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.     

Comparison of manual and inverse optimisation techniques in high dose rate intracavitary brachytherapy of cervical cancer: A dosimetric study

Aims and objectivesTo compare dosimetrically the manual optimisation with IPSA using dose volume histograms (DVH) among patients treated for carcinoma of cervix with intracavitary brachytherapy.BackgroundWith the advent of advanced imaging modalities, there has been a shift from conventional X-ray based planning to three-dimensional planning. Manual optimisation is widely used across various institutions but it is time consuming and operator dependant. Inverse planning simulated annealing (IPSA) is now available in various brachytherapy planning systems. But there is a paucity of studies comparing manual optimisation and IPSA in treatment of carcinoma cervix with intracavitary brachytherapy and hence this study.Materials and methodsFifteen consecutive patients treated between December 2013 and March 2014 with intracavitary brachytherapy for carcinoma of cervix were selected for this study. All patients were initially treated with external beam radiotherapy followed by intracavitary brachytherapy. The DVH was evaluated and compared between manually optimised plans and IPSA in the same set of patients.ResultsThere was a significant improvement in the HRCTV coverage, mean V100 of 87.75% and 82.37% (p = 0.001) and conformity index 0.67 and 0.6 (p = 0.007) for plans generated using IPSA and manual optimisation, respectively. Homogeneity index and dose to the OARs remained similar between the two groups.ConclusionThe use of inverse planning in intracavitary brachytherapy of cervix has shown a significant improvement in the target volume coverage when compared with manual planning.     

Dosimetric evaluation of image based brachytherapy using tandem ovoid and tandem ring applicators

Aim

The aim of the study is to evaluate the differences in dosimetry between tandem-ovoid and tandem-ring gynaecologic brachytherapy applicators in image based brachytherapy.

Clinical application of MOSkin dosimeters to rectal wall in vivo dosimetry in gynecological HDR brachytherapy

PurposeThree MOSkins dosimeters were assembled over a rectal probe and used to perform in vivo dosimetry during HDR brachytherapy treatments of vaginal cancer. The purpose of this study was to verify the applicability of the developed tool to evaluate discrepancies between planned and measured doses to the rectal wall.Materials and methodsMOSkin dosimeters from the Centre for Medical Radiation Physics are particularly suitable for brachytherapy procedures for their ability to be easily incorporated into treatment instrumentation. In this study, 26 treatment sessions of HDR vaginal brachytherapy were monitored using three MOSkin mounted on a rectal probe. A total of 78 measurements were collected and compared to doses determined by the treatment planning system.ResultsMean dose discrepancy was determined as 2.2 ± 6.9%, with 44.6% of the measurements within ±5%, 89.2% within ±10% and 10.8% higher than ±10%. When dose discrepancies were grouped according to the time elapsed between imaging and treatment (i.e., group 1: ≤90 min; group 2: >90 min), mean discrepancies resulted in 4.7 ± 3.6% and 7.1 ± 5.0% for groups 1 and 2, respectively. Furthermore, the position of the dosimeter on the rectal catheter was found to affect uncertainty, where highest uncertainties were observed for the dosimeter furthest inside the rectum.ConclusionsThis study has verified MOSkin applicability to in-patient dose monitoring in gynecological brachytherapy procedures, demonstrating the dosimetric rectal probe setup as an accurate and convenient IVD instrument for rectal wall dose verification. Furthermore, the study demonstrates that the delivered dose discrepancy may be affected by the duration of treatment planning.     

