Dosimetry of interface region near closed air cavities for Co-60, 6 MV and 15 MV photon beams using Monte Carlo simulations

Dose difference between anisotropic analytical algorithm (AAA) and Acuros XB (AXB) caused by target's air content for volumetric modulated arc therapy of head and neck cancer

Background

We clarified the dose difference between the anisotropic analytical algorithm (AAA) and Acuros XB (AXB) with increasing target's air content using a virtual phantom and clinical cases.

Materials and methods

Whole neck volumetric modulated arc therapy (VMAT) plan was transferred into a virtual phantom with a cylindrical air structure at the center. The diameter of the air structure was changed from 0 to 6 cm, and the target's air content defined as the air/planning target volume (PTV) in percent (air/PTV) was varied. VMAT plans were recalculated by AAA and AXB with the same monitor unit (MU) and multi-leaf collimator (MLC) motions. The dose at each air/PTV (5%-30%) was compared between each algorithm with D98%, D95%, D50% and D2% for the PTV. In addition, MUs were also compared with the same MLC motions between the D95% prescription with AAA (AAA_D95%), AXB_D95%, and the prescription to 100% minus air/PTV (AXB_D100%-air/PTV) in clinical cases of head and neck (HNC).

Results

When air/PTV increased (5-30%), the dose differences between AAA and AXB for D98%, D95%, D50% and D2% were 3.08-15.72%, 2.35-13.92%, 0.63-4.59%, and 0.14-6.44%, respectively. At clinical cases with air/PTV of 5.61% and 28.19%, compared to AAA_D95%, the MUs differences were, respectively, 2.03% and 6.74% for AXB_D95% and 1.80% and 0.50% for AXB_D100%-air/PTV.

Conclusion

The dose difference between AAA and AXB increased as the target's air content increased, and AXB_D95% resulted in a dose escalation over AAA_D95% when the target's air content was ≥ 5%. The D100%-air/PTV of PTV using AXB was comparable to the D95% of PTV using AAA.

Accuracy Evaluation of Collapsed Cone Convolution Superposition Algorithms for the Nasopharynx Interface in the Early Stage of Nasopharyngeal Carcinoma

This study combined the use of radiation dosimeteric measurements and a custom-made anthropomorphic phantom in order to evaluate the accuracy of therapeutic dose calculations at the nasopharyngeal air-tissue interface. The doses at the nasopharyngeal air-tissue interface obtained utilizing the Pinnacle and TomoTherapy TPS, which are based on collapsed cone convolution superposition (CCCS) algorithms, were evaluated and measured under single $10\times 10\text{c}{\text{m}}^2$ , $2\times 2\text{c}{\text{m}}^2$ , two parallel opposed $2\times 2\text{c}{\text{m}}^2$ and clinical fields for early stage of nasopharyngeal carcinoma by using EBT3, GR-200F, and TLD 100. At the air-tissue interface under a $10\times 10\text{c}{\text{m}}^2$ field, the TPS dose calculation values were in good agreement with the dosimeter measurement with all differences within 3.5%. When measured the single field $2\times 2\text{c}{\text{m}}^2$ , the differences between the average dose were measured at the distal interface for EBT3, GR-200F, and TLD-100 and the calculation values were -15.8%, -16.4%, and -4.9%, respectively. When using the clinical techniques such as IMRT, VMAT, and tomotherapy, the measurement results at the interface for all three techniques did not imply under dose. Small-field sizes will lead to dose overestimation at the nasopharyngeal air-tissue interface due to electronic disequilibrium when using CCCS algorithms. However, under clinical applications of multiangle irradiation, the dose errors caused by this effect were not significant.

Dosimetric effect of intensity-modulated radiation therapy for postoperative non-small cell lung cancer with and without air cavity in the planning target volume

