Chang Heon Choi

RapidArc® vs intensity-modulated radiation therapy for hepatocellular carcinoma: a comparative planning study

OBJECTIVE The purpose of this study is to compare the dose-volumetric results of RapidArc (RA Varian Medical Systems, Palo Alto, CA) with those of intensity-modulated radiation therapy (IMRT) for hepatocellular carcinoma. METHODS 20 patients previously treated for hepatocellular carcinoma were the subjects of this planning study. 10 patients were treated for portal vein tumour thrombosis (Group A), and 10 patients for primary liver tumour (Group B). Prescription dose to the planning target volume was 54 Gy in 30 fractions, and the planning goal was to deliver more than 95% of prescribed dose to at least 95% of planning target volume. RESULTS In Group A, mean doses to liver were increased with RA vs IMRT (22.9 Gy vs 22.2 Gy, p=0.0275). However, V(30 Gy) of liver was lower in RA vs IMRT (31.1% vs 32.1%, p=0.0283). In Group B, in contrast, neither mean doses nor V(30 Gy) of liver significantly differed between the two plans. V(35 Gy) of duodenum and V(20 Gy) of kidney were decreased with RA in Groups A and B, respectively (p=0.0058 and 0.0124, respectively). Both maximal doses to spinal cord and monitor unit were significantly lower in the RA plan, regardless of the group. CONCLUSION The dose-volumetric results of RA vs IMRT were different according to the different target location within the liver. In general, RA tended to be more effective in the sparing of non-liver organs at risk such as duodenum, kidney, and/or spinal cord. Moreover, RA was more efficient in the treatment delivery than IMRT in terms of total monitor unit used.

The dosimetric effect of mixed-energy IMRT plans for prostate cancer

We investigated the effect of mixing high‐ and low‐energy photon beams on the quality of intensity‐modulated radiation therapy (IMRT) plans for patients with prostate cancer. Three different plans for each of twenty patients were generated using either 6 MV or 15 MV alone, and both 6 and 15 MV beams. All the planning parameters, goals, and constraints were set to be identical except beam energy. The dose distributions were similar in terms of target coverage, conformity, and homogeneity regardless of beam energy. The V70Gy of rectal wall in 6 MV, 15 MV and mixed‐energy plans was 16.7%, 17.9%, and 16.3%, respectively, while V40Gy was 55.6%, 53.2%, and 50%. The mean dose to femoral heads in 6 MV, 15 MV, and mixed‐energy plans were 31.7 Gy, 26.3 Gy, and 26.2 Gy, respectively. The integral dose of 6 MV plans was 7% larger than those of 15 MV or mixed‐energy plans. These results indicated that mixed‐energy IMRT plans could take advantage of the dosimetric characteristics of low‐ and high‐energy beams. Even though the reduction of dose to the organs at risk may not be clinically relevant, mixing energy in an IMRT plan for deep‐seated tumors can improve the overall plan quality. PACS number: 87.55.ne

Photon energy-modulated radiotherapy: Monte Carlo simulation and treatment planning study

PURPOSE To demonstrate the feasibility of photon energy-modulated radiotherapy during beam-on time. METHODS A cylindrical device made of aluminum was conceptually proposed as an energy modulator. The frame of the device was connected with 20 tubes through which mercury could be injected or drained to adjust the thickness of mercury along the beam axis. In Monte Carlo (MC) simulations, a flattening filter of 6 or 10 MV linac was replaced with the device. The thickness of mercury inside the device varied from 0 to 40 mm at the field sizes of 5 × 5 cm(2) (FS5), 10 × 10 cm(2) (FS10), and 20 × 20 cm(2) (FS20). At least 5 billion histories were followed for each simulation to create phase space files at 100 cm source to surface distance (SSD). In-water beam data were acquired by additional MC simulations using the above phase space files. A treatment planning system (TPS) was commissioned to generate a virtual machine using the MC-generated beam data. Intensity modulated radiation therapy (IMRT) plans for six clinical cases were generated using conventional 6 MV, 6 MV flattening filter free, and energy-modulated photon beams of the virtual machine. RESULTS As increasing the thickness of mercury, Percentage depth doses (PDD) of modulated 6 and 10 MV after the depth of dose maximum were continuously increased. The amount of PDD increase at the depth of 10 and 20 cm for modulated 6 MV was 4.8% and 5.2% at FS5, 3.9% and 5.0% at FS10 and 3.2%-4.9% at FS20 as increasing the thickness of mercury from 0 to 20 mm. The same for modulated 10 MV was 4.5% and 5.0% at FS5, 3.8% and 4.7% at FS10 and 4.1% and 4.8% at FS20 as increasing the thickness of mercury from 0 to 25 mm. The outputs of modulated 6 MV with 20 mm mercury and of modulated 10 MV with 25 mm mercury were reduced into 30%, and 56% of conventional linac, respectively. The energy-modulated IMRT plans had less integral doses than 6 MV IMRT or 6 MV flattening filter free plans for tumors located in the periphery while maintaining the similar quality of target coverage, homogeneity, and conformity. CONCLUSIONS The MC study for the designed energy modulator demonstrated the feasibility of energy-modulated photon beams available during beam-on time. The planning study showed an advantage of energy-and intensity modulated radiotherapy in terms of integral dose without sacrificing any quality of IMRT plan.

