Shogo Hatanaka

Quantification of Cold Spots Caused by Geometrical Uncertainty in Field-in-field Techniques for Whole Breast Radiotherapy

OBJECTIVEnTo quantify the cold spot under geometrical uncertainties in field-in-field techniques for whole breast radiotherapy.nnnMETHODSnTen consecutive patients from both the left- and right-sided treatment site groups who received whole breast radiotherapy with the field-in-field technique were included. Virtual plans were made with moving isocenters to the posterior direction having two amplitudes (5 and 10 mm) and prescribing the same monitor unit as the original plan (FIF_5 and FIF_10). The planning target volume for evaluation was defined by subtracting the areas within 5 mm from the skin and within 5 mm from the lung from the whole breast. The differences in V90, V95 and D98 of planning target volume for evaluation were measured between the original and virtual plans. As a reference, the same measurements were taken for the wedge techniques (Wedge_5 and Wedge_10).nnnRESULTSnThe differences in V95 were -0.2% on FIF_5, -1.7% on FIF_10, -0.5% on Wedge_5 and -1.5% on Wedge_10. The differences in V90 were -0.02% on FIF_5, -0.3% on FIF_10, -0.05% on Wedge_5 and -0.1% on Wedge_10. The differences in D98 were 0 Gy on FIF_5, -0.1 Gy on FIF_10, -0.2 Gy on Wedge_5 and -0.4 Gy on Wedge_10. The differences in D98 between the original plans and virtual scenarios for field-in-field techniques were significantly smaller than those for wedge techniques, but there were no statically significant differences in V90 and V95.nnnCONCLUSIONSnThe quantity of the cold spots caused by the geometrical uncertainties in field-in-field techniques was similar to that for the wedge techniques and was acceptable.

Utility of Smart Arc CDR for intensity-modulated radiation therapy for prostate cancer.

Volumetric-modulated arc therapy (VMAT) is a widespread intensity-modulated radiation therapy (IMRT) method, however, VMAT requires adaptation of the radiation treatment planning system (RTPS) and linear accelerator (linac); these upgrades are quite expensive. The Smart Arc of Pinnacle3 (Philips), which is the software used in VMAT calculations, can select constant dose rate (CDR) mode. This approach has a low initial cost because the linac upgrade is not required. The objective of this study was to clarify the utility of CDR mode for prostate IMRT. Pinnacle3 and Clinac 21EX linac (Varian, 10 MV X-rays) were used for planning. The plans were created for 28 patients using a fixed multi-field IMRT (f-IMRT), VMAT and CDR techniques. The dose distribution results were classified into three groups: optimal, suboptimal and reject. For the f-IMRT, VMAT and CDR results, 25, 26 and 21 patients were classified as ‘optimal’, respectively. Our results show a significant reduction in the achievement rate of ‘optimal’ for a CDR when the bladder volume is <100 cm3. The total numbers of monitoring units (MUs) (average ± 1σ) were 469 ± 53, 357 ± 35 and 365 ± 33; the average optimization times were ∼50 min, 2 h and 2 h 40 min, and the irradiation times were ∼280 s, 60 s and 110 s, respectively. CDR can reduce the total MUs and irradiation time compared with f-IMRT, and CDR has a lower initial cost compared with VMAT. Thus, for institutions that do not currently perform VMAT, CDR is a useful option. Additionally, in the context of patient identification, bladder volume may be useful.

Effects of Geometrical Uncertainties on Whole Breast Radiotherapy: A Comparison of Four Different Techniques