To evaluate the dosimetric effect of intensity-modulated radiation therapy (IMRT) for postoperative non-small cell lung cancer (NSCLC), with and without the air cavity in the planning target volume (PTV). Two kinds of IMRT plans were made for 21 postoperative NSCLC patients. In Plan-0: PTV included the tracheal air cavity, and in Plan-1: the air cavity was subtracted from the PTV. For PTV, the dosimetric parameters, including D_mean, D₉₈, D₉₅, D₂, D_0.2, conformity index (CI), and homogeneity index (HI) were evaluated. For organs at risk (OARs), the evaluation indexes, included the V₅, V₂₀ and the mean lung dose (MLD) of total lung, the V₃₀, V₄₀, and the mean heart dose (MHD) of heart, the spinal cord D_max, and the V₃₅ and the mean esophageal dose (MED) of esophagus. The number of segments and MUs were also recorded. Additionally, the correlation between the Plan-1 dosimetric change value relative to Plan-0, the size of air cavity, and the volume proportion of the cavity in the PTV was also analyzed. The D_mean of PTV, D₂, D_0.2, HI and CI in Plan-1 decreased compared with those in Plan-0. For OARs, the V₃₀, MHD, and MED also decreased. The CI change value of Plan-1 relative to Plan-0 had a significantly negative correlation with the size and the volume proportion of air cavity, and the MED change value also had a significantly negative correlation with the air cavity size. The IMRT plans for patients with postoperative NSCLC can achieve a better target dose distribution and offer an improved sparing of the heart and esophagus by removing the PTV air cavity, while reducing the target conformity. The change value of CI and MED had a significantly negative correlation with the air cavity size.

Radiotherapy for Primary Tracheal Carcinoma: Experience at a Single Institution

Background:

There is limited understanding of tracheal carcinoma (TC) because of its rarity. We examined the efficacy of radiotherapy (RT) for patients with primary TC.

Methods:

We analyzed the records of 32 patients with primary TC who received RT at our center between November 1996 and December 2016.

Results:

Thirteen patients received adjuvant RT and 18 received definitive RT. Eight patients achieved complete remission (CR) after definitive RT. Among all patients, the 5-year overall survival (OS) rate was 46.9% and the locoregional progression free survival (LRPFS) rate was 68.1%. Univariate analysis indicated the 5-year OS was better in those with adenoid cystic adenocarcinoma than squamous cell carcinoma (P = 0.001); the 5-year LRPFS was better in patients who received surgical resection than those who did not (92.9% vs 46.4%, P = 0.013) and in patients who received postoperative RT than in those who received definitive RT (91.7% vs 50.1%, P = 0.038). A sub-group univariate analysis indicated the 5-year PFS was better for those who received at least 68 Gy of radiation (44.4% vs 13.0%, P = 0.044). Patients who achieved CR had a better 5-year PFS than those who did not (57.1% vs 10%, P = 0.006). No patients had a toxicity of grade 3 or more.

Conclusions:

Adjuvant and definitive RT are safe and effective treatments for TC. Patients who received dosages of 68 Gy or more and who had complete tumor regression following definitive RT seemed to have better long-term survival.

Quantifying and Assessing the Dosimetric Impact of Changing Gas Volumes Throughout the Course of VMAT Radiation Therapy of Upper Gastrointestinal Tumors

Purpose

This retrospective patient study assessed the consistency of abdominal gas presence throughout radiation therapy for patients with upper gastrointestinal cancer and determined the impact of variations in gas volume on the calculated dose distribution of volumetric modulated arc therapy.

Methods and Materials

Eight patients with pancreatic cancer were included for analysis. A plan library consisting of 3 reference plans per patient (Ref_0.0, Ref_0.5, and Ref_1.0) was created based on planning computed tomography (CT) with density overrides of 0.0, 0.5, and 1.0 applied to gas volumes, respectively. Corresponding cone beam CT (CBCT) data sets were obtained and density overrides were applied to enable fractional dose calculation. Variation in gas volume relative to initial volume determined from CT was assessed. Dose metrics for targets and organs at risk were compared between the accumulated CBCT dose and the planned dose of the 3 reference plans for each patient.

Results

There was a significant decrease in gas present from CT to treatment CBCT, with a mean decrease in volume of 48.6% for the entire cohort. Dosimetrically, all accumulated target and organ-at-risk parameters, aside from the kidneys, exhibited the smallest mean deviation from the Ref_0.0 plan and largest mean deviation from the Ref_1.0 plan. A statistically significant difference in mean accumulated dose to Ref_0.0 and Ref_1.0 was observed for the dose delivered to 95% of the planning target volume.

Conclusions

Significant variation in gas volumes from CT to treatment can occur throughout volumetric modulated arc therapy for pancreatic cancer. Through the use of a plan library, it was determined that initial assessment of a patient's treatment plan with an assigned gas density of 0.0 provided the most accurate prediction of the accumulated dose.

