Treatment volume from total reference air kerma (TRAK) in intracavitory applications and its comparison with ICRU reference volume

Relationship between biodosimetric parameters and treatment volumes in three types of prostate radiotherapy

Brachytherapy (BT) and external beam radiotherapy (EBRT) apply different dose rates, overall treatment times, energies and fractionation. However, the overall impact of these variables on the biological dose of blood is neglected. As the size of the irradiated volume influences the biological effect as well, we studied chromosome aberrations (CAs) as biodosimetric parameters, and explored the relationship of isodose surface volumes (ISVs: V_1%, V_1Gy, V_10%, V_10Gy, V_100%, V_150%) and CAs of both irradiation modalities. We performed extended dicentrics assay of lymphocytes from 102 prostate radiotherapy patients three-monthly for a year. Aberration frequency was the highest after EBRT treatment. It increased after the therapy and did not decrease significantly during the first follow-up year. We showed that various types of CAs 9 months after LDR BT, 3 months after HDR BT and in a long time-range (even up to 1 year) after EBRT positively correlated with ISVs. Regression analysis confirmed these relationships in the case of HDR BT and EBRT. The observed differences in the time points and aberration types are discussed. The ISVs irradiated by EBRT showed stronger correlation and regression relationships with CAs than the ISVs of brachytherapy.

Correlation of total reference air-kerma (TRAK) to prescription isodose surface volume in vaginal cylinder high-dose-rate brachytherapy

Purpose

The purpose of this technical note was to investigate correlation of total reference air-kerma (TRAK) with volume enclosed by the prescription isodose surface in vaginal cylinder high-dose-rate (HDR) brachytherapy.

Material and methods

Treatment plans of 175 gynecological cancer patients treated at our institution with iridium-192 (¹⁹²Ir) HDR brachytherapy using a single-channel vaginal cylinder applicator were retrospectively reviewed. Applicator size in diameter ranged from 20 mm to 40 mm. Treatment length ranged from 30 mm to 90 mm (median, 50 mm). Brachytherapy fractional dose was 5 Gy (Dose_Ref) prescribed to 5 mm distance from cylinder surface. Parameters TRAK (cGy), source activity during treatment (Ci), total treatment time (s), and prescription isodose surface volume ISV_Ref (cm³) were recorded from individual treatment plans. In each case, vaginal tissue volume (V_VT) enclosed by prescription isodose was calculated by subtracting cylinder volume enclosed by the prescription isodose from ISV_Ref.

Results

Total reference air-kerma correlated with the total volume enclosed by the prescription isodose via ISV_Ref = 4768 × (TRAK/Dose_Ref)^1.47. TRAK related linearly to the volume of vaginal tissue enclosed by the prescription dose via V_VT = ((138.3 × TRAK) – 8.2). Secondarily, TRAK related to the treatment time through time (s) = 882 (s/cGy) × TRAK (cGy), where 882 is (1/air-kerma strength) for 10 Ci apparent activity of ¹⁹²Ir source.

Conclusions

The correlation of TRAK to the vaginal tissue volume encompassed by the prescription dose surface yields a useful predictive equation. The TRAK treatment time relationship enables quick verification of planned treatment time by knowing TRAK in any HDR brachytherapy application.

Reporting of dose-volume specifications for "Martinez Universal Perineal Interstitial Template-based interstitial high-dose-rate brachytherapy for gynecological cancers" according to International Commission on Radiation Units and Measurements 58 with early clinical outcomes

PURPOSE

The study is an audit of reporting dose and volume specifications as per the ICRU 58 for MUPIT-based interstitial brachytherapy in gynecological cancers. Correlation between total reference air kerma (TRAK) and isodose surface was also evaluated to understand the intensity of treatment in interstitial brachytherapy.

METHODS AND MATERIALS

Forty-two patients underwent HDR MUPIT-based interstitial brachytherapy 20 Gy in five fractions after EBRT during 2017-2019. Treated volume, high and low-dose regions, mean central dose, Dose Homogeneity Index (DHI), organ at risk doses, and TRAK values were computed.

