Hippocampal dose during Linac-based stereotactic radiotherapy for brain metastases: An observational study

IntroductionAim of the present study is to evaluate homolateral and contralateral hippocampus (H-H, C-H, respectively) dose during Fractionated Stereotactic Radiotherapy (FSRT) or Radiosurgery (SRS) for brain metastases (BM).Materials & methodsPatients with BM < 5, size ≤ 30 mm, KPS ≥ 80 and a life expectancy > 3 months, were considered for SRS/FSRT (total dose 15–30 Gy, 1–5 fractions). For each BM, a Flattening Filter Free (FFF) Volumetric Modulated Arc Therapy (VMAT) plan was generated with one or two arcs. Hippocampi were not considered during optimizations phase and were contoured and evaluated retrospectively in terms of dose: the Dmedian, Dmean, D0.1cc and the V1Gy, V2Gy, V5Gy and V10Gy were analyzed.ResultsFrom April 2014 to December 2015, 81 BM were treated with FFF-FSRT/SRS. For the H-H, the average values of Dmedian, Dmean and D0.1cc were 1.5Gy, 1.54Gy and 2.2Gy, respectively, while the V1Gy, V2Gy, V5Gy and V10Gy values were 25%, 8.9%, 8.9% and 2.1%, respectively. For the C–H, the average Dmedian, Dmean and D0.1 cc were 0.7Gy, 0.7Gy, 0.9Gy, respectively, while the average values of V1Gy, V2Gy, V5Gy and V10Gy were 18%, 10.2%, 2.8% and 1.4%, respectively. Tumor dimension, tumor cranial-caudal length and the distance between BM and H-H were correlated to Dmedian, Dmean and D0.1cc. For C-H, only the distance from PTV was correlated with a dose reduction.ConclusionDuring FFF-FSRT/SRS, hippocampus received a negligible dose. Despite its clinical significance is still under evaluation, in patients with a long life expectancy, H-H should be considered during Linac-based FSRT/SRS.     

Effect of patient positioning on carbon-ion therapy planned dose distribution to pancreatic tumors and organs at risk

PurposePancreatic tumor treatment dose distribution variations associated with supine and prone patient positioning were evaluated.MethodsA total of 33 patients with pancreatic tumors who underwent CT in the supine and prone positions were analyzed retrospectively. Gross tumor volume (GTV), planning target volume (PTV), and organs at risk (OARs) (duodenum and stomach) were contoured. The prescribed dose of 55.2 Gy (RBE) was planned from four beam angles (0°, 90°, 180°, and 270°). Patient collimator and compensating boli were designed for each field. Dose distributions were calculated for each field in the supine and prone positions. To improve dose distribution, patient positioning was selected from supine or prone for each beam field.ResultsCompared with conventional beam angle and patient positioning, D2cc of 1st-2nd portion of duodenum (D1-D2), 3rd-4th portion of duodenum (D3-D4), and stomach could be reduced to a maximum of 6.4 Gy (RBE), 3.5 Gy (RBE), and 4.5 Gy (RBE) by selection of patient positioning. V10 of D1-D2, D3-D4, and stomach could be reduced to a maximum of 7.2 cc, 11.3 cc, and 11.5 cc, respectively. D95 of GTV and PTV were improved to a maximum of 6.9% and 3.7% of the prescribed dose, respectively.ConclusionsOptimization of patient positioning for each beam angle in treatment planning has the potential to reduce OARs dose maintaining tumor dose in pancreatic treatment.     

Gated carbon-ion scanning treatment for pancreatic tumour with field specific target volume and organs at risk