The Impact of Radiation Energy on Dose Homogeneity and Organ Dose in the Göttingen Minipig Total-Body Irradiation Model

Animal models of total-body irradiation (TBI) are used to elucidate normal tissue damage and evaluate the efficacy of medical countermeasures (MCM). The accuracy of these TBI models depends on the reproducibility of the radiation dose-response relationship for lethality, which in turn is highly dependent on robust radiation physics and dosimetry. However, the precise levels of radiation each organ absorbs can change dramatically when different photon beam qualities are used, due to the interplay between their penetration and the natural variation of animal sizes and geometries. In this study, we evaluate the effect of varying the radiation energy, namely cobalt-60 (Co-60); of similar penetration to a 4-MV polyenergetic beam), 6 MV and 15 MV, in the absorbed dose delivered by TBI to individual organs of eight Göttingen minipigs of varying weights (10.3-24.1 kg) and dimensions (17.5-25 cm width). The main organs, i.e. heart, lungs, esophagus, stomach, bowels, liver, kidneys and bladder, were contoured by an experienced radiation oncologist, and the volumetric radiation dose distribution was calculated using a commercial treatment planning system commissioned and validated for Co-60, 6-MV and 15-MV teletherapy units. The dose is normalized to the intended prescription at midline in the abdomen. For each animal and each energy, the body and organ dose volume histograms (DVHs) were computed. The results show that more penetrating photon energies produce dose distributions that are systematically and consistently more homogeneous and more uniform, both within individual organs and between different organs, across all animals. Thoracic organs (lungs, heart) received higher dose than prescribed while pelvic organs (bowel, bladder) received less dose than prescribed, due to smaller and wider separations, respectively. While these trends were slightly more pronounced in the smallest animals (10.3 kg, 19 cm abdominal width) and largest animals (>20 kg, ∼25 cm abdominal width), they were observed in all animals, including those in the 9-15 kg range typically used in MCM models. Some organs received an average absorbed dose representing <80% of prescribed dose when Co-60 was used, whereas all organs received average doses of >87% and >93% when 6 and 15 MV were used, respectively. Similarly, average dose to the thoracic organs reached as high as 125% of the intended dose with Co-60, compared to 115% for 15 MV. These results indicate that Co-60 consistently produces less uniform dose distributions in the Göttingen minipig compared to 6 and 15 MV. Moreover, heterogeneity of dose distributions for Co-60 is accentuated by anatomical and geometrical variations across various animals, leading to different absorbed dose delivered to organs for different animals. This difference in absorbed radiation organ doses, likely caused by the lower penetration of Co-60 and 6 MV compared to 15 MV, could potentially lead to different biological outcomes. While the link between the dose distribution and variation of biological outcome in the Göttingen minipig has never been explicitly studied, more pronounced dose heterogeneity within and between organs treated with Co-60 teletherapy units represents an additional confounding factor which can be easily mitigated by using a more penetrating energy.

Efficacy of MLC based cobalt-60 plans: A DVH comparison and analysis with 6 MV plans

The purpose of this study was to compare 3D treatment plans implemented using 6 MV Linac with a retrofitted multileaf collimator (MLC) based cobalt-60 plans. In this retrospective study, DVH analysis was used to compare homogeneity of dose within the target and the dose received by critical organs. A prototype MLC designed and developed as a retrofit to current cobalt-60 teletherapy machines with a dedicated 3D treatment planning system was used. Cases representing 5 tumor sites like head & neck, glottis, lung, gall bladder, stomach were taken for the study, which were planned using Eclipse treatment planning system and treated with 6 MV photon beams. The plans were re-planned using the retrofit cobalt-60 MLC with same beam arrangement and dose prescription in Radiation Oncology planning system (ROPS). For each case, DVH data was evaluated for both types of beam energies. Conformity index (CI) and homogeneity index (HI) for target were calculated and compared. The conformal plans created using cobalt MLC for five sites were found to be similar to those planned using 6 MV photon beams. CI values close to unity reflected dose uniformity in the target volume while HI evaluated the hotspots in the target volume. It was concluded that plans created using retrofit prototype MLC developed for cobalt-60 teletherapy machines can provide dose distributions comparable to 6 MV photon beams. The prototype MLC developed can provide a promising treatment option for existing telecobalt machines in implementing conformal therapy in developing countries.

Breast telecobalt beam therapy using multi-isocentric technique

AIM

To treat breast cancer patients in telecobalt unit with image based conformal radiotherapy planning using the multi-isocentric technique.

BACKGROUND

Breast cancer is the leading cancer among all the female cancers. With improved screening techniques, many patients are being diagnosed at an early stage and the need for radiotherapy in such patients has increased. The telecobalt machine is still a preferred machine in many of the low income countries as it is cost-effective and can offer uninterrupted treatment to large number of patients.