RESULTS

High-dose regions V150 mean was 12.4 cc and V200 was 4.58 cc; and low-dose region was 75.92 cc. The mean treated volume was 59.8 cc. The mean central dose was 3.7 Gy. DHI was 79%. The mean D2cm³ bladder and rectum were 2.9 Gy and 2.8 Gy. The mean TRAK was 0.16 cGy per fraction per hour at 1 m. TRAK values showed significant correlation with various isodose volumes (TRAK and V100: r = 0.943 p < 0.0005; and TRAK and V50: r = 0.953; p < 0.0005). A positive correlation was observed between TRAK and the number of needles (r = 0.746; p < 0.0005). At a median followup of 16 months, 4 of 42 patients (9.5%) had local recurrences.

CONCLUSIONS

Our study shows compliance with ICRU 58 recommendations along with certain deviations. Local recurrence rate is acceptable. TRAK shows correlation with surface isodose in MUPIT-based brachytherapy and should to be evaluated in future studies.

Formulation of normal tissue irradiation volumes in Co-60 and Ir-192 HDR ICBT of Ca cervix using Total Reference Air Kerma (TRAK)

Aim

The aim of this study was to formulate isodose volume relations encompassed by isodose surfaces in Co-60 and Ir-192 HDR intracavitary brachytherapy (ICBT) of cervix carcinoma using the Total Reference Air Kerma (TRAK).

Background

The TRAK and isodose volumes are radioactive source related. The formulated relations can easily estimate the irradiated isodose volume if the TRAK and dose are known. The C0-60 can also be used for brachytherapy because of its longer half life and comparable OAR doses to Ir-192.

Materials and methods

Isodose volumes encompassed by different isodose surfaces and TRAK were obtained from 22 Ca cervix ICBT treatment plans in Co-60 and Ir-192 HDR brachytherapy with 9 Gy prescription to point A. Isodose volume relations were formulated both for Co-60 and Ir-192 brachytherapy source from the slopes and intercepts of the linear fit in the plot between isodose volumes and TRAKs.

Results

The TRAK value of Co-60 was higher than Ir-192 by about 7.16%. The isodose volumes at low doses for Co-60 were higher than Ir-192. But no significant differences in the dose to the bladder and rectum were observed due to these sources. For dose to 2 cm³ bladder and rectum volume, the differences were 1.07% and 0.75%, respectively. The correlation coefficient with the 2-tailed significance of correlation (p value) between TPS measured isodose volume and calculated isodose volumes using the formulated relations at different dose values were statistically significant as p < 0.05.

Conclusion

Results show different isodose volumes for both sources but the dose to the bladder and rectum are nearly the same.

Dosimetric evaluation of rectum and bladder using image-based CT planning and orthogonal radiographs with ICRU 38 recommendations in intracavitary brachytherapy

The purpose is to compare CT-based dosimetry with International Commission on Radiation Units and Measurements (ICRU 38) bladder and rectum reference points in patients of carcinoma of uterine cervix treated with intracavitary brachytherapy (ICA). Twenty-two consecutive patients were evaluated. Orthogonal radiographs and CT images were acquired and transferred to PLATO planning system. Bladder and rectal reference points were identified according to ICRU 38 recommendations. Dosimetry was carried out based on Manchester system. Patient treatment was done using ¹⁹²Iridium high dose rate (HDR) remote after-loading machine based on the conventional radiograph-based dosimetry. ICRU rectal and bladder point doses from the radiograph plans were compared with D₂, dose received by 2 cm³ of the organ receiving maximum dose from CT plan. V₂, volume of organ receiving dose more than the ICRU reference point, was evaluated. The mean (±standard deviation) volume of rectum and bladder was 60 (±28) cm³ and 138 (±41) cm³ respectively. The mean reference volume in radiograph and CT plan was 105 (±7) cm³ and 107 (±7) cm³ respectively. It was found that 6 (±4) cm³ of rectum and 16 (±10) cm³ of bladder received dose more than the prescription dose. V₂ of rectum and bladder was 7 (±1.7) cm³ and 20.8 (±6) cm³ respectively. Mean D₂ of rectum and bladder was found to be 1.11 (±0.2) and 1.56 (±0.6) times the mean ICRU reference points respectively. This dosimteric study suggests that comparison of orthogonal X-ray-based and CT-based HDR ICA planning is feasible. ICRU rectal point dose correlates well with maximum rectal dose, while ICRU bladder point underestimates the maximum bladder dose.