ObjectiveTo assess the feasibility of treatment planning for pancreatic tumours subject to respiratory motion using field-specific target volumes (FTV) and field-specific organs at risk (FOAR) using four-dimensional computed tomography (4DCT).MethodsFourteen pancreatic cancer patients underwent 4DCT. Radiation oncologists contoured the gross tumour volume (GTV), clinical target volume (CTV), spinal cord, duodenum, kidneys, and stomach. The gating duty cycle was set to 30 % around exhalation. FTV and FOAR were calculated using the 4DCT dataset. Planning target volumes (PTV) and planning organs at risk volumes (PRV) were defined as equal to FTV and FOAR, respectively. A dose of 55.2 Gy relative biological effectiveness (RBE) was planned to target the PTV from four beam angles. A single field uniform dose (SFUD) plan was selected. The dose distribution, including intrafractional motion changes, was generated.ResultsThe mean volume of target receiving 95 % of the planned doses was 96.4 ± 4.1 % to the GTV and 94.7 ± 0.9 % to the CTV. The highest dose to 2 cc of duodenal volume was 27.5 Gy (RBE). The volume of the stomach receiving ⩾30 Gy (RBE) was <7.0 cc in all patients. All metrics for OARs satisfied dose constraints.ConclusionDose to the CTV was covered sufficiently by the 4DCT-generated FTV, and dose to OARs was reduced by 4DCT-generated FOAR. This methodology may prevent adverse reactions while preserving local tumour control.     

Segmented photon beams technique for irradiation of postmastectomy patients

AimTo present the segmented photon beams technique (SPBT) for irradiation of postmastectomy patients.BackgroundIn majority of techniques for irradiation of posmastectomy patients, a few adjacent photon or electron beams were usually implemented in order to encompass different parts of the target. In the presented SPBT technique, the radiotherapy plan consists of 6 isocentric photon beams and the area CTV includes both the chest wall and the supraclavicular area. This makes it possible to provide a uniform dose to the CTV with no hot and cold points and enables the determination of doses for the entire volume of critical organs.Methods and materialThe treatment forward-IMRT plan comprises six isocentric 4 and 15 MV photon beams. Modulation of the dose distribution for each field was obtained by applying three segments on average. The total dose of 45 Gy was administered in 20 fractions. Dose distributions in target volume and organs at risk were evaluated for 70 randomly chosen patients.ResultsOn average, 94.8% of the CTV volume received doses within 95–107% of the prescribed dose. The average volume of the heart receiving a dose of 30 Gy and lager was 2% for patients with left breast cancer. The average dose to the lung on the irradiation side was always lower than 15.5 Gy and the average V20 Gy was below 35.5%.ConclusionsThe SPBT complies with requirements for high dose homogeneity within the target volume and satisfactory level of sparing of organs at risk.     

In silico comparison of photons versus carbon ions in single fraction therapy of lung cancer

PurposeStereotactic body image guided radiation therapy (SBRT) shows good results for lung cancer treatment. Better normal tissue sparing might be achieved with scanned carbon ion therapy (PT). Therefore an in silico trial was conducted to find potential advantages of and patients suited for PT.MethodsFor 19 patients treated with SBRT, PT plans were calculated on 4D-CTs with simulated breathing motion. Prescribed single fraction dose was 24 Gy and OAR constraints used for photon planning were respected. Motion was mitigated by rescanning and range-adapted ITVs. Doses were compared to the original SBRT plans.ResultsCTV coverage was the same in SBRT and PT. The field-specific PTV including range margins for PT was 1.5 (median, 25–75% 1.3–2.1) times larger than for SBRT. Nevertheless, maximum point dose and mean dose in OARs were higher in SBRT by 2.8 (1.6–3.7) Gy and 0.7 (0.3–1.6) Gy, respectively. Patients with a CTV >2.5 cc or with multiple lung lesions showed larger differences in OAR doses in favor of PT.ConclusionsPatients receive less dose in critical OARs such as heart, spinal cord, esophagus, trachea and aorta with PT, while maintaining the same target coverage. Patients with multiple or larger lesions are particularly suited for PT.     