MATERIALS AND METHODS

Three hundred patients requiring radiotherapy had a computed tomography based planning. Patients were immobilized using a breast board with a thermoplastic mould. Three dimensional planning was done with the multi-isocentric technique. These patients were then simulated using a Nucletron Simulix digital simulator for field verification and were treated in a Theratron Phoenix telecobalt treatment unit.

RESULTS

The doses to the heart, ipsilateral lung and the conformity index were within the recommended values. The homogeneity index was not comparable; however, a section by section qualitative analysis was done and a final plan approved. As per the RTOG toxicity grading system, acute skin reaction grade 3 was observed in 3.6% of treatments to intact breast including nodal regions and in 3.5% of post mastectomy radiation patients.

CONCLUSION

Single isocenter technique was not feasible as the telecobalt unit did not have multileaf collimators and asymmetric jaws. With improved image based planning, a multi-isocentric technique was planned. By evaluating the dose distribution, beam modifications can be made and treatments can be given with acceptable toxicity.

Investigating the Effect of Air Cavities of Sinuses on the Radiotherapy Dose Distribution Using Monte Carlo Method

BACKGROUND

Considering that some vital organs exist in the head and neck region, the treatment of tumors in this area is a crucial task. The existence of air cavities, namely sinuses, disrupt the radiotherapy dose distribution. The study aims to analyze the effect of maxillary, frontal, ethmoid and sphenoid sinuses on radiotherapy dose distribution by Monte Carlo method.

MATERIAL AND METHODS

In order to analyze the effect of the cavities on dose distribution, the maxillary, frontal, ethmoid and sphenoid sinus cavities were simulated with (3×3.2×2) cm³, (2×2×3.2) cm³, (1×1×1.2) cm³ and (1×1×2) cm³ dimensions.

RESULTS

In the analysis of the dose distribution caused by cavities, some parameters were observed, including: inhomogeneity of dose distribution in the cavities, inhomogeneity of dose on the edges of the air cavities and dispersion of the radiations after the air cavity. The amount of the dose in various situations showed differences: before the cavity a 0.64% and a 2.76% decrease, a 12.06% and a 17.17% decrease in the air zone, and a 2.25% and a 5.9% increase after the cavity.

CONCLUSION

The results indicate that a drop in dose before the air cavities and in the air zone occurs due to the lack of scattered radiation. Furthermore, the rise in dose was due to the passage of more radiation from the air cavity and dose deposition after the air cavity. The changes in dose distribution are dependent on the cavity size and depth. As a result, this has to be noted in the treatment planning and MU calculations of the patient.

Evaluation of Lung Density and Its Dosimetric Impact on Lung Cancer Radiotherapy: A Simulation Study

BACKGROUND

The dosimetric parameters required in lung cancer radiation therapy are taken from a homogeneous water phantom; however, during treatment, the expected results are being affected because of its inhomogeneity. Therefore, it becomes necessary to quantify these deviations.

OBJECTIVE

The present study has been undertaken to find out inter- and intra- lung density variations and its dosimetric impact on lung cancer radiotherapy using Monte Carlo code FLUKA and PBC algorithms.

MATERIAL AND METHODS

Density of 100 lungs was recorded from their CT images along with age. Then, after PDD calculated by FLUKA MC Code and PBC algorithm for virtual phantom having density 0.2 gm/cm3 and 0.4 gm/cm3 (density range obtained from CT images of 100 lungs) using Co-60 10 x10 cm2 beams were compared.

RESULTS

Average left and right lung densities were 0.275±0.387 and 0.270±0.383 respectively. The deviation in PBC calculated PDD were (+)216%, (+91%), (+)45%, (+)26.88%, (+)14%, (-)1%, (+)2%, (-)0.4%, (-)1%, (+)1%, (+)4%, (+)4.5% for 0.4 gm/cm3 and (+)311%, (+)177%, (+)118%, (+)90.95%, (+)72.23%, (+)55.83% ,(+)38.85%, (+)28.80%, (+)21.79%, (+)15.95%, (+)1.67%, (-) 2.13%, (+)1.27%, (+)0.35%, (-)1.79%, (-)2.75% for 0.2 gm/cm3 density mediums at depths of 1mm, 2mm, 3mm, 4mm, 5mm, 6 mm, 7 mm, 8mm, 9mm,10mm, 15mm, 30mm, 40mm, 50mm, 80mm and 100 mm, respectively.

CONCLUSION

Large variations in inter- and intra- lung density were recorded. PBC overestimated the dose at air/lung interface as well as inside lung. The results of Monte Carlo simulation can be used to assess the performance of other treatment planning systems used in lung cancer radiotherapy.