Applying radiobiological plan ranking methodology to VMAT prostate SBRT

The impact of a rectal spacer and an increased near maximum target dose in VMAT prostate SBRT is studied.For a group of 11 patients (35 Gy-in-five-fractions VMAT prostate SBRT) a set of 4 plans were generated, namely two VMAT plans, with D2% ⩽ 37.5 Gy (Hom) and with D2% ⩽ 40.2 Gy (Het), were created for each of two CT scans taken before (NoSpc) and after (Spc) transperineal spacer insertion. Consequently the methodology for parameter invariant TCP (tumor control probability) plan ranking was applied for comparison of the plans in terms of tumor control. NTCPs (normal tissue complication probabilities) were calculated for rectum and bladder using Lyman’s model.For all 11 patients the TCP plan ranking has shown that the Het plans would perform considerably better in TCP terms than the Hom ones. The plans without rectal spacer were ranked worse compared to those with rectal spacer except for one set of Hom plans. The calculated NTCPs for rectum produced by the Het plans were quite similar to the NTCPs of the Hom ones. The rectal NTCPs of the Hom Spc plans were always lower than the NTCPs of the Hom NoSpc plans. The NTCP values for bladder were extremely low in all cases.The use of rectal spacer leads in general to lower risk of rectal complications, as expected, and even to better tumor control. Plans with increased near maximum target dose (D2% ⩽ 40.2 Gy) are expected to perform much better in terms of tumor control than those with D2% ⩽ 37.5 Gy.     

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.     

Application of Monte Carlo calculations for validation of a treatment planning system in high dose rate brachytherapy

AimThe accuracy of treatment planning systems is of vital importance in treatment outcomes in brachytherapy. In the current study the accuracy of dose calculations of a high dose rate (HDR) brachytherapy treatment planning system (TPS) was validated using the Monte Carlo method.Materials and methodsThree ⁶⁰Co sources of the GZP6 afterloading brachytherapy system were modelled using MCNP4C Monte Carlo (MC) code. The dose distribution around all the sources was calculated by MC and a dedicated treatment planning system. The results of both methods were compared.ResultsThere was good agreement (<2%) between TPS and MC calculated dose distributions except at a point near the sources (<1 cm) and beyond the tip of the sources.ConclusionsOur study confirmed the accuracy of TPS calculated dose distributions for clinical use in HDR brachytherapy.     

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.     

Influence of different Fletcher–Suit applicator geometries on sagittal dose distribution

IntroductionOne of the brachytherapy treatment modality of cervix carcinomas is insertion with Fletcher–Suit (FS) applicator. Depending on the patient anatomy and pathology and on the construction of the FS applicator different geometrical arrangements in ovoid separation, in ovoids' sagittal level with respect to the tandem were experienced. The multiple insertions show minor differences in applicator geometries. The aim of the study is to evaluate the influence of main geometrical parameters: the ovoid separation, symmetry and the ovoids' sagittal shift on dose distribution in different FS applicator arrangements. We tested the effect of dwell time settings in improvement of dose distribution of less adequate insertions. We also investigated the effect of inter-fractional variation of applicator geometry.Materials and methodsWe considered 73 treatment fractions of 22 patients. All insertions were performed by the same gynaecologist with the same type of FS applicator, while the treatment plans were generated by the same physicist using the same treatment planning method. We compared the sagittal dose distribution of different FS applicator geometries with dose levels at two applicator points, defined 2 cm apart from the tandem towards the bladder and rectum. We computed the Pearson correlation coefficients between the dose levels at the applicator points and the ovoid separation, symmetry and the ovoids' sagittal shift. We also investigated the effect of dwell time settings in ovoids in order to decrease the dose to organs at risk. The inter-fractional variation of the FS applicator geometries and the influence on the dose levels at the two applicator points were also tested.Results and conclusionsStrong correlation was found between the ovoid separation and dose values to applicator points defined in sagittal direction of FS applicator arrangements. Also strong correlation was between the ovoids' sagittal position with respect to the tandem and the applicator point defined towards the rectum, while the ovoid symmetry had no influence on the sagittal dose distribution. The standard deviations of inter-fractional variation of the ovoid separation and the ovoids' sagittal position were within ±5.2 mm and ±10.2 mm respectively. The inter-fractional variations in FS applicator geometry resulted in variation in dose levels at the applicator points ±0.8 Gy typically, while the largest value was ±1.6 Gy.     