Dose to pelvic lymph nodes in image based high dose rate brachytherapy of carcinoma cervix

AimThe aim of this study is to analyse the dosimetry to the pelvic lymph nodes and its correlation to point B using CT based high dose rate brachytherapy of carcinoma cervix.BackgroundConventionally, dose to pelvic lymph nodes from intracavitary brachytherapy was reported by point B and by the reference points of the lymphatic trapezoid.Materials and methods30 consecutive CT based high dose rate applications were reviewed between February and March 2016. The high risk clinical target volume and the organs at risk and the pelvic nodal groups were contoured. DVH parameters for the right and left obturator nodal group, right and left external iliac nodal group and right and left internal iliac nodal group were recorded. Right and left point B doses were also recorded.ResultsOn analysis of the combined dose, it was found that all the DVH parameters were significantly different from point B, except the D100 obturator and D2cc internal iliac lymph node. There was a significant correlation between all DVH parameters and point B, except D2cc, D1cc and D0.1cc of external iliac. The obturator group received the highest dose contribution from brachytherapy. The mean D90 dose received per fraction for the obturator, external iliac and internal iliac nodes was 2.7 Gy, 1.17 Gy and 1.41 Gy, respectively.ConclusionsThere is a significant dose contribution to the pelvic lymph nodal groups during intracavitary brachytherapy. There is a low degree of correlation between point B dose and dosimetric parameters of the individual nodal groups. Hence, it is important to analyse the dose delivered to individual nodal groups during intracavitary brachytherapy, at least in patients with enlarged lymph nodes to calculate the cumulative dose delivered.     

Dose distribution homogeneity in two TBI techniques—Analysis of 208 irradiated patients conducted in Stanislaw Leszczynski Memorial Hospital,Katowice

BackgroundTo analyze and compare dose distribution homogeneity in selected points (especially in the chest wall region) for patients irradiated with two different TBI techniques to achieve a uniform total dose (excluding lungs area) specified in the range of 11.4–14.0 Gy.Material and methodsFrom August 2000 to December 2009, a group of 158 patients was treated by the use of 15 MV photon irradiation consisting of six fractions: four opposed lateral and two anterior–posterior/posterior–anterior (AP/PA). Patients were irradiated with the fraction dose of 2 Gy twice a day for 3 consecutive days. The prescribed dose to PC point (specified at intersection of the beam axis with the mid-plane of the patient irradiated laterally) was 12 Gy. Since January 2010 until closing the study, another group of 50 patients was treated according to a modified protocol. The treatment was carried out in six lateral fractions only, twice a day, for three following days and a lateral lung shield was used for a part of total irradiation time. The measurements of doses in 20 selected points of patient's body were carried out by means of MOSFET detectors.ResultsThe modified TBI technique allows to achieve an expected homogenous dose in the points of interest similar to that obtained by using the initial protocol. The calculated and measured in vivo doses met the specified range of 11.4–14 Gy for both applied TBI protocols.ConclusionsOur results indicate that for all patients the homogenous dose distribution in the specified range was achieved.     

Local control rates in stereotactic body radiotherapy (SBRT) of lung metastases associated with the biologically effective dose

AimTo evaluate dose differences in lung metastases treated with stereotactic body radiotherapy (SBRT), and the correlation with local control, regarding the dose algorithm, target volume and tissue density.BackgroundSeveral studies showed excellent local control rates in SBRT for lung metastases, with different fractionation schemes depending on the tumour location or size. These results depend on the dose distributions received by the lesions in terms of the tissue heterogeneity corrections performed by the dose algorithms.Materials and methodsForty-seven lung metastases treated with SBRT, using intrafraction control and respiratory gating with internal fiducial markers as surrogates (ExacTrac, BrainLAB AG), were calculated using Pencil Beam (PB) and Monte Carlo (MC) (iPlan, BrainLAB AG).Dose differences between both algorithms were obtained for the dose received by 99% (D_99%) and 50% (D_50%) of the planning treatment volume (PTV). The biologically effective dose delivered to 99% (BED_99%) and 50% (BED_50%) of the PTV were estimated from the MC results. Local control was evaluated after 24 months of median follow-up (range: 3–52 months).ResultsThe greatest variations (40.0% in ΔD_99% and 38.4% in ΔD_50%) were found for the lower volume and density cases. The BED_99% and BED_50% were strongly correlated with observed local control rates: 100% and 61.5% for BED_99% > 85 Gy and <85 Gy (p < 0.0001), respectively, and 100% and 58.3% for BED_50% > 100 Gy and <100 Gy (p < 0.0001), respectively.ConclusionsLung metastases treated with SBRT, with delivered BED_99% > 85 Gy and BED_50% > 100 Gy, present better local control rates than those treated with lower BED values (p = 0.001).     

