Clinical dosimetric impact of Acuros XB and analytical anisotropic algorithm (AAA) on real lung cancer treatment plans : review

Dosimetric evaluation of different planning techniques based on flattening filter-free beams for central and peripheral lung stereotactic body radiotherapy

This study aimed to dosimetrically compare and evaluate the flattening filter-free (FFF) photon beam-based three-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiation therapy (IMRT), and volumetric modulated arc therapy (VMAT) for lung stereotactic body radiotherapy (SBRT). RANDO phantom computed tomography (CT) images were used for treatment planning. Gross tumor volumes (GTVs) were delineated in the central and peripheral lung locations. Planning target volumes (PTVs) was determined by adding a 5 mm margin to the GTV. 3DCRT, IMRT, and VMAT plans were generated using a 6-MV FFF photon beam. Dose calculations for all plans were performed using the anisotropic analytical algorithm (AAA) and Acuros XB algorithms. The accuracy of the algorithms was validated using the dose measured in a CIRS thorax phantom. The conformity index (CI), high dose volume (HDV), low dose location (D_2cm), and homogeneity index (HI) improved with FFF-VMAT compared to FFF-IMRT and FFF-3DCRT, while low dose volume (R_50%) and gradient index (GI) showed improvement with FFF-3DCRT. Compared with FFF-3DCRT, a drastic decrease in the mean treatment time (TT) value was observed with FFF-VMAT for different lung sites between 57.09% and 60.39%, while with FFF-IMRT it increased between 10.78% and 17.49%. The dose calculation with Acuros XB was found to be superior to that of AAA. Based on the comparison of dosimetric indices in this study, FFF-VMAT provides a superior treatment plan to FFF-IMRT and FFF-3DCRT in the treatment of peripheral and central lung PTVs. This study suggests that Acuros XB is a more accurate algorithm than AAA in the lung region.

Clinical verification of treatment planning dose calculation in lung SBRT with GATE Monte Carlo simulation code

PURPOSE

This study aims to use GATE/Geant4 simulation code to evaluate the performance of dose calculations with Anisotropic Analytical Algorithm (AAA) in the context of lung SBRT for complex treatments considering images of patients.

METHODS

Four cases of non-small cell lung cancer treated with SBRT were selected for this study. Irradiation plans were created with AAA and recalculated end to end using Monte Carlo (MC) method maintaining field configurations identical to the original plans. Each treatment plan was evaluated in terms of PTV and organs at risk (OARs) using dose-volume histograms (DVH). Dosimetric parameters obtained from DVHs were used to compare AAA and MC.

RESULTS

The comparison between the AAA and MC DVH using gamma analysis with the passing criteria of 3%/3% showed an average passing rate of more than 90% for the PTV structure and 97% for the OARs. Tightening the criteria to 2%/2% showed a reduction in the average passing rate of the PTV to 86%. The agreement between the AAA and MC dose calculations for PTV dosimetric parameters (V₁₀₀; V₉₀; Homogeneity index; maximum, minimum and mean dose; CI_Paddick and D_2cm) was within 18.4%. For OARs, the biggest differences were observed in the spinal cord and the great vessels.

CONCLUSIONS

In general, we did not find significant differences between AAA and MC. The results indicate that AAA could be used in complex SBRT cases that involve a larger number of small treatment fields in the presence of tissue heterogeneities.

Correlation of clinical outcome, radiobiological modeling of tumor control, normal tissue complication probability in lung cancer patients treated with SBRT using Monte Carlo calculation algorithm

Purpose/Background

We analyzed the predictive value of non‐x‐ray voxel Monte Carlo (XVMC)‐based modeling of tumor control probability (TCP) and normal tissue complication probability (NTCP) in patients treated with stereotactic body radiotherapy (SBRT) using the XVMC dose calculation algorithm.