Evaluating the predictive value of quantec rectum tolerance dose suggestions on acute rectal toxicity in prostate carcinoma patients treated with IMRT

AimTo investigate the predictive value of convenience of rectum dosimetry with Quantitative Analysis of Normal Tissue Effects in the Clinic (QUANTEC) dose limits, maximum rectum dose (Dmax), total rectal volume (TVrectum), rectal volume included in PTV (VrectumPTV) on Grade 2–3 acute rectal toxicity for utilization in clinical practice.BackgroundNumerous previous data have reported frequent acute proctitis after external-beam RT of prostate cancer. Predicting toxicity limited with dose information is inadequate in clinical practice due to comorbidities and medications used.Materials and MethodSixty-four non-metastatic prostate cancer patients treated with IMRT were enrolled. Patients were treated to a total dose of 70–76 Gy. Rectal dose volume histograms (DVH) of all patients were evaluated retrospectively, and a QUANTEC Score between 0 and 5 was calculated for each patient. The correlation between the rectal DVH data, QUANTEC score, TVrectum, VrectumPTV, rectum Dmax and Grade 2–3 rectal toxicity was investigated.ResultsIn the whole group grade 1, 2 and 3 acute rectal toxicities were 25%, 18.8% and 3.1%, respectively. In the DVH data, rectum doses of all patients were under RTOG dose limits. Statistically significant correlation was found between grade 2–3 rectal toxicity and TVrectum (p = 0,043); however. It was not correlated with QUANTEC score, VrectumPTV and Dmax.ConclusionOur results were not able to show any significant correlation between increasing convenience with QUANTEC limits and lower rectal toxicity. Conclusively, new dosimetric definitions are warranted to predict acute rectal toxicity more accurately in prostate cancer patients during IMRT treatment.     

A comparison of a moderately hypofractionated IMRT planning technique used in a randomised UK external beam radiotherapy trial with an in-house technique for localised prostate cancer

AimTo compare the radiotherapy technique used in a randomised trial with VMAT and an in-house technique for prostate cancer.BackgroundTechniques are evolving with volumetric modulated arc therapy (VMAT) commonly used. The CHHiP trial used a 3 PTV forward planned IMRT technique (FP_CH). Our centre has adopted a simpler two PTV technique with locally calculated margins.Materials and methods25 patients treated with FP_CH to 60 Gy in 20 fractions were re-planned with VMAT (VMAT_CH) and a two PTV protocol (VMAT_60/52 and VMAT_60/48). Target coverage, conformity index (CI), homogeneity index (HI), monitor units (MU) and dose to the rectum, bladder, hips and penile bulb were compared.ResultsPTV coverage was high for all techniques. VMAT_CH plans had better CI than FP_CH (p ≤  0.05). VMAT_60/52/48 plans had better CI than VMAT_CH. FP_CH had better HI and fewer MU than VMAT (p ≤  0.05). More favourable rectum doses were found for VMAT _CH than FP_CH (V_48.6, V_52.8, V₅₇, p ≤  0.05) with less difference for bladder (p ≥  0.05). Comparing VMAT_CH to VMAT_60/52/48 showed little differences for the bladder and rectum but VMAT_CH had larger penile bulb doses (V_40.8, V_48.6, mean, D_2, p ≤  0.05). Femoral head doses (V_40.8) were similarly low for all techniques (p = ≥ 0.05).ConclusionVMAT produced more conformal plans with smaller rectum doses compared to FP_CH albeit worse HI and more MU. VMAT_60/52 and VMAT_60/48 plans had similar rectal and bladder doses to VMAT_CH but better CI and penile bulb doses which may reduce toxicity.     

Characterization of irregular electron beam for boost dose after whole breast irradiation

Irradiating a tumor bed with boost dose after whole breast irradiation helps reducing the probability of local recurrence. However, the success of electron beam treatment with a small area aiming to cover a superficial lesion is a dual challenge as it requires an adequate dosimetry beside a double check for dose coverage with an estimation of various combined uncertainty of tumor location and losing lateral electron equilibrium within small field dimensions.Aim of workthis work aims to measure the electron beam fluence within different field dimensions and the deviation from measurement performed in standard square electron applicator beam flatness and symmetry, then to calculate the average range of the correction factor required to overcome the loss of lateral electron equilibrium.Material and methodthe electron beam used in this work generated from the linear accelerator model ELEKTA Precise and dosimetry system used were a pair of PTW Pin Point ion chambers for electron beam dosimetry at standard conditions and assessment of beam quality at a reference depth of measurement, with an automatic water phantom, then a Roos ion chamber was used for absolute dose measurement, and PTW 2Darray to investigate the beam fluence of four applicators 6, 10, 14 and 20 cm² and 4 rectangular cutouts 6 × 14, 8 × 14, 6 × 17 and 8 × 17 cm², the second part was clinical application which was performed in a precise treatment planning system and examined boost dose after whole breast irradiation.Resultsrevealed that lower energy (6MeV and 8MeV) showed the loss of lateral electron equilibrium and deviation from measurements of a standard applicator more than the high energy (15 MeV) which indicated that the treatment of superficial dose with 6MeV required higher monitor unit to allow for the loss of lateral electron equilibrium and higher margin as well.     

