Margins for geometric uncertainty around organs at risk in radiotherapy Alan McKenzie a, * , Marcel van Herk b , Ben Mijnheer b a Medical Physics Department, Bristol Oncology Centre, Horfield Road, Bristol BS2 8ED, UK b Radiotherapy Department, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands Received 5 September 2001; received in revised form 24 December 2001; accepted 5 February 2002 Abstract Background and purpose: ICRU Report 62 suggests drawing margins around organs at risk (ORs) to produce planning organ at risk volumes (PRVs) to account for geometric uncertainty in the radiotherapy treatment process. This paper proposes an algorithm for drawing such margins, and compares the recommended margin widths with examples from clinical practice and discusses the limitations of the approach. Method: The use of the PRV defined in this way is that, despite the geometric uncertainties, the dose calculated within the PRV by the treatment planning system can be used to represent the dose in the OR with a certain confidence level. A suitable level is where, in the majority of cases (90%), the dose-volume histogram of the PRV will not under-represent the high-dose components in the OR. In order to provide guidelines on how to do this in clinical practice, this paper distinguishes types of OR in terms of the tolerance doses relative to the prescription dose and suggests appropriate margins for serial-structure and parallel-structure ORs. Results: In some instances of large and parallel ORs, the clinician may judge that the complication risk in omitting a margin is acceptable. Otherwise, for all types of OR, systematic, treatment preparation uncertainties may be accommodated by an OR ! PRV margin width of 1.3S. Here, S is the standard deviation of the combined systematic (treatment preparation) uncertainties. In the case of serial ORs or small, parallel ORs, the effects of blurring caused by daily treatment execution errors (set-up and organ motion) should be taken into account. Near a region of high dose, blurring tends to shift the isodoses away from the unblurred edge as shown on the treatment planning system by an amount that may be represented by 0.5s. This margin may be used either to increase or to decrease the margin already calculated for systematic uncertainties, depending upon the size of the tolerance dose relative to the detailed planned dose distribution. Where the detailed distribution is unknown before the OR is delineated, then the overall margin for serial or small parallel ORs should be 1.3S 1 0.5s. Examples are given where the application of this algorithm leads to margin widths around ORs similar to those in use clinically. Conclusions: Using PRVs is appropriate both for forward and inverse planning. Dose-volume histograms of PRVs for serial- and parallel- structure ORs require careful interpretation. Nevertheless, use of the proposed algorithms for drawing margins around both serial and parallel ORs can alert the dosimetrist/radiation oncologist to the possibility of high-dose complications in individual treatment plans, which might be missed if no such margins were drawn. q 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Radiotherapy; Organs at risk; Geometric uncertainty; Margins; Treatment planning 1. Introduction Many workers have addressed the problem of geometric uncertainty in radiotherapy by determining the widths of margins to draw around clinical target volumes (CTVs) [1–3,6,7,9,11,13–15]. The converse problem is how to deal with organs at risk (ORs). ICRU Report 62 [5] suggests drawing margins around organs at risk (ORs) to produce planning organ at risk volumes (PRVs) to account for geometric uncertainty in the radiotherapy treatment process. In that report it is recommended that an integrated margin be added to the OR to compensate for any movements of the OR during the treatment, as well as uncertainties in the set- up during the whole treatment course. Some clinical exam- ples (treatment of breast cancer, cancer of the prostate and lung cancer) are given in which PRVs of various organs are shown. While a few workers have considered the effect of geometric uncertainty on the dose to ORs (for example, refs [4,14]), there is still a scarcity of advice in the literature dealing with the question of how to draw the margins recommended by the ICRU. Even in the clinical examples given in ICRU Report 62, no details are given about the origin of the margins drawn around the ORs. This paper suggests an approach to that problem which effectively mirrors that used in constructing margins around the CTV. The need to draw margins around a CTV arises because Radiotherapy and Oncology 62 (2002) 299–307 0167-8140/02/$ - see front matter q 2002 Elsevier Science Ireland Ltd. All rights reserved. PII: S0167-8140(02)00015-4 www.elsevier.com/locate/radonline * Corresponding author.