Secondary cancer risk from modern external-beam radiotherapy of prostate cancer patients: Impact of fractionation and dose distribution

Modern radiotherapy (RT) uses altered fractionation, long beam-on time and image-guided procedure. This study aimed to compare secondary cancer risk (SCR) associated with primary field, scatter/leakage radiations and image-guided procedure in prostate treatment using intensity-modulated RT (IMRT), CyberKnife stereotactic body RT (CK-SBRT) in relative to 3-dimensional conformal RT (3D-CRT). Prostate plans were generated for 3D-CRT, IMRT (39 fractions of 2 Gy), and CK-SBRT (five fractions of 7.25 Gy). Excess absolute risk (EAR) was calculated for organs in the primary field using Schneider's mechanistic model and concept of organ equivalent dose (OED) to account for dose inhomogeneity. Doses from image-guided procedure and scatter/leakage radiations were determined by phantom measurements. The results showed that hypofractionation relative to conventional fractionation yielded lower SCR for organs in primary field (p ≤ 0.0001). SCR was further modulated by dose-volume distribution. For organs near the field edge, like the rectum and pelvic bone, CK-SBRT plan rendered better risk profiles than IMRT and 3D-CRT because of the absence of volume peak in high dose region (relative risk [RR]: 0.65, 0.22, respectively, p ≤ 0.0004). CK-SBRT and IMRT generated more scatter/leakage and imaging doses than 3D-CRT (p ≤ 0.0002). But primary field was the major contributor to SCR. EAR estimates (risk contributions, primary field: scatter/leakage radiations: imaging procedure) were 7.1 excess cases per 104 person-year (PY; 3.64:2.25:1) for CK-SBRT, 9.93 (7.32:2.33:1) for IMRT and 8.24 (15.99:2.35:1) for 3D-CRT (p ≤ 0.0002). We conclude that modern RT added more but small SCR from scatter/leakage and imaging doses. The primary field is a major contributor of risk which can be mitigated by the use of hypofractionation.

MALAT1 Decreases the Sensitivity of Head and Neck Squamous Cell Carcinoma Cells to Radiation and Cisplatin

BACKGROUND/AIM

Two-thirds of head and neck squamous cell carcinoma (HNSCC) patients present with locally advanced (LA) stages and have a poor survival rate. The aim of this study was to investigate the roles of the long non-coding RNAs MALAT1 on radiation and cisplatin sensitivity of HNSCC cells.

MATERIALS AND METHODS

Clonogenic, cell viability, and apoptosis assays were performed in cells following MALAT1 knockdown using CRISPR/Cas9 system.

RESULTS

MALAT1 was overexpressed in HNSCC cell lines as compared to a non-tumorigenic cell line. The number of colonies formed after radiation was significantly reduced in MALAT1 knockdown cells. The IC₅₀ value of cisplatin in MALAT1 knockdown cells was lower than that of the control cells. MALAT1 knockdown resulted in cell cycle arrest at G₂/M phase, DNA damage and apoptotic cell death.

CONCLUSION

MALAT1 knockdown enhanced the sensitivity of HNSCC cells to radiation and cisplatin partly through the induction of G₂/M cell cycle arrest resulting in DNA damage and apoptosis.

Multi-institutional evaluation using the end-to-end test for implementation of dynamic techniques of radiation therapy in Thailand

AIM

In this study, an accuracy survey of intensity-modulated radiation therapy (IMRT) and volumetric arc radiation therapy (VMAT) implementation in radiotherapy centers in Thailand was conducted.

BACKGROUND

It is well recognized that there is a need for radiotherapy centers to evaluate the accuracy levels of their current practices, and use the related information to identify opportunities for future development.

MATERIALS AND METHODS

An end-to-end test using a CIRS thorax phantom was carried out at 8 participating centers. Based on each center's protocol for simulation and planning, linac-based IMRT or VMAT plans were generated following the IAEA (CRP E24017) guidelines. Point doses in the region of PTVs and OARs were obtained from 5 ionization chamber readings and the dose distribution from the radiochromic films. The global gamma indices of the measurement doses and the treatment planning system calculation doses were compared.

RESULTS

The large majority of the RT centers (6/8) fulfilled the dosimetric goals, with the measured and calculated doses at the specification points agreeing within ±3% for PTV and ±5% for OARS. At 2 centers, TPS underestimated the lung doses by about 6% and spinal cord doses by 8%. The mean percentage gamma pass rates for the 8 centers were 98.29 ± 0.67% (for the 3%/3 mm criterion) and 96.72 ± 0.84% (for the 2%/2 mm criterion).

CONCLUSIONS

The 8 participating RT centers achieved a satisfactory quality level of IMRT/VMAT clinical implementation.

In vivo whole body dosimetry measurement technique of total body irradiation: a 12-year retrospective study result from one institute in Thailand

OBJECTIVE

The main problem of total body irradiation (TBI) is how to maintain radiation dose homogeneity throughout the body during a treatment course. The simple set up treatment with non-complicated in vivo dosimetry measurement technique is the ideal method to solve this problem. For this reason, the authors have reported the results of in vivo dosimetry measurement method to prove the reliability of dose distribution from the authors' TBI technique.

