Comparing treatment uncertainty for ultra- vs. standard-hypofractionated breast radiation therapy based on in-vivo dosimetry

In vivo dose measurements for tangential field-in-field ultra-hypofractionated breast radiotherapy

INTRODUCTION

Ultra-hypofractionated radiotherapy (UHF-RT) mandates more accuracy in each part of the treatment cycle to maximize cure rates and minimize toxicities. In vivo dosimetry is a direct method for verifying overall treatment accuracy. This study evaluated uncertainties in the delivered dose of Hypofractionated (HF) and UHF Whole Breast Irradiation (WBI) and to analyze the accuracy of the workflow to pave the way for a wide-scale use of UHF-RT.

METHODS

Thirty-three breast cancer cases, including 16 HF-WBI and 17 UHF-WBI were treated with 3D conformal Radiotherapy (3D-CRT), where 79 fields were analyzed for dose verification. The measurement point was set at the beam entrance (1.5 cm depth). The expected dose at Dmax was calculated via TPS. Before in vivo measurements, diode detectors were tested and calibrated. We developed initial validation measurements for UHF-RT on an anthropomorphic breast phantom for the first time.

RESULTS

For RANDO phantom, the percentage difference between measured and calculated doses showed an average of -0.52 ± 5.4%, in addition to an excellent dose reproducibility within 0.6%. The overall in vivo measurements for studied cases showed that 83.5% of the measured doses were within ±5% and only 1.8% of the measured doses were greater than ±10% of the calculated doses. The percentage accuracy was slightly larger for UHF cohort (84.2%) compared to HF cohort (83.2%). The maximum percentage difference between them was less than 1%.

CONCLUSION

Breast in vivo dosimetry is an adequate tool for treatment verification that improves the accuracy of the treatment cycle. UHF-RT may contribute in reducing the long waiting lists, increasing patient convenience, and saving the available resources for breast cancer patients.

Validation of an in vivo transit dosimetry algorithm using Monte Carlo simulations and ionization chamber measurements

PURPOSE

Transit dosimetry is a safety tool based on the transit images acquired during treatment. Forward-projection transit dosimetry software, as PerFRACTION, compares the transit images acquired with an expected image calculated from the DICOM plan, the CT, and the structure set. This work aims to validate PerFRACTION expected transit dose using PRIMO Monte Carlo simulations and ionization chamber measurements, and propose a methodology based on MPPG5a report.

METHODS

The validation process was divided into three groups of tests according to MPPG5a: basic dose validation, IMRT dose validation, and heterogeneity correction validation. For the basic dose validation, the fields used were the nine fields needed to calibrate PerFRACTION and three jaws-defined. For the IMRT dose validation, seven sweeping gaps fields, the MLC transmission and 29 IMRT fields from 10 breast treatment plans were measured. For the heterogeneity validation, the transit dose of these fields was studied using three phantoms: 10 , 30 , and a 3 cm cork slab placed between 10 cm of solid water. The PerFRACTION expected doses were compared with PRIMO Monte Carlo simulation results and ionization chamber measurements.

RESULTS

Using the 10 cm solid water phantom, for the basic validation fields, the root mean square (RMS) of the difference between PerFRACTION and PRIMO simulations was 0.6%. In the IMRT fields, the RMS of the difference was 1.2%. When comparing respect ionization chamber measurements, the RMS of the difference was 1.0% both for the basic and the IMRT validation. The average passing rate with a γ(2%/2 mm, TH = 20%) criterion between PRIMO dose distribution and PerFRACTION expected dose was 96.0% ± 5.8%.

CONCLUSION

We validated PerFRACTION calculated transit dose with PRIMO Monte Carlo and ionization chamber measurements adapting the methodology of the MMPG5a report. The methodology presented can be applied to validate other forward-projection transit dosimetry software.

Robustness analysis of surface-guided DIBH left breast radiotherapy: personalized dosimetric effect of real intrafractional motion within the beam gating thresholds

PURPOSE

The robustness of surface-guided (SG) deep-inspiration breath-hold (DIBH) radiotherapy (RT) for left breast cancer was evaluated by investigating any potential dosimetric effects due to the residual intrafractional motion allowed by the selected beam gating thresholds. The potential reduction of DIBH benefits in terms of organs at risk (OARs) sparing and target coverage was evaluated for conformational (3DCRT) and intensity-modulated radiation therapy (IMRT) techniques.