Materials/Methods

We conducted an IRB‐approved retrospective analysis in patients with lung tumors treated with XVMC‐based lung SBRT. For TCP, we utilized tumor size‐adjusted biological effective dose (s‐BED) TCP modeling validated in non‐MC dose calculated SBRT to: (1) verify modeling as a function of s‐BED in patients treated with XVMC‐based SBRT; and (2) evaluate the predictive potential of different PTV dosimetric parameters (mean dose, minimum dose, max dose, prescription dose, D95, D98, and D99) for incorporation into the TCP model. Correlation between observed local control and TCPs was assessed by Pearson's correlation coefficient. For NTCP, Lyman NTCP Model was utilized to predict grade 2 pneumonitis and rib fracture.

Results

Eighty‐four patients with 109 lung tumors were treated with XVMC‐based SBRT to total doses of 40 to 60 Gy in 3 to 5 fractions. Median follow‐up was 17 months. The 2‐year local and local‐regional control rates were 91% and and 78%, respectievly. All estimated TCPs correlated significantly with 2‐year actuarial local control rates (P < 0.05). Significant corelations between TCPs and tumor control rate according to PTV dosimetric parameters were observed. D99 parameterization demonstrated the most robust correlation between observed and predicted tumor control. The incidences of grade 2 pneumonitis and rib fracture vs. predicted were 1% vs. 3% and 10% vs. 13%, respectively.

Conclusion

Our TCP results using a XVMC‐based dose calculation algorithm are encouraging and yield validation to previously described TCP models using non‐XVMC dose methods. Furthermore, D99 as potential predictive parameter in the TCP model demonstrated better correlation with clinical outcome.

On the use of AAA and AcurosXB algorithms for three different stereotactic ablative body radiotherapy (SABR) techniques: Volumetric modulated arc therapy (VMAT), intensity modulated radiation therapy (IMRT) and 3D conformal radiotherapy (3D-CRT)

Aim

The purpose of this study was to investigate the dosimetric characteristics of three stereotactic ablative body radiotherapy (SABR) techniques using the anisotropic analytical algorithm (AAA) and Acuros XB algorithm. The SABR techniques include coplanar volumetric modulated arc therapy (C-VMAT), non-coplanar intensity modulated radiation therapy (NC-IMRT) and non-coplanar three-dimensional conformal radiotherapy (NC-3D CRT).

Background

SABR is a special type of radiotherapy where a high dose of radiation is delivered over a short time. The treatment outcome and accuracy of the dose delivered to cancer patients highly depend on the dose calculation algorithm and treatment technique.

Materials and methods

Twelve lung cancer patients underwent 4D CT scanning, and three different treatment plans were generated: C-VMAT, NC-IMRT, NC-3D CRT. Dose calculation was performed using the AAA and Acuros XB algorithm. The dosimetric indices, such as conformity index (CI), homogeneity index, dose fall-off index, doses received by organs at risk and planning target volume, were used to compare the plans. The accuracy of AAA and Acuros XB (AXB) algorithms for the lung was validated against measured dose on a CIRS thorax phantom.

Results

The CIs for C-VMAT, NC-IMRT and NC-3D CRT were 1.21, 1.28 and 1.38 for the AAA, respectively, and 1.17, 1.26 and 1.36 for the Acuros XB algorithm, respectively. The overall dose computed by AcurosXB algorithm was close to the measured dose when compared to the AAA algorithm. The overall dose computed by the AcurosXB algorithm was close to the measured dose when compared to the AAA algorithm.

Conclusion

This study showed that the treatment planning results obtained using the Acuros XB algorithm was better than those using the AAA algorithm in SABR lung radiotherapy.

Calculation of absorbed dose in radiotherapy by solution of the linear Boltzmann transport equations

Over the last decade, dose calculations which solve the linear Boltzmann transport equations have been introduced into clinical practice and are now in widespread use. However, knowledge in the radiotherapy community concerning the details of their function is limited. This review gives a general description of the linear Boltzmann transport equations as applied to calculation of absorbed dose in clinical radiotherapy. The aim is to elucidate the principles of the method, rather than to describe a particular implementation. The literature on the performance of typical algorithms is then reviewed, in many cases with reference to Monte Carlo simulations. The review is completed with an overview of the emerging applications in the important area of MR-guided radiotherapy.