Monte Carlo study on the secondary cancer risk estimations for patients undergoing prostate radiotherapy: A humanoid phantom study

AimThe aim of this study was to estimate the secondary malignancy risk from the radiation in FFB prostate linac-based radiotherapy for different organs of the patient.BackgroundRadiation therapy is one of the main procedures of cancer treatment. However, the application the radiation may impose dose to organs of the patient which can be the cause of some malignancies.Materials and methodsMonte Carlo (MC) simulation was used to calculate radiation doses to patient organs in 18 MV linear accelerator (linac) based radiotherapy. A humanoid MC phantom was used to calculate the equivalent dose s for different organs and probability of secondary cancer, fatal and nonfatal risk, and other risks and parameters related to megavoltage radiation therapy. In out-of-field radiation calculation, it could be seen that neutrons imparted a higher dose to distant organs, and the dose to surrounding organs was mainly due to absorbed scattered photons and electron contamination.ResultsOur results showed that the bladder and skin with 54.89 × 10⁻³ mSv/Gy and 46.09 × 10⁻³ mSv/Gy, respectively, absorbed the highest equivalent dose s from photoneutrons, while a lower dose was absorbed by the lung at 3.42 × 10⁻³ mSv/Gy. The large intestine and bladder absorbed 55.00 × 10⁻³ mSv/Gy and 49.08 × 10⁻³, respectively, which were the highest equivalent dose s due to photons. The brain absorbed the lowest out-of-field dose, at 1.87 × 10⁻³ mSv/Gy.ConclusionsWe concluded that secondary neutron portion was higher than other radiation. Then, we recommended more attention to neutrons in the radiation protection in linac based high energy radiotherapy.     

Nasopharyngeal carcinoma in Slovenia, 1990–2003 (results of treatment with conventional two-dimensional radiotherapy)

AimTo review the treatment results and identify prognostic factors for disease control and survival in a cohort of nasopharyngeal carcinoma (NPC) patients from a non-endemic population in Slovenia, diagnosed between 1990 and 2003.BackgroundIn Caucasians, nasopharyngeal carcinoma is a rare malignant tumor. Its diagnosis and treatment are complex and have been dramatically impacted by recent technological advances.Materials and methodsIn the Cancer Registry of Slovenia database, a total of 126 patients with NPC were identified, 93 of whom were available for analysis. All patients were treated with conventional two-dimensional radiotherapy (RT) and 29.3% underwent chemotherapy (ChT).ResultsThe median follow-up time for those alive at the last follow-up examination was 74.5 months. Disease recurred locally in 17 patients, regionally in 4 patients and at distant sites in 18 patients, resulting in 5-year locoregional control (LRC), distant failure-free survival (DFFS) and disease-free survival (DFS) of 73.7%, 78.6% and 59.3%, respectively. Disease-specific survival at 5 years was 59% and overall survival (OS) was 49.7%. In a multivariate analysis, LRC was favorably affected (P < 0.05) by an undifferentiated histology (hazard ratio [HR] = 2.86), DFFS through the absence of neck metastases (HR = 0.28), DFS by younger age (HR = 0.46), and more intensive RT (expressed as the isoeffective dose, EQD_2,T; HR = 2.08). The independent prognosticator for OS was age (≤55 years vs. >55 years, HR = 0.39); in the ≤55 years subgroup, an improved OS was connected to a more intensive RT regimen of EQD_2,T ≥ 66 Gy (HR = 4.17).ConclusionsOur results confirm an independent and favorable effect from an undifferentiated histology, the absence of neck metastases, a younger patient age at diagnosis, and more intensive RT regimens for disease control and survival.     