METHODS

A total of 192 fractions of SGRT DIBH left breast 3DCRT treatment for 12 patients were analyzed. For each fraction, the average of the real-time displacement between the isocenter on the daily reference surface and on the live surface ("SGRT shift") during beam-on was evaluated and applied to the original plan isocenter. The dose distribution for the treatment beams with the new isocenter point was then calculated and the total plan dose distribution was obtained by summing the estimated perturbed dose for each fraction. Then, for each patient, the original plan and the perturbed one were compared by means of Wilcoxon test for target coverage and OAR dose-volume histogram (DVH) metrics. A global plan quality score was calculated to assess the overall plan robustness against intrafractional motion of both 3DCRT and IMRT techniques.

RESULTS

Target coverage and OAR DVH metrics did not show significant variations between the original and the perturbed plan for the IMRT techniques. 3DCRT plans showed significant variations for the left descending coronary artery (LAD) and the humerus only. However, none of the dose metrics exceeded the mandatory dose constraints for any of the analyzed plans. The global plan quality analysis indicated that both 3DCRT and IMRT techniques were affected by the isocenter shifts in the same way and, generally, the residual isocenter shifts more likely tend to worsen the plan in all cases.

CONCLUSION

The DIBH technique proved to be robust against residual intrafractional isocenter shifts allowed by the selected SGRT beam-hold thresholds. Small-volume OARs located near high dose gradients showed significant marginal deteriorations in the perturbed plans with the 3DCRT technique only. Global plan quality was mainly influenced by patient anatomy and treatment beam geometry rather than the technique adopted.

The use of in-vivo dosimetry to identify head and neck cancer patients needing adaptive radiotherapy

BACKGROUND AND PURPOSE

Head and neck cancer (HNC) patients experiencing anatomical changes during their radiotherapy (RT) course may benefit from adaptive RT (ART). We investigated the sensitivity of an electronic portal imaging device (EPID)-based in-vivo dosimetry (EIVD) systemto detect patients that require ART and identified its limitations.

MATERIALS AND METHODS

A retrospective study was conducted for 182 HNC patients: laryngeal cancer without elective lymph nodes (group A), postoperative RT (group B) and primary RT including elective lymph nodes (group C). The effect of anatomical changes on the dose distribution and volumetric changes was quantified. The receiver operating characteristic curve was used to obtain the optimal cut-off value for the gamma passing rate (%GP) with a dose difference of 3% and a distance to agreement of 3mm.

RESULTS

Fifty HNC patients receiving ART were analyzed: 1 in group A, 10 in group B and 39 in group C. Failed fractions (FFs) occurred in 1/1, 6/10 and 23/39 cases before ART in group A, B and C respectively. In the four cases in group B without FFs, only minor dosimetric changes were observed. One of the cases in group C without FFs had significant dosimetric changes (false negative). Three cases received ART because of clinical reasons that cannot be detected by EIVD. The optimal cut-off value for the %GP was 95%/95.2% for old/new generation machines respectively.

CONCLUSION

EIVD in combined with 3D imaging techniques can be synergistic in the detection of anatomical changes in HNC patients who benefit from ART.

Clinical implementation of the first Cherenkov imaging system in a community-based hospital

Purpose

To document experiences with one year of clinical implementation of the first Cherenkov imaging system and share the methods that we developed to utilize Cherenkov imaging to improve treatment delivery accuracy in real-time.

Methods

A Cherenkov imaging system was installed commissioned and calibrated for clinical use. The optimal room lighting conditions and imaging setup protocols were developed to optimize both image quality and patient experience. The Cherenkov images were analyzed for treatment setup and beam delivery verification.

Results

We have successfully implemented a clinical Cherenkov imaging system in a community-based hospital. Several radiation therapy patient setup anomalies were found in 1) exit dose to the contralateral breast, 2) dose to the chin due to head rotation for a supraclavicular field, 3) intrafractional patient motion during beam delivery, and 4) large variability (0.5 cm to 5 cm) in arm position between fractions. The system was used to deliver deep inspiration breath hold (DIBH) treatment delivery of an electron treatment beam. Clinical process and procedures were improved to mitigate the identified issues to ensure treatment delivery safety and to improve treatment accuracy.

Conclusion

The Cherenkov imaging system has proven to be a valuable clinical tool for the improvement of treatment delivery safety and accuracy at our hospital. With only minimal training the therapists were able to adjust or correct treatment positions during treatment delivery as needed. With future Cherenkov software developments Cherenkov imaging systems could provide daily surface guided radiotherapy (SGRT) and real time treatment delivery quality control for all 3D and clinical setup patients without adding additional radiation image dose as in standard kV, MV and CBCT image verifications. Cherenkov imaging can greatly improve clinical efficiency and accuracy, making real time dose delivery consistency verification and SGRT a reality.