Dosimetrical and radiobiological approach to manage the dosimetric shift in the transition of dose calculation algorithm in radiation oncology: how to improve high quality treatment and avoid unexpected outcomes?

BACKGROUND

For a given prescribed dose of radiotherapy, with the successive generations of dose calculation algorithms, more monitor units (MUs) are generally needed. This is due to the implementation of successive improvements in dose calculation: better heterogeneity correction and more accurate estimation of secondary electron transport contribution. More recently, there is the possibility to report the dose-to-medium, physically more accurate compared to the dose-to-water as the reference one. This last point is a recent concern and the main focus of this study.

METHODS

In this paper, we propose steps for a general analysis procedure to estimate the dosimetric alterations, and the potential clinical changes, between a reference algorithm and a new one. This includes dosimetric parameters, gamma index, radiobiology indices based on equivalent uniform dose concept and statistics with bootstrap simulation. Finally, we provide a general recommendation on the clinical use of new algorithms regarding the dose prescription or dose limits to the organs at risks.

RESULTS

The dosimetrical and radiobiological data showed a significant effect, which might exceed 5-10%, of the calculation method on the dose the distribution and clinical outcomes for lung cancer patients. Wilcoxon signed rank paired comparisons indicated that the delivered dose in MUs was significantly increased (> 2%) using more advanced dose calculation methods as compared to the reference one.

CONCLUSION

This paper illustrates and explains the use of dosimetrical, radiobiologcal and statistical tests for dosimetric comparisons in radiotherapy. The change of dose calculation algorithm may induce a dosimetric shift, which has to be evaluated by the physicists and the oncologists. This includes the impact on tumor control and on the risk of toxicity based on normal tissue dose constraints. In fact, the alteration in dose distribution makes it hard to keep exactly the same tumor control probability along with the same normal tissue complication probability.

The choice of multi-beam IMRT for whole breast radiotherapy in early-stage right breast cancer

The aim of this study was to identify a rational strategy for the selection of multi-beam IMRT in patients with right breast cancer through the comparison of dosimetric parameters of the planning target volume (PTV) and organs at risk (OARs) using five different radiotherapy modalities. This was a retrospective study using computed tomography scans from ten patients with early-stage right breast cancer who had been treated previously. Three dimensional conformal radiotherapy (3DCRT), forward-planned IMRT (for-IMRT), inverse-planned IMRT (inv-IMRT), helical tomotherapy (HT), and volumetric-modulated arc therapy (VMAT) were planned for each patient. The plans were compared according to dose–volume histogram analysis. The most significant impact of inverse-planned multi-beam modalities for right breast cancer was the reduction of D_max, D_mean, V_53.5 and prescribed dose volume (cc) outside of the PTV (breast) (OB-V₅₀) of the PTV. HT decreased the ipsilateral OAR volumes receiving higher doses. In exchange, HT also increased the volumes receiving low doses, which is known to lead to an increased rate of radiation-induced secondary malignancies. The heart, LAD, and contralateral doses for 3DCRT and for-IMRT were significantly lower than those for inv-IMRT, HT, and VMAT. In addition, inv-IMRT demonstrated an increase in exposed volume of heart, LAD, ipsilateral lung, and contralateral lung compared with those parameters for HT or VMAT. Although it is known to reduce cardiac toxicity with breath hold technique in left sided breast cancer, similarly it is possible for 3DCRT and for-IMRT techniques in right sided breast cancer even in free breathing.