The effect of photon energy on intensity-modulated radiation therapy (IMRT) plans for prostate cancer

Purpose To evaluate the effect of common three photon energies (6-MV, 10-MV, and 15-MV) on intensity-modulated radiation therapy (IMRT) plans to treat prostate cancer patients. Materials and Methods Twenty patients with prostate cancer treated locally to 81.0 Gy were retrospectively studied. 6-MV, 10-MV, and 15-MV IMRT plans for each patient were generated using suitable planning objectives, dose constraints, and 8-field setting. The plans were analyzed in terms of dose-volume histogram for the target coverage, dose conformity, organs at risk (OAR) sparing, and normal tissue integral dose. Results Regardless of the energies chosen at the plans, the target coverage, conformity, and homogeneity of the plans were similar. However, there was a significant dose increase in rectal wall and femoral heads for 6-MV compared to those for 10-MV and 15-MV. The V20 Gy of rectal wall with 6-MV, 10-MV, and 15-MV were 95.6%, 88.4%, and 89.4% while the mean dose to femoral heads were 31.7, 25.9, and 26.3 Gy, respectively. Integral doses to the normal tissues in higher energy (10-MV and 15-MV) plans were reduced by about 7%. Overall, integral doses in mid and low dose regions in 6-MV plans were increased by up to 13%. Conclusion In this study, 10-MV prostate IMRT plans showed better OAR sparing and less integral doses than the 6-MV. The biological and clinical significance of this finding remains to be determined afterward, considering neutron dose contribution.

Assessment of potential jaw-tracking advantage using control point sequences of VMAT planning

This study aims to evaluate the potential jaw‐tracking advantage using control point sequences of volume volumetric‐modulated arc therapy (VMAT) planning. VMAT plans for patients with prostate and head and neck (H&N) cancers were converted into new static arc (SA) plans. The SA plan consisted of a series of static fields at each control point of the VMAT plan. All other machine parameters of the SA plan were perfectly identical to those of the original VMAT plan. The jaw‐tracking static arc (JTSA) plans were generated with fields that closed the jaws of each SA field into the multileaf collimators (MLCs) aperture. The dosimetric advantages of JTSA over SA were evaluated in terms of a dose‐volume histogram (DVH) of organ at risk (OAR) after renormalizing both plans to make the same target coverage. Both plans were delivered to the MatriXX‐based COMPASS system for 3D volume dose verification. The average jaw size reduction of the JTSA along the X direction was 3.1±0.9 cm for prostate patients and 6.9±1.9 cm for H&N patients. For prostate patients, the organs far from the target showed larger sparing (3.7%—8.1% on aver‐age) in JTSA than the organs adjacent to the target (1.1%—1.5%). For the H&N plans, the mean dose reductions for all organs ranged from 4.3% to 11.9%. The dose reductions were more significant in the dose regions of D80,D90, and D95 than the dose regions of D5,D10, and D20 for all patients. Likewise, the deliverability and reproducibility of jaw‐tracking plan were validated. The measured dosimetric advantage of JTSA over SA coincided with the calculated one above. PACS numbers: 87.55.D‐, 87.55.ne

Quality of tri-Co-60 MR-IGRT treatment plans in comparison with VMAT treatment plans for spine SABR

