Dose specification and quality assurance of radiation therapy oncology group protocol 95-17; a cooperative group study of iridium-192 breast implants as sole therapy

Abstract

Purpose: The Radiation Therapy Oncology Group (RTOG) protocol 95-17 was a Phase I/II trial to evaluate multicatheter brachytherapy as the sole method of adjuvant breast radiotherapy for Stage I/II breast carcinoma after breast-conserving surgery. Low- or high-dose-rate sources were allowed. Dose prescription and treatment evaluation were based on recommendations in the International Commission on Radiation Units and Measurements (ICRU), Report 58 and included the parameters mean central dose (MCD), average peripheral dose, dose homogeneity index (DHI), and the dimensions of the low- and high-dose regions.

Methods and materials: Three levels of quality assurance were implemented: (1) credentialing of institutions was required before entering patients into the study; (2) rapid review of each treatment plan was conducted before treatment; and (3) retrospective review was performed by the Radiological Physics Center in conjunction with the study chairman and RTOG dosimetry staff.

Results: Credentialing focused on the accuracy of dose calculation algorithm and compliance with protocol guidelines. Rapid review was designed to identify and correct deviations from the protocol before treatment. The retrospective review involved recalculation of dosimetry parameters and review of dose distributions to evaluate the treatment. Specifying both central and peripheral doses resulted in uniform dose distributions, with a mean dose homogeneity index of 0.83 +/- 0.06.

Conclusions: Vigorous quality assurance resulted in a high-quality study with few deviations; only 4 of 100 patients were judged as representing minor variations from protocol, and no patient was judged as representing major deviation. This study should be considered a model for quality assurance of future trials.

Conflict of interest statement

Conflicts of Interest Notification No potential conflicts of interest exist.

Figures

1

Sketch illustrating the determination of Mean Central Dose, MCD, as used in RTOG protocol 95-17. The centroids of the triangles formed by adjacent seed trains (catheters) correspond to the centers of the regions of low dose and low dose gradient between the seed trains, called Geometric Center Dose, GCD. The MCD was defined in the central plane of the implant and is the mean of the GCDs.

2

Sketch illustrating the definition of Average Peripheral Dose (APD). The TV was depicted as an ellipse determined by the clips, and the axes were approximately centered on the TV. The four peripheral doses were averaged in both the central and sagittal planes.

3

An idealized two-plane implant used as the benchmark case for RTOG protocol 95-17. The tylectomy cavity was marked by clips and provisions for both HDR and LDR were included.