Assessment of Monte Carlo algorithm for compliance with RTOG 0915 dosimetric criteria in peripheral lung cancer patients treated with stereotactic body radiotherapy

The purpose of the study was to evaluate Monte Carlo‐generated dose distributions with the X‐ray Voxel Monte Carlo (XVMC) algorithm in the treatment of peripheral lung cancer patients using stereotactic body radiotherapy (SBRT) with non‐protocol dose‐volume normalization and to assess plan outcomes utilizing RTOG 0915 dosimetric compliance criteria. The Radiation Therapy Oncology Group (RTOG) protocols for non‐small cell lung cancer (NSCLC) currently require radiation dose to be calculated using tissue density heterogeneity corrections. Dosimetric criteria of RTOG 0915 were established based on superposition/convolution or heterogeneities corrected pencil beam (PB‐hete) algorithms for dose calculations. Clinically, more accurate Monte Carlo (MC)‐based algorithms are now routinely used for lung stereotactic body radiotherapy (SBRT) dose calculations. Hence, it is important to determine whether MC calculations in the delivery of lung SBRT can achieve RTOG standards. In this report, we evaluate iPlan generated MC plans for peripheral lung cancer patients treated with SBRT using dose‐volume histogram (DVH) normalization to determine if the RTOG 0915 compliance criteria can be met. This study evaluated 20 Stage I‐II NSCLC patients with peripherally located lung tumors, who underwent MC‐based SBRT with heterogeneity correction using X‐ray Voxel Monte Carlo (XVMC) algorithm (Brainlab iPlan version 4.1.2). Total dose of 50 to 54 Gy in 3 to 5 fractions was delivered to the planning target volume (PTV) with at least 95% of the PTV receiving 100% of the prescription dose (V100%≥95%). The internal target volume (ITV) was delineated on maximum intensity projection (MIP) images of 4D CT scans. The PTV included the ITV plus 5 mm uniform margin applied to the ITV. The PTV ranged from 11.1 to 163.0 cc (mean=46.1±38.7 cc). Organs at risk (OARs) including ribs were delineated on mean intensity projection (MeanIP) images of 4D CT scans. Optimal clinical MC SBRT plans were generated using a combination of 3D noncoplanar conformal arcs and nonopposing static beams for the Novalis‐TX linear accelerator consisting of high‐definition multileaf collimators (HD‐MLCs: 2.5 mm leaf width at isocenter) and 6 MV‐SRS (1000 MU/min) beam. All treatment plans were evaluated using the RTOG 0915 high‐ and intermediate‐dose spillage criteria: conformity index (R100%), ratio of 50% isodose volume to the PTV (R50%), maximum dose 2 cm away from PTV in any direction (D2cm), and percent of normal lung receiving 20 Gy V20 or more. Other OAR doses were documented, including the volume of normal lung receiving 5 Gy V5 or more, dose to <0.35 cc of spinal cord, and dose to 1000 cc of total normal lung tissue. The dose to <1 cc, <5 cc, <10 cc of ribs, as well as maximum point dose as a function of PTV, prescription dose, and a 3D distance from the tumor isocenter to the proximity of the rib contour were also examined. The biological effective dose (BED) with α/β ratio of 3 Gy for ribs was analyzed. All 20 patients either fully met or were within the minor deviation dosimetric compliance criteria of RTOG 0915 while using DVH normalization. However, only 5 of the 20 patients fully met all the criteria. Ten of 20 patients had minor deviations in R100% (mean=1.25±0.09), 13 in R50% (mean=4.5±0.6), and 11 in D2cm (mean=61.9±8.5). Lung V20, dose to 1000 cc of normal lung, and dose to <0.35 cc of spinal cord were met in accordance with RTOG criteria in 95%, 100%, and 100%, respectively, with exception of one patient who exhibited the largest PTV (163 cc) and experienced a minor deviation in lung V20 (mean=4.7±3.4%). The 3D distance from the tumor isocenter to the proximal rib contour strongly correlated with maximum rib dose. The average values of BED3Gy for maximum point dose and dose to <1 cc of ribs were higher by a factor of 1.5 using XVMC compared to RTOG 0915 guidelines. The preliminary results for our iPlan XVMC dose analyses indicate that the majority (i.e., 75% of patient population) of our patients had minor deviations when compared to the dosimetric guidelines set by RTOG 0915 protocol. When using an exclusively sophisticated XVMC algorithm and DVH normalization, the RTOG 0915 dosimetric compliance criteria such as R100%, R50%, and D2cm may need to be revised. On average, about 7% for R100%, 13% for R50%, and 14% for D2cm corrections from the mean values were necessary to pass the RTOG 0915 compliance criteria. Another option includes rescaling of the prescription dose. No further adjustment is necessary for OAR dose tolerances including normal lung V20 and total normal lung 1000 cc. Since all the clinical MC plans were generated without compromising the target coverage, rib dose was on the higher side of the protocol guidelines. As expected, larger tumor size and proximity to ribs correlated to higher absolute dose to ribs. These patients will be clinically followed to determine whether delivered MC‐computed dose to PTV and the ribs dose correlate with tumor control and severe chest wall pain and/or rib fractures. In order to establish new specific MC‐based dose parameters, further dosimetric studies with a large cohort of MC lung SBRT patients will need to be conducted.