Purpose The purpose of this study was to quantify the target coverage, homogeneity, and robustness of the dose distributions against geometrical uncertainties associated with four whole breast radiotherapy techniques. Methods The study was based on the planning-computed tomography-datasets of 20 patients who underwent whole breast radiotherapy. A total of four treatment plans (wedge, field-in-field [FIF], hybrid intensity-modulated radiotherapy [IMRT], and full IMRT) were created for each patient. The hybrid IMRT plans comprised two opposed tangential open beams plus two IMRT beams. Setup errors were simulated by moving the beam isocenters by 5 mm in the anterior or posterior direction. Results With the original plan, the wedge technique yielded a high volume receiving ≥107% of the prescription dose (V107; 7.5%±4.2%), whereas the other three techniques yielded excellent target coverage and homogeneity. A 5 mm anterior displacement caused a large and significant increase in the V107 (+5.2%±4.1%, p<0.01) with the FIF plan, but not with the hybrid IMRT (+0.4%±1.2%, p=0.11) or full IMRT (+0.7%±1.8%, p=0.10) plan. A 5-mm posterior displacement caused a large decrease in the V95 with the hybrid IMRT (-2.5%±3.7%, p<0.01) and full IMRT (-4.3%±5.1%, p<0.01) plans, but not with the FIF plan (+0.1%±0.7%, p=0.74). The decrease in V95 was significantly smaller with the hybrid IMRT plan than with the full IMRT plan (p<0.01). Conclusion The FIF, hybrid IMRT, and full IMRT plans offered excellent target coverage and homogeneity. Hybrid IMRT provided better robustness against geometrical uncertainties than full IMRT, whereas FIF provided comparable robustness to that of hybrid IMRT.

Gas in the rectum tends to reduce during radical external beam radiotherapy for localised prostate cancer

This study aims to clarify the time‐course of gas accumulation in the rectum during treatment as guidance for the management of rectal volumes.

Identifying patients who are unsuitable for accelerated partial breast irradiation using three-dimensional external beam conformal techniques.

PURPOSEnSeveral recent studies reported that severe late toxicities including soft-tissue fibrosis and fat necrosis are present in patients treated with accelerated partial breast irradiation (APBI) and that these toxicities are associated with the large volume of tissue targeted by high-dose irradiation. The present study was performed to clarify which patients are unsuitable for APBI to avoid late severe toxicities.nnnMETHODS AND MATERIALSnStudy subjects comprised 50 consecutive patients with Stage 0-II unilateral breast cancer who underwent breast-conserving surgery, and in whom five or six surgical clips were placed during surgery. All patients were subsequently replanned using three-dimensional conformal radiotherapy (3D-CRT) APBI techniques according to the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-39 and Radiation Therapy Oncology Group (RTOG) 0413 protocol. The beam arrangements included mainly noncoplanar four- or five-field beams using 6-MV photons alone.nnnRESULTSnDose-volume histogram (DVH) constraints for normal tissues according to the NSABP/RTOG protocol were satisfied in 39 patients (78%). Multivariate analysis revealed that only long craniocaudal clip distance (CCD) was correlated with nonoptimal DVH constraints (p = 0.02), but that pathological T stage, anteroposterior clip distance (APD), site of ipsilateral breast (IB) (right/left), location of the tumor (medial/lateral), and IB reference volume were not. DVH constraints were satisfied in 20% of patients with a long CCD (≥5.5 cm) and 92% of those with a short CCD (p < 0.0001). Median IB reference volume receiving ≥50% of the prescribed dose (IB-V(50)) of all patients was 49.0% (range, 31.4-68.6). Multivariate analysis revealed that only a long CCD was correlated with large IB-V(50) (p < 0.0001), but other factors were not.nnnCONCLUSIONnPatients with long CCDs (≥5.5 cm) might be unsuitable for 3D-CRT APBI because of nonoptimal DVH constraints and large IB-V(50).

Determination of the appropriate physical density of internal metallic ports in temporary tissue expanders for the treatment planning of post-mastectomy radiation therapy