OBJECTIVE To investigate the plan quality of tri-Co-60 intensity-modulated radiation therapy (IMRT) plans for spine stereotactic ablative radiotherapy (SABR). METHODS A total of 20 patients with spine metastasis were retrospectively selected. For each patient, a tri-Co-60 IMRT plan and a volumetric-modulated arc therapy (VMAT) plan were generated. The spinal cords were defined based on MR images for the tri-Co-60 IMRT, while isotropic 1-mm margins were added to the spinal cords for the VMAT plans. The VMAT plans were generated with 10-MV flattening filter-free photon beams of TrueBeam STx™ (Varian Medical Systems, Palo Alto, CA), while the tri-Co-60 IMRT plans were generated with the ViewRay™ system (ViewRay inc., Cleveland, OH). The initial prescription dose was 18 Gy (1 fraction). If the tolerance dose of the spinal cord was not met, the prescription dose was reduced until the spinal cord tolerance dose was satisfied. RESULTS The mean dose to the target volumes, conformity index and homogeneity index of the VMAT and tri-Co-60 IMRT were 17.8 ± 0.8 vs 13.7 ± 3.9 Gy, 0.85 ± 0.20 vs 1.58 ± 1.29 and 0.09 ± 0.04 vs 0.24 ± 0.19, respectively. The integral doses and beam-on times were 16,570 ± 1768 vs 22,087 ± 2.986 Gy cm3 and 3.95 ± 1.13 vs 48.82 ± 10.44 min, respectively. CONCLUSION The tri-Co-60 IMRT seems inappropriate for spine SABR compared with VMAT. Advances in knowledge: For spine SABR, the tri-Co-60 IMRT is inappropriate owing to the large penumbra, large leaf width and low dose rate of the ViewRay system.

Correlation analysis between 2D and quasi-3D gamma evaluations for both intensity-modulated radiation therapy and volumetric modulated arc therapy

The aim of this work was to investigate correlations between 2D and quasi-3D gamma passing rates. A total of 20 patients (10 prostate cases and 10 head and neck cases, H&N) were retrospectively selected. For each patient, both intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans were generated. For each plan, 2D gamma evaluation with radiochromic films and quasi-3D gamma evaluation with fluence measurements were performed with both 2%/2 mm and 3%/3 mm criteria. Gamma passing rates were grouped together according to delivery techniques and treatment sites. Statistical analyses were performed to examine the correlation between 2D and quasi-3D gamma evaluations. Statistically significant difference was observed between delivery techniques only in the quasi-3D gamma passing rates with 2%/2 mm. Statistically significant differences were observed between treatment sites in the 2D gamma passing rates (differences of less than 8%). No statistically significant correlations were observed between 2D and quasi-3D gamma passing rates except the VMAT group and the group including both IMRT and VMAT with 3%/3 mm (r = 0.564 with p = 0.012 for theVMAT group and r = 0.372 with p = 0.020 for the group including both IMRT and VMAT), however, those were not strong. No strong correlations were observed between 2D and quasi-3D gamma evaluations.

A Patient-Specific Polylactic Acid Bolus Made by a 3D Printer for Breast Cancer Radiation Therapy.

Purpose The aim of this study was to assess the feasibility and advantages of a patient-specific breast bolus made using a 3D printer technique. Methods We used the anthropomorphic female phantom with breast attachments, which volumes are 200, 300, 400, 500 and 650 cc. We simulated the treatment for a right breast patient using parallel opposed tangential fields. Treatment plans were used to investigate the effect of unwanted air gaps under bolus on the dose distribution of the whole breast. The commercial Super-Flex bolus and 3D-printed polylactic acid (PLA) bolus were applied to investigate the skin dose of the breast with the MOSFET measurement. Two boluses of 3 and 5 mm thicknesses were selected. Results There was a good agreement between the dose distribution for a virtual bolus generated by the TPS and PLA bolus. The difference in dose distribution between the virtual bolus and Super-Flex bolus was significant within the bolus and breast due to unwanted air gaps. The average differences between calculated and measured doses in a 200 and 300 cc with PLA bolus were not significant, which were -0.7% and -0.6% for 3mm, and -1.1% and -1.1% for 5 mm, respectively. With the Super-Flex bolus, however, significant dose differences were observed (-5.1% and -3.2% for 3mm, and -6.3% and -4.2% for 5 mm). Conclusion The 3D-printed solid bolus can reduce the uncertainty of the daily setup and help to overcome the dose discrepancy by unwanted air gaps in the breast cancer radiation therapy.

