Abstract Book

S124

ESTRO 37

painting initiatives. Cases in which the dose distribution notably changes as a result of geometric errors (e.g. in particle therapies) go beyond the PTV’s capabilities. Risks of PTV deprecation As the CTV and PTV margins simply add up, an overly tight CTV may be compensated for by a correct, yet overly safe PTV around it, and therefore go unnoticed. Substitution of this PTV by a method that more aptly addresses the geometrical errors, leading to a more concentrated dose, may reveal the underlying CTV error, and lead to reduced tumor control. Besides a tool to shape radiation dose distributions, the PTV is also used to evaluate a treatment plan. As long as the various implementations of probability based techniques differ in their principles and details, the plans generated by different initiatives are difficult to compare. Typically, an optimized plan will always outperform all other plans when evaluated for the quality it was optimized for (PTV coverage of a PTV-based plan, TCP of a TCP optimized plan). An objective method of plan evaluation and comparison in terms of tumor control (regardless optimization strategy) needs to be established, otherwise deprecation of the PTV might bring chaos. Practicalities A PTV-less technique has been developed to act as a drop-in replacement, providing the same minimum CTV dose confidence, but able to shape the dose to the individual patient anatomy. This leads to a clinical benefit, but increases the complexity of a customized radiotherapy chain. Commercial support would lower this threshold, however commercial partners need a guaranteed market before investments can be made. Conclusion While there is ample reason to replace the PTV, it will take a joint effort. SP-0241 Coverage probability based dose optimization - first clinical applications M. Alber 1 1 Heidelberg University Clinic, Department of Radiooncology, Stuttgart, Germany Abstract text The planning target volume (PTV) concept is a device to enforce an acceptable dose coverage of the clinical target volume (CTV) in treatment planning. It consists of two elements: an envelope volume and a dose prescription to every point of this volume. The prescription is unconditional: every point is treated the same. The coverage probability concept (CovP) of dose optimization makes the dose prescription conditional on the probability that the volume of interest occupies this point in a number of geometric scenarios. In other words, the concept does not alter the prescription dose for each point, but the strength with which this prescription is enforced during dose optimization. Points with low CovP are given up more readily when conflicting dose prescriptions demand. This is an attempt at quantifying the frequent practice to accept more readily cold spots in the outer shell of a PTV, especially in the vicinity of organs at risk. CovP brings the fuzzyness to the PTV concept that it always had in practice. Strictly speaking, CovP ought only be applied for non- superficial tumours in a homogeneous density environment and for the compensation of systematic organ displacement- and setup errors. It fails where the PTV concept also fails, with tumour volumes extending to the patient surface, or with sizeable dose changes as a consequence of geometrical uncertainties. Since CovP can mediate PTV underdosages based on uncertainty probabilities, it is important to quantify these. Estimates about target volume shifts and setup errors can be derived for populations and individuals by repeated

Symposium: What could replace the PTV?

SP-0240 The challenging path towards clinical adoption of probabilistic planning M. Witte 1 1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands

Abstract text Introduction

Over the past decades, technological innovations allowed the incidental radiation dose delivered outside designated treatment volumes to be reduced. Conventional box fields became conformal radiotherapy, and then IMRT. Treatment room image guidance has reduced geometric uncertainties in the dose delivery, allowing smaller treatment volumes. In parallel, advanced imaging techniques have been under development, providing an increasing insight into the heterogeneous nature of tumors. Investigations about the extent of subclinical disease surrounding the tumor are being performed based on surgical series, and indirectly by correlating delivered radiation dose patterns with outcome. Potentially, modern radiotherapy can match the highly heterogeneous nature of invasive cancer growth and the geometrical intricacies of fractionated treatment delivery to a moveable patient with a high level of flexibility to modulate the dose. Nonetheless, the current day standard of care relies on the definition of binary volumes (GTV, CTV, PTV) along with uniform dose prescriptions. This practice provides a high confidence that the intended dose will in fact be delivered to the tumor, however at the cost of treating large volumes to high dose. Thus, dose limiting constraints of surrounding healthy structures prevent the delivery of high tumor doses. In the pre-IMRT era, the adequacy of this practice was dictated by the limitations of the planning and delivery systems. Nowadays, the ingrained routine of subdividing a patient volume into either target or not, and the flat dose paradigm to accompany it, may become obstacles in the further development of the field. Extending the GTV concept, dose painting initiatives have emerged to deliver intentionally non-uniform tumor dose distributions, based on biological and/or functional imaging. Initiatives to replace the CTV are still hampered by the largely unknown distributions of microscopic disease, however now that information about geometric uncertainties is abundant through IGRT, the definition of a PTV could be considered obsolete as soon as better plan optimization techniques based on probabilities become available. Reasons to deprecate the PTV Foremost, one should consider the potential clinical benefit of PTV-less planning. While the PTV is intended to provide a certain confidence of minimum tumor dose, there is very limited clinical evidence about the impact of geometric miss on local tumor control. Therefore, it is difficult to estimate the potential benefit of an alternative planning technique which trades a higher dose to the bulk of the tumor against an increased risk to miss a small part of the tumor. We will explore the balance between these competing effects for a spherical target volume and idealized 3D dose distributions, concentrating on two aspects inherent to the PTV: its geometrical rigidity (each volume element in the PTV is considered equally important, even though small geometrical errors are more likely that larger ones) and its dose-effect simplicity (increased/residual effect of dose above/below prescription level is not acknowledged). Also, the practice of margin expansion does not easily mix with the emerging dose painting by numbers, or biological dose