Abstract Some patients undergoing breast reconstruction require post-mastectomy radiation therapy, but the metallic ports used in temporary tissue expanders attenuate the X-rays. In this study, we evaluated by the film method, the attenuation of 4 MV and 6 MV X-rays after passing through a metallic port, with the aim of identifying a useful method for determining the appropriate density to use in the radiation treatment planning system (RTPS), taking into account the distance between the metallic port and the targets. Radiochromic film was used to measure depth doses after the X-rays passed through the metallic port. The physical density allotted to the metal port portion was varied on the RTPS within the range 1–16 g/cm3, and the physical density values were calculated that best reproduced the depth–dose distribution extrapolated from the film method. When the metallic port was orientated perpendicularly, the attenuation of the X-rays peaked at ~7% at both 4 MV and 6 MV. In the parallel orientation, the X-rays were attenuated by up to ~40% at 4 MV and by up to ~30% at 6 MV. We estimated the optimum physical density to be 9.8 g/cm3, which yielded the best fit with the actual measurements. We demonstrated the most likely range for the target depth from the CT images of actual patients and, within this range, we identified the optimum physical density at which the measured and calculated values were most consistent with each other.

Retrospective analysis of multi-institutional, patient-specific treatment planning results of high-dose-rate intracavitary brachytherapy for gynecological cancer using V100%

The objective of this study was to clarify the usefuleness of the K parameters of the independent verification method using V100% (the volume of water receiving 100% of the prescription dose) for institutions implementing the high-dose-rate (HDR) intracavitary brachytherapy for gynecological cancer. The data of 249 plans of 11 institutions in Japan were used, and the constant K value obtained by a parameter fit for single-192Ir, two-192Ir, and three-192Ir systems was calculated. The predicted total dwell time calculated using the constant K value was defined as Tpr, and the total dwell time calculated using a radiation treatment planning system was defined as TRTP. The ratio of Tpr and TRTP for each plan was calculated. The constant K values (95% CI) obtained for each system outlined above were 1233 (1227–1240), 1205 (1199–1211), and 1171 (1167–1175), respectively. Regarding the Tpr/TRTP, the entire data were within 0.9–1.1. For accurate verification, it was clarified that constant K values should be calculated for each system. The Nuclear Regulatory Commission considers a difference of 20% between the prescribed total dose and the administered total dose as a reportable medical event. There is a need for a quick method to verify the accuracy with a minimum of 10% threshold of a plan. The constant K values in this study were obtained from multiple institutions, and the variation in the values among these institutions was small. The data obtained by this study may be used as a parameter of this verification method employed by numerous institutions, particularly those who have recently initiated HDR brachytherapy. In addition, for institutions already using this method, this data might be useful for the validation of the parameters which were used in such institutions.

Dose calculation accuracies in whole breast radiotherapy treatment planning: a multi-institutional study

AbstractnOur objective in this study was to evaluate the variation in the doses delivered among institutions due to dose calculation inaccuracies in whole breast radiotherapy. We have developed practical procedures for quality assurance (QA) of radiation treatment planning systems. These QA procedures are designed to be performed easily at any institution and to permit comparisons of results across institutions. The dose calculation accuracy was evaluated across seven institutions using various irradiation conditions. In some conditions, there was a >3xa0% difference between the calculated dose and the measured dose. The dose calculation accuracy differs among institutions because it is dependent on both the dose calculation algorithm and beam modeling. The QA procedures in this study are useful for verifying the accuracy of the dose calculation algorithm and of the beam model before clinical use for whole breast radiotherapy.

Quantitative analysis of in-air output ratio

Output factor (Scp) is one of the important factors required to calculate monitor unit (MU), and is divided into two components: phantom scatter factor (Sp) and in-air output ratio (Sc). Generally, Sc for arbitrary fields are calculated using several methods based on Sc determined by the absorbed dose measurement for several square fields. However, there are calculation errors when the treatment field has a large aspect ratio and the opening of upper and lower collimator are exchanged. To determine Sc accurately, scattered photons from the treatment head and backscattered particles into the monitor chamber must be analyzed individually. In this report, a simulation model that agreed well with measured Sc was constructed and dose variation by scattered photons from the treatment head and by backscattered particles into the monitor chamber was analyzed quantitatively. The results showed that the contribution of scattered photons from the primary collimator was larger than that of the flattening filter, and backscattered particles were affected by not only the upper jaw but also the lower jaw. In future work, a new Sc determination algorism based on the result of this report will be proposed.