Bladder dose–surface maps and urinary toxicity: Robustness with respect to motion in assessing local dose effects

The purpose of this study was to quantify the impact of inter-fraction modifications of bladder during RT of prostate cancer on bladder dose surface maps (DSM).Eighteen patients treated with daily image-guided Tomotherapy and moderate hypofractionation (70–72.8 Gy at 2.5–2.6 Gy/fr in 28 fractions and full bladder) were considered. Bladder contours were delineated on co-registered daily Megavoltage CT (MVCT) by a single observer and copied on the planning CT to generate dose–volume/surface histograms (DVH/DSH) and bladder DSMs. Discrepancies between planned and daily absorbed doses were analyzed through the average of individual systematic errors, the population systematic errors and the population random errors for the DVH/DSHs and DSMs.In total, 477 DVH/DSH and 472 DSM were available. DSH and DVH showed small population systematic errors of absolute surfaces (<3.4 cm²) and volumes (<8.4 cm³) at the highest doses.The dose to the posterior bladder base assessed on DSMs showed a mean systematic error below 1 Gy, with population systematic and random errors within 4 and 3 Gy, respectively. The region surrounding this area shows higher mean systematic errors (1–3 Gy), population systematic (8–11 Gy) and random (5–7 Gy) errors.In conclusion, DVH/DSH and DSMs are quite stable with respect to inter-fraction variations in the high-dose region, within about 2 cm from bladder base. Larger systematic variations occur in the anterior portion and cranially 2.5–3.5 cm from the base.Results suggest that dose predictors related to the high dose area (including the trigone dose) are likely to be sufficiently reliable with respect to the expected variations due to variable bladder filling.     

A dosimetric evaluation of IGART strategies for cervix cancer treatment

PurposeImage guided adaptive radiotherapy (IGART) strategies can be used to include the temporal aspects of radiotherapy treatment. A dosimetric evaluation of on- and off-line adaptive strategies are done in this study.MethodsA library of equivalent uniform dose (EUD)-based Intensity Modulated Radiotherapy Treatment plans with incrementally increasing clinical target volume (CTV)-to-planning target volume (PTV) margins were developed for 10 patients. Utilizing daily computed tomography (CT) images an on-line strategy using a margin-of-the-day (MOD) concept that selects the best plan from the library was employed. This was compared to an off-line strategy with full analysis of accumulated dose between fractions where dosimetric deviations from the treatment intent triggered plan adaptation. A fixed margin treatment approach was used as benchmark.ResultsUsing fixed margins of <15 mm lead to under-dosages of more than 5 Gy in total delivered dose. The average CTV EUD for the off-line and on-line strategy was 50.0 ± 5.0 Gy and 50.4 ± 2.0 Gy respectively and OAR doses were comparable.ConclusionA fixed margin treatment approach yields a significant probability of CTV under-dosage. Using EUD dose metrics CTV coverage can be restored in both the off-line and on-line adaptive strategies at acceptable OAR dose levels. Considering the workload and time on the treatment machine, the off-line strategy proves to be sufficient and more practical.     

Dosimetric impact of 4DCT artifact in carbon-ion scanning beam treatment: Worst case analysis in lung and liver treatments

IntroductionWe evaluated the impact of 4DCT artifacts on carbon-ion pencil beam scanning dose distributions in lung and liver treatment.Methods & materials4DCT was performed in 20 liver and lung patients using area-detector CT (original 4DCT). 4DCT acquisition by multi-detector row CT was simulated using original 4DCT by selecting other phases randomly (plus/minus 20% phases). Since tumor position can move over the respiratory range in original 4DCT, mid-exhalation was set as reference phase. Total prescribed dose of 60 Gy (RBE) was delivered to the clinical target volume (CTV). Reference dose distribution was calculated with the original CT, and actual dose distributions were calculated with treatment planning parameters optimized using the simulated CT (simulated dose). Dose distribution was calculated by substituting these parameters into the original CT.ResultsFor liver cases, CTV-D95 and CTV-Dmin values for the reference dose were 97.6 ± 0.5% and 89.8 ± 0.6% of prescribed dose, respectively. Values for the simulated dose were significantly degraded, to 88.6 ± 14.0% and 46.3 ± 26.7%, respectively. Dose assessment results for lung cases were 84.8 ± 12.8% and 58.0 ± 24.5% for the simulated dose, showing significant degradation over the reference dose of 95.1 ± 1.5% and 87.0 ± 2.2%, respectively.Conclusions4DCT image quality should be closely checked to minimize degradation of dose conformation due to 4DCT artifacts. Medical staff should pay particular attention to checking the quality of 4DCT images as a function of respiratory phase, because it is difficult to recognize 4DCT artifact on a single phase in some cases     