MATERIAL AND METHOD

The authors reviewed the data of dose measurement record from 53 patients' treatment files to report the uniformity of absorbed in vivo dose distribution throughout the whole body from TBI with semiconductor detectors and ionization chamber with the accepted homogeneity within +/- 10% of the prescribed dose. The result was reported in the term of mean and standard deviation of absorbed dose difference from the prescribed dose.

RESULTS

The uniformity of radiation dose distribution throughout the whole body of all patients calculated from semiconductors was accepted with mean difference value of -3.2 +/- 2.5% from the prescribed dose and the difference of mean absorbed dose value at midline point between semiconductor and ionization chamber was 4 +/- 3.3%.

CONCLUSION

This TBI dosimetry measurement technique has been proved to exhibit the reliability of dose homogeneity throughout the whole body within the accepted value. This could be applied for use at any institute that has some limitation in resources and small treatment room.

Matchline dosimetry in step and shoot IMRT fields: a film study

The Varian millennium 120 multileaf collimator has curved leaf ends. Transmission through the leaf ends generates a small asymmetric penumbral dose effect. This design can lead to hot spots between neighbouring beam segments during step and shoot IMRT dose delivery. We have observed some matchlines with film for clinical beams optimized using the pinnacle radiotherapy treatment planning system; hence we sought to verify the optimum leaf offset required to minimize the matchline effect. An in-house program was created to control the MLC leaf banks in 2 cm steps with a 2 cm gap. The gap was varied by the following offset values from 0.0 to 0.1 cm. Two types of radiographic films (Kodak EDR and XV films) and a radiochromic film (Gafchromic MD-55-2) were used to measure the optical density maps. The films were positioned in a solid water phantom perpendicular to the beam axis and irradiated at d(max) using a 6 MV photon beam. An ion chamber (IC4) was used to measure point doses for normalization in a beam umbral minima position. The relative mean peak to valley dose ratios measured with no leaf offset were 1.31, 1.30 and 1.31 for the XV, EDR2 and Gafchromic films, respectively. For a 0.07 cm gap per leaf and a performance of end leaf repeatability of 0.01 cm, the central matchline was reduced to about 1.0 for all dosimeters, with two mini-peaks measured as 1.05, 1.05 and 1.08 each side of the matchline, for XV, EDR2 and Gafchromic, respectively. The average relative dose across the umbra for this offset was XO-mat V = 1.01, EDR = 1.01 and radiochromic film = 1.02, respectively. While we expected the beam penumbral tails from segment neighbours to cause overprediction of the dose in the central valley regions due to the energy response of radiographic films, by normalizing all dosimeters to an ion chamber reading in the minimum we could not observe any major shape distortion between the radiographic film and radiochromic film results. In conclusion, relative doses measured by radiographic and radiochromic films agree well with IC4 within +/-2%.

Intensity-modulated radiation therapy: overlapping co-axial modulated fields

The Varian multi-leaf collimator has a 14.5 cm leaf extension limit from each carriage. This means the target volumes in the head and neck region are sometimes too wide for standard width-modulated fields to provide adequate dose coverage. A solution is to set up asymmetric co-axial overlapping fields. This protects the MLC carriage while in return the MLC provides modulated dose blending in the field overlap region. Planar dose maps for coincident fields from the Pinnacle radiotherapy treatment planning system are compared with planar dose maps reconstructed from radiographic film and electronic portal images. The film and portal images show small leaf-jaw matchlines at each field overlap border. Linear profiles taken across each image show that the observed leaf-jaw matchlines from the accelerator images are not accounted for by the treatment planning system. Dose difference between film reconstructed electronic portal images and planning system are about 2.5 cGy in a modulated field at d(max). While the magnitude of the dose differences are small improved round end leaf modelling combined with a finer dose calculation grid may minimize the discrepancy between calculated and delivered dose.

Intensity modulated radiation therapy: Film verification of planar dose maps

A new slow response radiographic film (Kodak EDR) is compared with a more traditional faster responding dosimetry film (Kodak XV) in an intensity modulated radiation therapy x-ray field. Dose profiles derived from the two films are compared with doses obtained using a radiotherapy dose planning system (Philips-Pinnacle) which calculates planar dose using a collapsed cone convolution algorithm. Comparisons of the dose maps delivered from film with the Pinnacle dose maps are useful to ensure accurate dose delivery. The Pinnacle dose maps agreed with measurement using both film types to within +/- 3% (in the umbral region) at depths ranging from d(max) to 15 cm. Both XV and EDR films can be used to verify IMRT. EDR film is better suited for dosimetry for combined field dose maps due to its useful dose range of 1-5 Gy. We found XV to be more suitable for individual field dose maps (dose range 0-1 Gy) as no scaling of monitor units were required to achieve acceptable optical density response.