This workshop was not intended to reach any conclusions as to the superiority of one technology over another, nor did the participants feel that was possible in most cases due to a paucity of data. The workshop summarized the state of the science as of the end of 2006 (some references have been updated, however, in the discussion that follows), emphasized the need for developing quality assurance procedures and technologies suitable for clinical trials and clinical practice, and prompted vigorous discussion as to what clinical trials were possible and necessary before the new technologies entered routine medical practice.
Challenges Posed by the Advanced Technologies
Determining the ‘Correct’ Size and Shape of the Target Volume
The advanced technologies can conform the radiation dose very closely to the CTV and spare the organs at risk. In order to conform the radiation dose closely to the CTV, however, it is crucial to know the precise size and shape of the target volume. That can be a challenge because current imaging tools are often inadequate for determining the “correct” target volume, as evidenced by the fact that concordance among target volumes drawn by different experts on the same patients’ images was low—not only for covering microscopic disease extensions beyond the tumor visible on imaging studies but, in some cases, even for the gross tumor volume.[5-7]
Preventing Excessive Dose Heterogeneity Within the CTV
In a recent study involving 803 patients, whose IMRT treatment plans were done by experienced physicists (each of whom had already planned at least 50 IMRT cases) at five different institutions, it was discovered that in nearly one-half of the patients, the plan delivered to the CTV a maximum dose that was more than 10% higher than prescribed by the physician (it was 40% higher in the worst case). Furthermore, in nearly two-thirds of patients, the plan delivered to the CTV a minimum dose at least 10% lower than prescribed (it was 100% lower, ie, zero, in the worst case). Those authors did not report the outcomes (tumor control or toxicity) relative to the doses, but in our current state of knowledge those “hot” and “cold” spots are rather alarming since we do not yet have the ability to identify “subvolumes” within the clinical target volume that should receive doses substantially higher or lower than prescribed by the physician.
Preventing Errors in Treatment Delivery
In another recent study, investigators at 128 Radiation Therapy Oncology Group (RTOG) member institutions, in order to be credentialed for participation in NCI-sponsored clinical trials employing IMRT, imaged an anthropomorphic phantom, developed an IMRT treatment plan, and then treated that phantom. The goal was to deliver to the CTV a dose within 7% of the planned dose, with 4‑mm agreement between the high-dose gradient and the edge of the critical organs at risk to be spared.
Approximately one-third of those institutions failed this test on the first attempt. It was discovered that the dose delivered differed from the planned dose by up to 22%, while the high-dose region was off by up to 1.5 cm. The sources of error were many, including:
• Inaccurate positioning of the phantom
• Inaccurate modeling by the treatment planning system (TPS) algorithm of field sizes formed by the multileaf collimator leaves
• Inaccurate handling by the TPS of inhomogeneity corrections
• Variable handling of cost-function optimization by algorithms that could not be controlled by the user
• Incorrect data input into the TPS
• Indexing errors in the table movement system
• Incorrect monitor unit settings