The aim of this study which was carried out at Radiation Therapy Department of Karachi Institute of Radiotherapy and Nuclear Medicine (KIRAN), was to assess the impact of patient transition from 15 MV to 6 MV in body sites where the use of high-energy beams has been preferred (like pelvic region). The important consideration in any such transition was to keep in mind the tumor control probability (TCP) and the dose to the organs at risk (OAR). Prior to site selection a literature survey was carried[1-4].

Single slice body contours were obtained for each patient and the oncologist drew the target area and OAR. These marked contours were digitized. One better alternative to the above step was to use CT scans instead of manual contour taking; manual contours were not taken for those patients who had already undergone a CT scan. ‘Vidar VXR-12 Plus film Digitizer' was used to scan these CT slices to treatment planning system. Target localization with the help of CT scan was more accurate. But there were very few patients available with CT scans. Optimal treatment plans were prepared for 6 MV and 15 MV photon beams for each patient. The treatment planning software used was DSS (Decision Support System) by Multidata. This software closely resembled the properties of a 2.5D modern treatment planning system. Finally both the plans were reviewed by the radiation oncologist.

Treatment plans have been provided for one patient from each pelvic site (because of restriction of space). In addition the mean data in the form of tables for each pelvic site has been furnished as well.

Unless and otherwise mentioned, three field technique was used.

Bladder
Seven bladder patients were studied. The total dose prescribed to the tumor was 6000 cGy delivered in daily fraction of 200 cGy (30 fractions). In all cases 15 MV photon beams were superior to 6 MV in terms of quality index, global maximum doses, doses to the organs at risk (OAR), target uniformity and the entrance doses (table 1).Comparison of entrance doses amongst 15 MV and 6 MV revealed that from the anterior field an additional 20% dose was delivered in case of 6 MV photons. Direct transition (DT) resulted in 10% extra dose from the anterior field, and also delivered high doses from lateral aspects. However no such dose concentration was seen for 15 MV and 6 MV optimum plans (Figures 1 & 2). In case of DT the entrance dose from left lateral side was higher by 13.3% and 10.9% as compared to 15 MV and 6 MV respectively, where as from right lateral side in case of DT the entrance dose was up by 13.6% and 10.4% from 15 MV and 6 MV respectively (Figures 2 & 3).

Table 1: Bladder patient mean data

Fig 1: Bladder 15 MV plan

Fig 2: Bladder 6 MV plan

Fig 3: Bladder DT plan

The above trends indicated that careful patient planning was required before making any transition from 15 MV to 6 MV incase of bladder, other wise there were chances of skin reaction from the lateral sides, normally above 3000 cGy skin reactions are very common. Mean quality index for tumor (Table 1) indicated that in case of 15 MV the area under DVH was more and target coverage was better, because quality index is the ratio of area under the DVH and the total area. Mean dose to rectum for 15 MV was less by 10 cGy that implied that in 30 fractions a total reduction of 300 cGy was made to bladder dose, which was clinically significant. The TD5/5 dose for bladder is 6000 cGy. For bladder and other pelvic sites to be discussed in the following sections, superiority of high-energy beams in various parameters of interest was because of the reason that lateral body separation in pelvic region was greater as compared to any other body site, which favored the use of higher energy beams.

Prostate
Three prostate patients were studied. 7000 cGy was delivered in 35 fractions (200 cGy per fraction). In all cases 15 MV photon beams were superior to 6 MV in terms of quality index, global maximum doses, doses to the organs at risk (OAR), target uniformity and entrance doses (Table 2). The 50% isodose curve for 15 MV photon beam was closer to the tumor both from anterior and posterior ends as compared to 6 MV and DT cases (Figures 4,5 & 6). Comparison of entrance doses between 15 MV and 6 MV showed that from the anterior field an additional 19.2% dose was delivered in case of 6 MV photons. DT resulted in an additional 6.8% dose from the anterior field. From left lateral side in case of direct transition the entrance dose was up by 13.2% and 9.7% as compared to 15 MV and 6 MV respectively. For right lateral side in case of direct transition the entrance dose was up by 13.3% and 10.2% as compared to 15 MV and 6 MV respectively. As high as 85% isodose curve was visible from the lateral sides (Figure 6), where no such dose concentration was visible for 15 MV and 6 MV (Figures 4 & 5) Mean dose to rectum for 15 MV beam was lower by 7 cGy as compared to 6 MV (Table 2). This suggested a reduction of 245 cGy in bladder dose during 35 fractions. Quality index (Table 2), global maximum doses, and doses to rectum and target uniformity followed similar trends as explained in section[7,8].