Hyperbaric oxygen therapy for radiation-induced brachial plexopathy,a case report and literature review

AimTo report a case of radiation-induced brachial plexopathy (RIBP) with significant radiographic and clinical improvement after a course of hyperbaric oxygen (HBO).BackgroundRIBP is a rare complication after radiotherapy to the neck and axilla. There are no standard treatment options, with empirical use pharmacotherapy being predominately used, which has had mixed results.HBO is efficacious for the treatment of other severe radiation-induced side effects, however, its benefit in RIBP has conflicting reports.Case PresentationA 45-year-old male, with a 33 pack-year smoking history, presented with a 6-month history of a progressive left neck mass. The final diagnosis was unknown primary squamous cell carcinoma of the head and neck. He received intensity-modulated radiation therapy (IMRT) with 70 Gy prescribed to the gross tumor volume (PTV HR) and 56 Gy to the oropharynx, nasopharynx, and bilateral lymphatics (PTV SR) in 35 daily fractions with three cycles of concurrent cisplatin at 100 mg/m².Fifteen months following therapy completion, the patient began to endorse symptoms of left brachial plexopathy. Decadron was prescribed for 2 weeks, trental and vitamin E for 6 months, and HBO. The patient returned for follow-up 2 months after completing 30 dives of HBO at 2.4 atmospheres for 2 hours per session. He reported pain resolution and full range of motion of his left arm.ConclusionsThe best management strategy of RIBP is prevention by reducing total RT doses and close follow-up. However, when RIBP occurs, we recommend treatment with HBO therapy, steroids, trental, and vitamin E as tolerable.     

Radiation therapy for the management of painful bone metastases: Results from a randomized trial

AimThe aim of this study was to compare the effectiveness of two radiotherapy schedules in patients with bone metastases.BackgroundWe analyzed the need for re-irradiation, rates of pain control, pathological fractures, and functionality in patients randomized to single-fraction (8 Gy 1×) or multiple-fraction radiotherapy (3 Gy 10×) with at least 12 months follow-up, during five years. The hypothesis was that the two radiotherapy schedules are equally effective.Materials and methodsNinety patients with painful skeletal metastases were randomized to receive single fraction (8 Gy) or multiple fraction (3 Gy 10×) radiotherapy.ResultsIn the single-fraction group, seven pathological fractures occurred (15.5%) versus two (4.4%) in the multiple-fraction group. There was no statistically significant difference between the time it took to suffer a pathological fracture in both groups (p = 0.099). Patients in the single-fraction group received twelve re-irradiations (26.6%), four in the multiple-fraction group (8.8%), with no significant difference between time elapsed before the first re-irradiation (p = 0.438).ConclusionThis study shows no difference between the two groups for the majority of patients with painful bone metastases. Patients were followed up during five years, and the trial showed no disadvantage for 8 Gy 1× compared to 3 Gy 10×. Despite the fact that the pathological fracture rate is 3.75 times higher in the single-fraction group, this schedule is considered more convenient for patients and more cost-effective for radiotherapy departments.     

Utilization of cone-beam CT for offline evaluation of target volume coverage during prostate image-guided radiotherapy based on bony anatomy alignment