PACS number(s): 87.55.k

Evaluation of the dosimetric impact of applying flattening filter-free beams in intensity-modulated radiotherapy for early-stage upper thoracic carcinoma of oesophagus

INTRODUCTION

Flattening filter-free (FFF) radiation beams have recently become clinically available on modern linear accelerators in radiation therapy. This study aimed to evaluate the dosimetric impact of using FFF beams in intensity-modulated radiotherapy (IMRT) for early-stage upper thoracic oesophageal cancer.

METHODS

Eleven patients with primary stage upper thoracic oesophageal cancer were recruited. For each patient, two IMRT plans were computed using conventional beams (Con-P) and FFF beams (FFF-P), respectively. Both plans employed a five-beam arrangement and were prescribed with 64 Gy to (planning target volume) PTV1 and 54 Gy to PTV2 in 32 fractions using 6 MV photons. The dose parameters of the target volumes and organs at risks (OARs), and treatment parameters including the monitor units (MU) and treatment time (TT) for Con-P and FFF-P were recorded and compared.

RESULTS

The mean D 5 of PTV1 and PTV2 were higher in FFF-P than Con-P by 0.4 Gy and 0.3 Gy, respectively. For the OARs, all the dose parameters did not show significant difference between the two plans except the mean V 5 and V 10 of the lung in which the FFF-P was lower (46.7% vs. 47.3% and 39.1% vs. 39.6%, respectively). FFF-P required 54% more MU but 18.4% less irradiation time when compared to Con-P.

CONCLUSION

The target volume and OARs dose distributions between the two plans were comparable. However, FFF-P was more effective in sparing the lung from low dose and reduced the mean TT compared with Con-P. Long-term clinical studies are suggested to evaluate the radiobiological effects of FFF beams.

Dosimetric study of the AAA algorithm for the VMAT technique using an anthropomorphic phantom in the pelvic region

Purpose

The objective of this work was to investigate the accuracy of AAA dose calculation algorithm for RapidArc volumetric modulated technique (VMAT) in the presence of anatomical heterogeneities in the pelvic region.

Material and methods

An anthropomorphic phantom was used to simulate a prostate case, delineating planning target volumes (PTVs) and organs at risk. VMAT plans were optimised in eclipse (v10·0) treatment planning system (TPS). The dose distributions were calculated by the AAA dose calculation algorithm. A total of 49 thermoluminiscent dosimeters were inserted into the anthropomorphic phantom and dose measurements were compared with the predicted TPS doses.

Results

The average dose variation was −1·5% for planning target volume corresponding to the prostate and −0·3% for planning target volume corresponding to the pelvic nodes, −0·2% for the rectum, +2·4% for the bladder, −2·0% for the femoral heads and +1·0% for the intestinal package.

Conclusion

AAA is a reliable dose calculation for the treatment with VMAT in the anatomy of the pelvis.