Total reference air kerma: to what extent can it predict intracavitary volume enclosed by isodose surfaces during multiple high-dose rate brachytherapy?

BACKGROUND

The International Commission on Radiation Units and Measurements (ICRU) report 38 recommends reporting of total reference air kerma (TRAK) and reference ICRU isodose volumes during intracavitary brachytherapy (ICBT) in cancer of the cervix. The present study attempts to estimate the volumes enclosed by isodose surfaces from TRAK and evaluate its utility to represent doses to organs of interest.

MATERIAL AND METHODS

Volumes encompassed by isodose surfaces of 3 Gy, 6 Gy, 9 Gy, and 12 Gy were obtained for 90 high-dose rate (HDR) ICBT procedures. These were used to derive a relation between isodose volumes, TRAK/dose (K/D), and rectal and bladder doses.

RESULTS

Actual volumes (V) encompassed by isodose surfaces were reflected as a quadratic function of K/D (r(2)=0.998) and the expression, V=-23.09+1295.99(K/D)+5661.65(K/D)(2) gave the best estimates for various volumes. No correlation was observed between TRAK and bladder (r(2)=0.086) or rectal doses (r(2)=0.082).

CONCLUSIONS

Estimates of volumes encompassed by different dose levels from TRAK could be derived with reasonable certainty. However, TRAK fails to correlate with rectal and bladder doses.

Comparison of radiography- and computed tomography-based treatment planning in cervix cancer in brachytherapy with specific attention to some quality assurance aspects

INTRODUCTION

A modern approach in treatment planning for cervix carcinoma is based on a series of computed tomography (CT) sections and 3D dose computation. When these techniques were not yet available, dose evaluation was based on orthogonal radiographs. The CT based planning provides information on target and organ volumes and dose-volume histograms. The radiography based planning provides only dimensions and doses at selected points. The aim of the presented study is to correlate the information obtained with the two approaches for high dose-rate (HDR) brachytherapy of cervix carcinoma.

METHODS

For the study 28 patients with 35 applications receiving HDR treatment with Ir-192 were investigated. The planning system PLATO (Nucletron) was used. The different aspects of available data, results and inaccuracies regarding quality assurance were looked at.

RESULTS

From the CT based planning, the volume, location and dose-volume histograms were calculated for the CTV, rectum and bladder. From the radiography-based planning, the dose to point A (prescription), point B, rectum and bladder ICRU reference points [14], points related to the bony structures could be evaluated as well as volumes receiving different dose levels. These two sets of information were compared and following mean values derived. For a dose prescription of 7 Gy at point A, as an average, 83% (44 cm(3)) of the clinical target volume (CTV) receives at least 7 Gy. The mean dose at the rectum ICRU reference point is 4.3 Gy, and 12% (9 cm(3)) of the rectum is encompassed by the 4.3 Gy isodose. The mean dose at the bladder ICRU reference point is 5.8 Gy, and 8% (16 cm(3)) of the bladder is encompassed by the 5.8 Gy isodose. The maximum dose to the rectum is 1.5 times higher than the dose at the ICRU reference point, and for the bladder 1.4 times higher. Uncertainties caused by the reconstruction of the applicator and merging of isodoses could be evaluated.

DISCUSSION

The subdivision of different approaches and the transfer from point doses to volumes in treatment planning is possible and practical for the treatment of cervix carcinoma in brachytherapy.