Intensity-modulated radiotherapy at the Royal Marsden NHS Trust

Authors
EJ Adams, DJ Convery, AP Warrington, S Webb steve@icr.ac.uk
Joint Department of Physics, The Royal Marsden NHS Trust and Institute of Cancer Research.

INTRODUCTION

IMRT is likely to offer significant advantages for patients who have prostate cancer with high risk of spread to regional lymph nodes. It is difficult to deliver a radical radiotherapy treatment to these patients using conventional techniques due to the presence of the dose-limiting small bowel, which is enclosed within the pelvic nodes planning target volume (PTV), as shown in Fig. 1. An IMRT technique has therefore been implemented to allow doses of 70Gy and 50Gy to be delivered to the prostate and pelvic nodes respectively, without exceeding the small bowel tolerance dose of 45Gy. It is intended to escalate the pelvic nodes dose to 55Gy and then 60Gy as part of a phase I study. Other organs-at-risk (OARs) which are considered include the rectum and bladder.

Fig 1: 3D representation of organs for a typical patient. The small bowel (yellow) sits in the concavity formed by the pelvic nodes (orange). Also shown are the prostate (red) and rectum (green)

TREATMENT PLANNING

CT scans are taken at 5mm intervals, and prostate, pelvic node and OAR volumes are contoured. Treatment planning is carried out using TMS v6.0 (MDS-Nordion). Five coplanar fields are used, typically with gantry angles of 30°, 90°, 180°, 270° and 330°. The user enters dose-volume constraints for the target organs and critical structures (Fig. 2), and can also specify the maximum number of segments allowed per beam. The TMS system uses a gradient-descent inverse-planning algorithm with a dose-based objective function (Gustafsson A, Lind BK & Brahme A. Med Phys 21: 343-356, 1994). The leaf sequences for a step-and-shoot multileaf collimator (MLC) delivery are calculated as part of the optimisation process, thus ensuring that the optimised plan is representative of that delivered in practice.

A typical plan is shown in Fig. 2. The dose distribution conforms well to the target volumes, sparing the small bowel, bladder and rectum. Average dose statistics are shown in Table 1. The generated modulations are smooth (Fig. 3), requiring 10 – 15 segments per beam. Once an acceptable plan has been generated, it is transferred to an Elekta linear accelerator via a Dicom link.

Fig. 2a: Dose distribution through nodes. Isodoses shown are 55Gy (orange), 50Gy (green), 45Gy, 40Gy (light blue), 30Gy, 20Gy, 10Gy (dark blue).

Fig. 2b: Dose distribution through prostate. Isodoses shown are 70Gy, 67Gy (green), 65Gy, 60Gy, 50Gy (light blue), 40Gy, 30Gy, 20Gy, 10Gy (dark blue).

Table 1: Average dose statistics for 4 test patients. D95% is the dose received by 95% of the volume. V40Gy is the volume receiving ³40Gy.

Fig 3: Intensity modulation for a posterior field from a 5-field prostate + pelvic nodes plan

Before the first clinical use of the system, a variety of validation measurements were carried out for a series of test patients. IMRT beams from a patient plan can be exported and then imported onto a phantom using the DICOM facilities of TMS. This allowed verification of both single fields and complete plans.

Single field verification

• Single IMRT beams imported onto homogeneous phantom.
• Comparison with film measurements generally within 3% or 3mm.
• Output factor measurements (0.2cc ionisation chamber) within 2%.

Complete treatment verification

• 5-field IMRT plan imported onto CT scan of anthropomorphic phantom.
• TLD chips loaded into phantom for absolute dose verification; mean discrepancies within 1-2% of the prescription dose (SD 3-4%)
• Films inserted between the slices RANDO phantom to verify dose distribution; good agreement with prediction (see Fig. 4)

Fig. 4: Comparison of measured (solid) and predicted (dotted) isodoses for a 5-field prostate + pelvic nodes plan, at the level of the pelvic nodes

Following these results, clinical use of the system commenced in September 2001. Prior to each patient’s treatment, dosimetric validation of the treatment plan is carried out, including film, TLD and ion chamber measurements. This QA program is quite extensive, and may be streamlined as experience and confidence in the system increase.