Technical Note: Dosimetric evaluation of Monte Carlo algorithm in iPlan for stereotactic ablative body radiotherapy (SABR) for lung cancer patients using RTOG 0813 parameters

For stereotactic ablative body radiotherapy (SABR) in lung cancer patients, Radiation Therapy Oncology Group (RTOG) protocols currently require radiation dose to be calculated using tissue heterogeneity corrections. Dosimetric criteria of RTOG 0813 were established based on the results obtained from non-Monte Carlo (MC) algorithms, such as superposition/convolutions. Clinically, MC-based algorithms are now routinely used for lung SABR dose calculations. It is essential to confirm that MC calculations in lung SABR meet RTOG guidelines. This report evaluates iPlan MC plans for SABR in lung cancer patients using dose-volume histogram normalization per current RTOG 0813 compliance criteria. Eighteen Stage I-II non-small cell lung cancer (NSCLC) patients with centrally located tumors, who underwent MC-based lung SABR with heterogeneity correction using X-ray Voxel Monte Carlo (XVMC) algorithm (BrainLAB iPlan version 4.1.2), were analyzed. Total dose of 60 Gy in 5 fractions was delivered to planning target volume (PTV) with at least V100% = 95%. Internal target volumes (ITVs) were delineated on maximum intensity projection (MIP) images of 4D CT scans. PTV (ITV + 5 mm margin) volumes ranged from 10.0 to 99.9 cc (mean = 36.8 ± 20.7 cc). Organs at risk (OARs) were delineated on average images of 4D CT scans. Optimal clinical MC SABR plans were generated using a combination of non-coplanar conformal arcs and beams for the Novalis-TX consisting of high definition multileaf collimators (MLCs) and 6 MV-SRS (1000 MU/min) mode. All plans were evaluated using the RTOG 0813 high and intermediate dose spillage criteria: conformity index (R100%), ratio of 50% isodose volume to the PTV (R50%), maximum dose 2 cm away from PTV in any direction (D2 cm), and percent of normal lung receiving 20 Gy (V20) or more. Other organs-at-risk (OARs) doses were tabulated, including the volume of normal lung receiving 5 Gy (V5), maximum cord dose, dose to < 15 cc of heart, and dose to <5 cc of esophagus. Only six out of 18 patients met all RTOG 0813 compliance criteria. Eight of 18 patients had minor deviations in R100%, four in R50%, and nine in D2 cm. However, only one patient had minor deviation in V20. All other OARs doses, such as maximum cord dose, dose to < 15 cc of heart, and dose to < 5 cc of esophagus, were satisfactory for RTOG criteria, except for one patient, for whom the dose to < 15 cc of heart was higher than RTOG guidelines. The preliminary results for our limited iPlan XVMC dose calculations indicate that the majority (i.e., 2/3) of our patients had minor deviations in the dosimetric guidelines set by RTOG 0813 protocol in one way or another. When using an exclusive highly sophisticated XVMC algorithm, the RTOG 0813 dosimetric compliance criteria such as R100% and D2 cm may need to be revisited. Based on our limited number of patient datasets, in general, about 6% for R100% and 9% for D2 cm corrections could be applied to pass the RTOG 0813 compliance criteria in most of those patients. More patient plans need to be evaluated to make recommendation for R50%. No adjustment is necessary for OAR dose tolerances, including normal lung V20. In order to establish new MC specific dose parameters, further investigation with a large cohort of patients including central, as well as peripheral lung tumors, is anticipated and strongly recommended.

A dosimetric comparison of three-dimensional conformal radiotherapy, volumetric-modulated arc therapy, and dynamic conformal arc therapy in the treatment of non-small cell lung cancer using stereotactic body radiotherapy