Table 2: Prostate patient mean data

Fig 4: Prostate 15 MV plan

Fig 5: Prostate 6 MV plan

Fig 6: Prostate DT plan

Uterine cervix
Two uterine cervix cases were studied (high and low energy for both 3-F and box techniques, Table 3). The prescribed dose was 6000 cGy with 200 cGy per fraction and a total of 30 fractions, the organs at risk were bladder and rectum and the TD5/5 for both these organs was 6000 cGy. For 3-F technique entrance doses from anterior side for 15 MV photons was reduced by 26% and 10% as compared to 6 MV and DT cases respectively (Figures 7, 8 & 9). For DT the doses from right lateral sides were higher by 15.5% and 12% as compared to 15 MV and 6 MV respectively. For left lateral side the entrance dose was up by 9.75% and 6.75% as compared to 15 MV and 6 MV. Mean dose to rectum and bladder were reduced by 8.5 cGy and 27 cGy respectively for 15 MV beams in case of 3-F technique, resulting in a reduction of 255 cGy and 810 cGy to rectum and bladder doses respectively during 30 fractions.

Table 3: Cervix patient mean data

Fig 7: Uterine cervix 15 MV 3-F plan

Fig 8: Uterine cervix 6 MV 3-F plan

Fig 9: Uterine cervix DT 3-F plan

For box technique entrance doses from anterior side was up by 8.25% and 5.75% for 6 MV and DT cases respectively as compared to 15 MV. From posterior side dose was up by 14.5% and 6.5% for 6 MV and DT cases respectively In case of DT the doses from right lateral side were up by 14.5% and 7.75% in comparison with 15 and 6 MV respectively. For left lateral side the entrance dose was up by 9.75% and 6.75% as compared to 15 MV and 6 MV respectively. Mean doses to rectum and bladder were lower by 10 cGy and 13 cGy respectively for 15 MV beams in case of 3-F technique, resulting in a reduction of 260 cGy and 200 cGy to rectum and bladder doses respectively in 30 fractions. Directly shifting the patients from 15 MV to 6 MV was not feasible as there were high doses from lateral aspect and the doses to femur head was quite significant.

Data in table 3 also suggested that dose to bladder and rectum in box as well as 3-F techniques was quite reduced in case of 15 MV beams and especially in case of 3-F technique the dose to bladder was significantly reduced; therefore DT would result in unacceptable doses to bladder and there fore should be avoided. The quality index in case of high- energy beams was improved. (Table 3). Comparison of doses on femur head
While irradiating the pelvis, femur head doses are important; it is desired to keep these doses as low as possible. The doses for femur head were calculated on a single patient, where the femur heads were marked on the CT scan so as to determine their exact location. Doses on left and right femur heads for 15 MV photons were 42% & 43% of the prescribed dose respectively. For 6 MV left and right femur heads received 58% & 59.5% of the prescribed dose. This trend could be verified from the isodose distributions (Table 4, Figures 10, 11 & 12). Therefore before shifting a patient directly from 15 MV to 6 MV proper planning for femur head doses was required.

Table 4: Femur head patient mean data

Fig 10: Femur head 15 MV plan

Fig 11: Femur head 6 MV plan

Fig 12: Femur head DT plan

Despite the fact that quality index for all cases were the, still the lateral doses were high and therefore re-planning was necessary.

1) Khan FM, Potish RA. Treatment Planning in Radiation Oncology. Williams & Wilkins, USA, 1998.

2) Bentel GC. Radiation Therapy Treatment Planning. 2nd Edition. McGraw-Hill Health Professions Division, 1996.

3) Chao KSC, Perez CA, Brady LW. Radiation Oncology: Management Decisions. Philadelphia: Lippincott Williams & Wilkins, 1999,2002.

4) Dobbs J, Barrett A, Ash D. Practical Radiotherapy Planning. 3rd edition. London, 1999.

5) ICRU Report 50, Prescribing, Recording, and Reporting Photon Beam Therapy. International commission on radiation units and measurements, Maryland 20614 U.S.A, 1993.

6) Khan FM. The Physics of Radiation Therapy. 3rd Edition, Philadelphia: Lippincott Williams & Wilkins, 2003.

7) Review of Radiation Oncology Physics: A Handbook for Teachers and Students. Podgorsak EB, editor. International Atomic Energy Agency, Austria 2003.

8) ICRU Report 62, Prescribing, Recording and Reporting Photon Beam Therapy (Supplement to ICRU Report 50). International commission on radiation units and measurements, Maryland 20614 U.S.A, 1999.