Confidence limits for patient-specific IMRT dose QA: a multi-institutional study in Korea

This study aims to investigate tolerance levels for patient-specific IMRT dose QA (DQA) using the confidence limits (CL) determined by a multi-institutional study. Eleven institutions participated in the multi-institutional study in Korea. A total of 155 DQA measurements, consisting of point-dose differences (high- and low-dose regions) and gamma passing rates (composite and per-field) for IMRT patients with brain, head and neck (H&N), abdomen, and prostate cancers were examined. The Shapiro-Wilk test was used to evaluate the normality of data grouped by the treatment sites and the DQA methods. The confidence limit coefficients in cases of the normal distribution, and the two-sided Students t-distribution were applied to determine the confidence limits for the grouped data. The Spearmans test was applied to assess the sensitivity of DQA results within the limited groups. The differences in CLs between the two confidence coefficients based on the normal and t-distributions were negligible for the point-dose data and the gamma passing rates with 3%/3 criteria. However, with 2%/2 criteria, the difference in CLs were 1.6% and 2.2% for composite and per-field measurements, respectively. This resulted from the large standard deviation and the more sensitive criteria of 2%/2. There was no noticeable correlation among the different QA methods. Our multi-institutional study suggested that the CL was not a suitable metric for defining the tolerance level when the statistics of the sample group did not follow the normality and had a large standard deviation. PACS number: 87.55.Qr.This study aims to investigate tolerance levels for patient‐specific IMRT dose QA (DQA) using the confidence limits (CL) determined by a multi‐institutional study. Eleven institutions participated in the multi‐institutional study in Korea. A total of 155 DQA measurements, consisting of point‐dose differences (high‐ and low‐dose regions) and gamma passing rates (composite and per‐field) for IMRT patients with brain, head and neck (H&N), abdomen, and prostate cancers were examined. The Shapiro‐Wilk test was used to evaluate the normality of data grouped by the treatment sites and the DQA methods. The confidence limit coefficients in cases of the normal distribution, and the two‐sided Students t‐distribution were applied to determine the confidence limits for the grouped data. The Spearmans test was applied to assess the sensitivity of DQA results within the limited groups. The differences in CLs between the two confidence coefficients based on the normal and t‐distributions were negligible for the point‐dose data and the gamma passing rates with 3%/3 criteria. However, with 2%/2 criteria, the difference in CLs were 1.6% and 2.2% for composite and per‐field measurements, respectively. This resulted from the large standard deviation and the more sensitive criteria of 2%/2. There was no noticeable correlation among the different QA methods. Our multi‐institutional study suggested that the CL was not a suitable metric for defining the tolerance level when the statistics of the sample group did not follow the normality and had a large standard deviation. PACS number: 87.55.Qr

A comparison of treatment plan quality between Tri-Co-60 intensity modulated radiation therapy and volumetric modulated arc therapy for cervical cancer

PURPOSE To investigate the plan quality of tri-Co-60 intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) for cervical cancer. METHODS A total of 20 patients who received postoperative radiotherapy for cervical cancer were selected. For each patient, a tri-Co-60 IMRT plan for which the target volume was the planning target volume (PTV) generated by adding 1mm isotropic margins from the clinical target volume (CTV) and a VMAT plan for which the target volume was the PTV generated by adding 7mm and 10mm margins from the CTV were generated. The tri-Co-60 IMRT plans were generated with the ViewRay™ system while the VMAT plans were generated with 15-MV photon beams from a linear accelerator (prescription dose=50.4Gy in 28 fractions). RESULTS The average volumes of the PTVs and CTVs were 704.9cc±87.8cc and 271.6cc±51.6cc, respectively. No noticeable differences in the dose-volumetric parameters for the target volumes were observed between the tri-Co-60 IMRT and VMAT plans. The values of V40Gy for the small bowel and rectal wall, V45Gy of the bladder, and V35Gy of the femoral heads for the VMAT plans were 14.6%±7.8%, 54.4%±4.2%, 30.0%±4.7%, and 8.9%±3.3%, respectively. Those of the tri-Co-60 IMRT plans were 2.8%±2.1%, 23.0%±8.9%, 17.1%±6.1%, and 0.3%±0.4%, respectively. CONCLUSIONS Owing to the target margin reduction capability, the tri-Co-60 IMRT plans were more favorable than the VMAT plans for cervical cancer.