3% to 1.9%, respectively. RapidPlan knowledge-based treatment plans were comparable to CP if the patient's OAR-planning target volume geometry was within the range of those included in the models. EG2 results showed that a model including swallowing-muscle and oral-cavity sparing can be applied to patients with only salivary gland sparing. This may allow model library sharing between institutes. Optimal detection of inadequate plans and population of model libraries requires further investigation.

SU‐FF‐T‐130: Radiotherapy Treatment Plans with RapidArc for Head‐And‐Neck Cancer

Purpose: The purpose of this study is to compare RapidArc plans and conventional IMRT plans for head‐and‐neck cancer treatment involving bilateral nodes. Methods and Materials: This study included 10 locally advanced head‐and‐neck cancer patients. The planning target volume (PTV) included a 3‐mm margin onto the clinical target volume. The prescription was 50Gy to the PTV (2Gy/fraction). Right/left parotids, oral cavity, larynx, brainstem, spinal cord and mandible were contoured as critical organs. For each patient, 9‐fixed‐field IMRT and 3‐arc RapidArc plans were generated. For IMRT plans, the beams were spaced 40° apart and the collimator jaws were fixed to avoid shoulders. RapidArc plans had a total of 600° rotation, one full rotation and two anterior 120° rotations (179°–181°, 60°–300° and 300°–60°). All plans were normalized to cover 90% of PTV with 100% of the prescribed dose. Dosimetric results and treatment time were compared. Results: The homogeneity index of PTV was (1.07; 1.06) and the normal‐tissue receiving dose greater than 110% of the prescription dose was (0.11%; 0.01%) for (RapidArc; IMRT). The average median doses were (right parotid; 46.6%; 43.1%), (left parotid; 39.9%; 34.5%), (oral cavity; 50.0%; 51.7%) and (larynx; 45.7%; 41.4%) for (structure; RapidArc; IMRT). The average maximum doses were (brainstem; 45.5%; 49.9%), (spinal cord; 52.9%; 65.6%) and (mandible; 108.5%; 107.1%) for (organ; RapidArc; IMRT). The statistical analysis showed that the differences in homogeneity index and median doses to right/left parotids and spinal cords were statistically significant (p&lt;0.05), and all others were not. The MU values were reduced by 56% in average and the total treatment time was reduced by 7 minutes when RapidArc was compared to IMRT. Conclusions: IMRT performs better for parotid glands whereas RapidArc performs better for spinal cord. IMRT and RapidArc provide comparable plans for other organs. Treatment time with RapidArc is much reduced compared to IMRT.