1012 Quantification of variation in organ position and volume during conformal radiotherapy of the prostate

250

I. J. Radiation

Oncology

0 Biology

0 Physics

Volume

36, Number

1, Supplement,

1996

1012 Quantification

of Variation

D. Creant, Herks, and

0. Lyassl, M.J. G.J. Kutcherl

‘Memorial Sloan-Kettering Boston; and 3Netherlands

in Organ

Position

Zelefskyt,

Cancer Cancer

and Volume

S. Built,

Center, Institute,

G.S.

New York; Amsterdam

During Mageras

*Joint

Conformal

Radiotherapy

1, V. Lucciot, Center

for

Z. Fuksl,

Radiation

Therapy,

Purpose: A previous study of internal organ motion (1). in which a group of twelve patients with prostate days, showed variations in the position of the prostate and seminal vesicles, with larger variations observed seminal vesicle positions were observed to be significantly correlated with changes in bladder volume. For amount of bladder tilling prior to scanning was not controlled, and large variations in bladder volume were these results, a second study in a larger group of patients has been undertaken, to examine the variability in organs when the amount of bladder filling is controlled.

of the Prostate S.A.

Leibel’, Harvard

H.M. Medical

Kooy*,

M.

van

School,

cancer received CT scans on different in the latter. In addition, prostate and most of the patients in the study the observed for some patients. In light of the position of the prostate and nearby

Materials and Methods: Patients underwent four serial CT scans, consisting of an initial planning scan and a subsequent scan at the beginning, middle and end of the treatment course. Each scan was performed under the same conditions as for treatment, with the patient scanned in the prone position with an immobilization device. In order to minimize the variability in rectal and bladder filling, no bowel contrast was used and patients were asked to void immediately prior to each scan. Contours of the prostatic capsule, seminal vesicles, rectum, bladder and small bowel were drawn on the axial CT slices of each scan by a physician. The subsequent scans were matched to the planning scan by aligning the pelvic bones with a chamfer matching algorithm (2). Variability in target and nontarget organ positions and volumes were determined using the contours from the different scans. Results: An analysis has been performed on the data from the first sixteen patients and compared to the earlier study. The one standard deviation displacement of the prostate center of mass from the planning position was 0.19 cm and 0.32 cm in the lateral and anterior-posterior (AP) directions, respectively, which was comparable to the previous measurements (0.11 cm and 0.25 cm, respectively). For the seminal vesicles the standard deviation of the displacement of the center of mass was 0.20 cm laterally and 0.50 cm in the AP direction, the latter being somewhat less than in the previous study (0.14 cm and 0.72 cm, respectively). Significant correlation was observed between changes in rectal volume and prostate AP displacement (probability of no correlation p=O.O25), as well as with seminal vesicle AP displacement (p
correlations

with

(I) E. Melian, G. Kutcher, S. Leibel et al. Int. J. Radiat. Oncol. Biol. Phys. 27: 137; 1993. (2) M. van Herk, H.M. Kooy. Med. Phys. 21: 1163-l 178; 1994.

1013 CLINICAL MUSCLE

VARIABILITY OF TARGET VOLUME INVASIVE BLADDER CANCER

DESCRIPTION

AND TREATMENT

John P L@uc’, Carole L Sharmck’. Richard. A. Cowan’. Graham Read’. Julie Mam’.

PLANS

IN CONFORMAL

RADRYIHERAPY

IN

David Mat?.

Depamncnts ofClinical Oacology ’ and Medical Physics’ ,CJmstie Hospital, Maachester. M20 4BX. England U.K.

Purpose/ Objective: The delineation of tumor and the production of a treatment plan to encompass this is the prime step in radiotherapy planning. Conformal radiotherapy is developing rapidly and although plentiful research has addressed the impIetnentation of the radiotherapy prescription, scam attention has been made to the fundamental step of production, by the clinician, of an appropriate target volume. As part of an ongoing randomized trial of conformal radiotherapy, in bladder cancer, we have therefore assessed the ltttetphysiciart variability of radiologists and radiation oncologists (RO) in assessing Gross Tumor VoIume(GTV) (ICRU 50) and the adherence of the radiation oncologists to the study protocol of producing a Planning Target Volume (PTV). Materials MR scans GTV onto the GTV. comparison

and Methods: Four patients with T3 carcinoma of bladder who had been entered into the trial were identified. The clinical details, and CT scans were made available. Eight RO and 3 dedicated diagnostic oncology radiologists were invited to directly outline the CT images on a planning computer consul. The RO in addition created a PTV following the trial protocol of l5mm margin around Three RO sub-specialized in Urological radiotherapy; all RO had completed training. Volumes were produced, for each clinician, and of these volumes and their isocenters were analyzed. In addition the margins allowed were measured aud compared.

Results: There was a maximum variation ratio (largest to smallest volume outlined) of the GTV in the four cases of I .74 among radiologists and 3.74 among oncologists. There was a significant difference( p= 0.01) in mean GTV between RO and the radiologists. The mean GTV ofthe RO exceeded the radiologists by a factor of 1.29 with a mean difference of 13.4 cm ’ The between observer variance within speciality comprised only 9.9 % of the total variance in the data having accounted for case and observers speciality The variation ratio in PTV among oncologists ranged from 1. 25 to 3.33. There was no significant difference in mean PI’V values between the two groups of RO. The between observer variance was, however, 42.2%. The variance in location of the isocenter was from I .37mm to 24.14mm from the centroid of the radiologists volume.Median margin per case ranged from 14.75 to 18.75mm. Mium margins allowed in each case varied from minus 7 mm to 9mm between RO. Conclusion: This study demonstrates signiticant interphysician variability in producing target volumes and radiation plans for conformal radiotherapy. The scale of this dillerence is clearly of significance with over threefold variation in volume arid with differences in positioning the isocenter greater than the margins allowed in this protocol. The factors leading to these differences will be further addressed.The existence of such variability. however. clearly needs to be accepted as a factor in the overall uncertainty analysis in conformal radiotherapy planning