AimTo assess target volume coverage during prostate image-guided radiotherapy based on bony anatomy alignment and to assess possibility of safety margin reduction.BackgroundImplementation of IGRT should influence safety margins. Utilization of cone-beam CT provides current 3D anatomic information directly in irradiation position. Such information enables reconstruction of the actual dose distribution.Materials and methodsSeventeen prostate patients were treated with daily bony anatomy image-guidance. Cone-beam CT (CBCT) scans were acquired once a week immediately after bony anatomy alignment. After the prostate, seminal vesicles, rectum and bladder were contoured, the delivered dose distribution was reconstructed. Target dose coverage was evaluated by the proportion of the CTV encompassed by the 95% isodose. Original plans employed a 1 cm safety margin. Alternative plans assuming a smaller 7 mm margin between CTV and PTV were evaluated in the same way. Rectal and bladder volumes were compared with the initial ones. Rectal and bladder volumes irradiated with doses higher than 75 Gy, 70 Gy, 60 Gy, 50 Gy and 40 Gy were analyzed.ResultsIn 12% of reconstructed plans the prostate coverage was not sufficient. The prostate underdosage was observed in 5 patients. Coverage of seminal vesicles was not satisfactory in 3% of plans. Most of the target underdosage corresponded to excessive rectal or bladder filling. Evaluation of alternative plans assuming a smaller 7 mm margin revealed 22% and 11% of plans where prostate and seminal vesicles coverage, respectively, was compromised. These were distributed over 8 and 7 patients, respectively.ConclusionSufficient dose coverage of target volumes was not achieved for all patients. Reducing of safety margin is not acceptable. Initial rectal and bladder volumes cannot be considered representative for subsequent treatment.     

Quantification of residual dose estimation error on log file-based patient dose calculation

PurposeThe log file-based patient dose estimation includes a residual dose estimation error caused by leaf miscalibration, which cannot be reflected on the estimated dose. The purpose of this study is to determine this residual dose estimation error.Methods and materialsModified log files for seven head-and-neck and prostate volumetric modulated arc therapy (VMAT) plans simulating leaf miscalibration were generated by shifting both leaf banks (systematic leaf gap errors: ±2.0, ±1.0, and ±0.5 mm in opposite directions and systematic leaf shifts: ±1.0 mm in the same direction) using MATLAB-based (MathWorks, Natick, MA) in-house software. The generated modified and non-modified log files were imported back into the treatment planning system and recalculated. Subsequently, the generalized equivalent uniform dose (gEUD) was quantified for the definition of the planning target volume (PTV) and organs at risks.ResultsFor MLC leaves calibrated within ±0.5 mm, the quantified residual dose estimation errors that obtained from the slope of the linear regression of gEUD changes between non- and modified log file doses per leaf gap are in head-and-neck plans 1.32 ± 0.27% and 0.82 ± 0.17 Gy for PTV and spinal cord, respectively, and in prostate plans 1.22 ± 0.36%, 0.95 ± 0.14 Gy, and 0.45 ± 0.08 Gy for PTV, rectum, and bladder, respectively.ConclusionsIn this work, we determine the residual dose estimation errors for VMAT delivery using the log file-based patient dose calculation according to the MLC calibration accuracy.     

Efficacy of tangential irradiation with volumetric modulated arc therapy on scalp angiosarcoma using medical linac

PurposeTo increase the superficial dose and reduce the brain dose for radiotherapy of scalp angiosarcoma, we propose a novel irradiation technique of tangential irradiation volumetric modulated arc therapy (TI-VMAT).MethodsTI-VMAT and the conventional VMAT treatment plans for thirteen scalp angiosarcoma patients were created with a prescribed dose of 70 Gy. Each treatment was normalized to cover 95% of the planning target volume (PTV) with its prescribed dose. To realize TI-VMAT, an avoidance structure (AS) function was applied. AS was defined as a contour subtracted PTV by a certain space from the brain contour. TI-VMAT treatment plans for six different spaces between PTV and AS were developed and compared with the conventional VMAT treatment plan with respect to the following dosimetric parameters: homogeneity index (HI) and conformity index (CI) of the PTV, mean brain dose, and brain volume irradiated with 20% (V_20% [cc]), 40% (V_40% [cc]), 60% (V_60% [cc]), 80% (V_80% [cc]), and 100% (V_100% [cc]) of the prescribed dose.ResultsHI and CI were comparable between TI-VMAT and the conventional VMAT, the mean brain dose for TI-VMAT with AS defined by a space of 2.0 cm and jaw tracking was 14.27 Gy, which was significantly lower than that for the conventional VMAT (21.20 Gy). In addition, dosimetric parameters such as V_20% [cc] were significantly suppressed compared to those for high doses.ConclusionOur proposed irradiation technique TI-VMAT shows the potential to reduce radiation doses in the brain with maintaining higher dose coverage on the PTV.