Dosimétrie rénale du 177Lu-Dotatate : mise en place au sein du Groupement hospitalier Est des Hospices Civils de Lyon

IntroductionThe kidney is considered as a critical dose-limiting organ with ¹⁷⁷Lu-Dotatate. Renal dosimetry could play a role in optimizing treatment. We present a feedback on the implementation of renal dosimetry in our medical center.Material and methodThe renal dosimetry of the 1st administration of ¹⁷⁷Lu-Dotatate (approximately 7.4 GBq) has been performed for seven patients. The reference dosimetry strategy included 4 post-therapeutic SPECT/CT at 6 h, 24 h, 72 h and 168 h and anatomical renal volume delineation (VOI). Alternative dosimetric strategies consisted of 72 h or 168 h time point eviction (time sampling A or B) and delimitation of 1 or 3 spherical VOIs (3 mL each) per kidney (“1 sVOI” or “3 sVOI” methods). The quantitative scintigraphic processing was performed by 4 operators using Dosimetry Toolkit®. The renal dose was calculated with OLINDA/EXM® 2.0.ResultsThe calculated mean absorbed renal dose was 3.68 ± 0.68 Gy with the reference method, with no significant impact of interoperator variability (P = 0.41). It was in satisfactory agreement with time sampling A or B. The “1 sVOI” and “3 sVOI” methods overestimated the renal dose (5.01 ± 0.94 Gy and 4.91 ± 0.79 Gy respectively), with a significant impact on interoperator variability (P < 0.05), despite a reduction in processing time.ConclusionThe main logistic constraint of ¹⁷⁷Lu-Dotatate renal dosimetry in our center is the time-consumption due to SPECT/CT acquisitions. A possible approach supported by our preliminary results is a reduction in the number of scintigraphic acquisitions.     

Novel 6 MV X-ray photoneutron detection and dosimetry of medical accelerators

PurposeDosimetry of fast, epithermal and thermal photoneutrons in 6 MV X-ray beams of two medical accelerators were studied by novel dosimetry methods.MethodsA Siemens ONCOR and an Elekta COMPACT medical accelerators were used. Fast, epithermal and thermal photoneutron dose equivalents in 10 cm × 10 cm 6 MV X-rays fields were determined in air and on surface of a polyethylene phantom in X and Y directions. Polycarbonate dosimeters as bare or with enriched ¹⁰B convertors (with or without cadmium covers) were used applying a 50 Hz-HV electrochemical etching method.ResultsFast, epithermal and thermal photoneutron dose equivalents were efficiently determined respectively as ∼1145.8, ∼45.3 and ∼170.6 μSv in air and ∼1888.5, ∼96.1 and ∼640.6 μSv on phantom per 100 Gy X-rays at the isocenter of Siemens ONCOR accelerator in air. The dose equivalent is maximum at the isocenter which decreases as distance from it increases reaching a constant level. Tissue-to-air ratios are constants up to 15 cm from the isocenter. No photoneutrons was detected in the Elekta COMPACT accelerator.ConclusionsFast, epithermal and thermal photoneutron dosimetry of 6 MV X-rays were made by novel dosimetry methods in a Siemens ONCOR accelerator with sum dose equivalent per Gy of ∼0.0014% μSv with ∼0.21 MeV mean energy at the isocenter; i.e. ∼150 times smaller than that of 18 MV X-rays. This observation assures clinical safety of 6 MV X-rays in particular in single-mode machines like Elekta COMPACT producing no photoneutrons due to no “beryllium exit window” in the head structure.     

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).     

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.