This study evaluates three‐dimensional conformal radiotherapy (3D CRT), volumetric‐ modulated arc therapy (VMAT), and dynamic conformal arc therapy (DCAT) planning techniques using dosimetric indices from Radiation Therapy Oncology Group (RTOG) protocols 0236, 0813, and 0915 for the treatment of early‐stage non‐small cell lung cancer (NSCLC) using stereotactic body radiotherapy (SBRT). Twenty‐five clinical patients, five per lung lobe, previously treated for NSCLC using 3D CRT SBRT under respective RTOG protocols were replanned with VMAT and DCAT techniques. All plans were compared using respective RTOG dosimetric indices. High‐ and low‐dose spillage improved for VMAT and DCAT plans, though only VMAT was able to improve dose to all organs at risk (OARs). DCAT was only able to provide a minimal improvement in dose to the heart and ipsilateral brachial plexus. Mean bilateral, contralateral, and V20 (percentage of bilateral lung receiving at least 20 Gy dose) doses were reduced with VMAT in comparison with respective 3D CRT clinical plans. Though some of the DCAT plans had values for the above indices slightly higher than their respective 3D CRT plans, they still were able to meet the RTOG constraints. VMAT and DCAT were able to offer improved skin dose by 1.1% and 11%, respectively. Monitor units required for treatment delivery increased with VMAT by 41%, but decreased with DCAT by 26%. VMAT and DCAT provided improved dose distributions to the PTV, but only VMAT was consistently superior in sparing dose to OARs in all the five lobes. DCAT should still remain an alternative to 3D CRT in facilities that do not have VMAT or intensity‐modulated radiotherapy (IMRT) capabilities.

PACS numbers: 87.53.Ly, 87.55.dk, 87.55.D‐

Dosimetric study of uniform scanning proton therapy planning for prostate cancer patients with a metal hip prosthesis, and comparison with volumetric-modulated arc therapy

The main purposes of this study were to 1) investigate the dosimetric quality of uniform scanning proton therapy planning (USPT) for prostate cancer patients with a metal hip prosthesis, and 2) compare the dosimetric results of USPT with that of volumetric‐modulated arc therapy (VMAT). Proton plans for prostate cancer (four cases) were generated in XiO treatment planning system (TPS). The beam arrangement in each proton plan consisted of three fields (two oblique fields and one lateral or slightly angled field), and the proton beams passing through a metal hip prosthesis was avoided. Dose calculations in proton plans were performed using the pencil beam algorithm. From each proton plan, planning target volume (PTV) coverage value (i.e., relative volume of the PTV receiving the prescription dose of 79.2 CGE) was recorded. The VMAT prostate planning was done using two arcs in the Eclipse TPS utilizing 6 MV X‐rays, and beam entrance through metallic hip prosthesis was avoided. Dose computation in the VMAT plans was done using anisotropic analytical algorithm, and calculated VMAT plans were then normalized such that the PTV coverage in the VMAT plan was the same as in the proton plan of the corresponding case. The dose‐volume histograms of calculated treatment plans were used to evaluate the dosimetric quality of USPT and VMAT. In comparison to the proton plans, on average, the maximum and mean doses to the PTV were higher in the VMAT plans by 1.4% and 0.5%, respectively, whereas the minimum PTV dose was lower in the VMAT plans by 3.4%. The proton plans had lower (or better) average homogeneity index (HI) of 0.03 compared to the one for VMAT (HI = 0.04). The relative rectal volume exposed to radiation was lower in the proton plan, with an average absolute difference ranging from 0.1% to 32.6%. In contrast, using proton planning, the relative bladder volume exposed to radiation was higher at high‐dose region with an average absolute difference ranging from 0.4% to 0.8%, and lower at low‐ and medium‐dose regions with an average absolute difference ranging from 2.7% to 10.1%. The average mean dose to the rectum and bladder was lower in the proton plans by 45.1% and 22.0%, respectively, whereas the mean dose to femoral head was lower in VMAT plans by an average difference of 79.6%. In comparison to the VMAT, the proton planning produced lower equivalent uniform dose (EUD) for the rectum (43.7 CGE vs. 51.4 Gy) and higher EUD for the femoral head (16.7 CGE vs. 9.5 Gy), whereas both the VMAT and proton planning produced comparable EUDs for the prostate tumor (76.2 CGE vs. 76.8 Gy) and bladder (50.3 CGE vs. 51.1 Gy). The results presented in this study show that the combination of lateral and oblique fields in USPT planning could potentially provide dosimetric advantage over the VMAT for prostate cancer involving a metallic hip prosthesis.

PACS number: 87.55.D‐, 87.55.ne, 87.55.dk